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Seagrasses are the only
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four
families Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
(
Posidoniaceae ''Posidonia'' is a genus of flowering plants. It contains nine species of marine plants ("seagrass"), found in the seas of the Mediterranean and around the south coast of Australia. The APG system (1998) and APG II system (2003) accept this gen ...
,
Zosteraceae Zosteraceae (one of the four seagrasses families, Kubitzki ed. 1998) is a family of Marine (ocean), marine Perennial plant, perennial flowering plants found in temperate and subtropical coastal waters, with the highest diversity located around Ko ...
,
Hydrocharitaceae Hydrocharitaceae is a flowering plant family including 16 known genera with a total of ca 135 known species (Christenhusz & Byng 2016), that including a number of species of aquatic plant, for instance the Vallisneria spiralis, tape-grasses, the ...
and
Cymodoceaceae Cymodoceaceae is a Family (biology), family of flowering plants, sometimes known as the "manatee-grass family", which includes only marine species. The 2016 APG IV system, APG IV does recognize Cymodoceaceae and places it in the order Alismatale ...
), all in the order
Alismatales The Alismatales (alismatids) are an order of flowering plants including about 4,500 species. Plants assigned to this order are mostly Tropical vegetation, tropical or Aquatic plant, aquatic. Some grow in fresh water, some in marine habitats. ...
(in the clade of
monocotyledon Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. Th ...
s). Seagrasses evolved from
terrestrial plant A terrestrial plant is a plant that grows on, in, or from land. Other types of plants are aquatic plant, aquatic (living in water), epiphytic (living on trees) and lithophytes, lithophytic (living in or on rocks). The distinction between aquatic ...
s which recolonised the ocean 70 to 100 million years ago. The name ''seagrass'' stems from the many species with long and narrow
leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
, which grow by
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary bud ...
extension and often spread across large "
meadows A meadow ( ) is an open habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species ...
" resembling
grassland A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other Herbaceous plant, herbs. Grassl ...
; many species superficially resemble terrestrial
grass Poaceae () or Gramineae () is a large and nearly ubiquitous Family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species culti ...
es of the family
Poaceae Poaceae () or Gramineae () is a large and nearly ubiquitous Family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species culti ...
. Like all
autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
plants, seagrasses photosynthesize, in the submerged
photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine
pollination Pollination is the transfer of pollen from an Stamen, anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents can ...
and complete their life cycle underwater. While it was previously believed this pollination was carried out without pollinators and purely by sea current drift, this has been shown to be false for at least one species, ''
Thalassia testudinum ''Thalassia testudinum'', commonly known as turtlegrass, is a species of Marine (ocean), marine seagrass. It forms meadows in shallow sandy or muddy locations in the Caribbean Sea and the Gulf of Mexico. Turtle grass and other seagrasses form sea ...
'', which carries out a mixed biotic-abiotic strategy. Crustaceans (such as crabs, '' Majidae zoae'', '' Thalassinidea zoea'') and
syllid Syllidae is a Family (biology), family of small to medium-sized polychaete worms. Syllids are distinguished from other polychaetes by the presence of a muscular region of the anterior digestive tract known as the ''proventricle''. Syllid worms ...
polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...
worm larvae have both been found with pollen grains, the plant producing nutritious mucigenous clumps of pollen to attract and stick to them instead of nectar as terrestrial flowers do. Seagrasses form dense underwater
seagrass meadow A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and ...
s which are among the most productive ecosystems in the world. They function as important
carbon sink A carbon sink is anything, natural or otherwise, that accumulates and stores some carbon-containing chemical compound for an indefinite period and thereby removes carbon dioxide () from the atmosphere. Globally, the two most important carbon si ...
s and provide habitats and food for a diversity of
marine life Marine life, sea life, or ocean life is the aquatic plant, plants, aquatic animal, animals and other organisms that live in the seawater, salt water of seas or oceans, or the brackish water of coastal estuary, estuaries. At a fundamental leve ...
comparable to that of
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s.


Overview

Seagrasses are a paraphyletic group of marine
angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s which evolved in parallel three to four times from land plants back to the sea. The following characteristics can be used to define a seagrass species. It lives in an
estuarine An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...
or in the
marine environment Marine habitats are habitats that support marine life. Marine life depends in some way on the seawater, saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or Natur ...
, and nowhere else. The
pollination Pollination is the transfer of pollen from an Stamen, anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents can ...
takes place underwater with specialized pollen. The seeds which are dispersed by both biotic and abiotic agents are produced underwater. Material was copied from this source, which is available under
Creative Commons Attribution 3.0 International License
The seagrass species have specialized leaves with a reduced
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in th ...
, an
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...
which lacks
stomata In botany, a stoma (from Greek language, Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. ...
and is the main
photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
tissue. The
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary bud ...
or underground stem is important in anchoring. The roots can live in an anoxic environment and depend on oxygen transport from the leaves and rhizomes but are also important in the
nutrient A nutrient is a Chemical substance, substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungus, fungi, and protists. Nutrients can be incorporated into cells for me ...
transfer processes. Seagrasses profoundly influence the physical, chemical, and biological environments of coastal waters. Though seagrasses provide invaluable
ecosystem service Ecosystem services are the many and varied benefits to humans provided by the natural environment and healthy ecosystems. Such ecosystems include, for example, agroecosystems, forest ecosystem, grassland ecosystems, and aquatic ecosystems. Th ...
s by acting as breeding and nursery ground for a variety of organisms and promote commercial fisheries, many aspects of their physiology are not well investigated. Several studies have indicated that seagrass habitat is declining worldwide. Ten seagrass species are at elevated risk of extinction (14% of all seagrass species) with three species qualifying as
endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and invas ...
. Seagrass loss and degradation of seagrass
biodiversity Biodiversity or biological diversity is the variety and variability of life, life on Earth. Biodiversity is a measure of variation at the Genetics, genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem d ...
will have serious repercussions for marine biodiversity and the human population that depends upon the resources and ecosystem services that seagrasses provide. Seagrasses form important coastal ecosystems. The worldwide endangering of these sea meadows, which provide food and habitat for many marine species, prompts the need for protection and understanding of these valuable resources.


Evolution

Around 140 million years ago, seagrasses evolved from early monocots which succeeded in conquering the marine environment.
Monocot Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. Th ...
s are grass and grass-like
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s (angiosperms), the seeds of which typically contain only one embryonic leaf or
cotyledon A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germination, germinating see ...
.
Terrestrial plant A terrestrial plant is a plant that grows on, in, or from land. Other types of plants are aquatic plant, aquatic (living in water), epiphytic (living on trees) and lithophytes, lithophytic (living in or on rocks). The distinction between aquatic ...
s evolved perhaps as early as 450 million years ago from a group of
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
. Seagrasses then evolved from terrestrial plants which migrated back into the ocean. Between about 70 million and 100 million years ago, three independent seagrass lineages (
Hydrocharitaceae Hydrocharitaceae is a flowering plant family including 16 known genera with a total of ca 135 known species (Christenhusz & Byng 2016), that including a number of species of aquatic plant, for instance the Vallisneria spiralis, tape-grasses, the ...
,
Cymodoceaceae Cymodoceaceae is a Family (biology), family of flowering plants, sometimes known as the "manatee-grass family", which includes only marine species. The 2016 APG IV system, APG IV does recognize Cymodoceaceae and places it in the order Alismatale ...
complex, and
Zosteraceae Zosteraceae (one of the four seagrasses families, Kubitzki ed. 1998) is a family of Marine (ocean), marine Perennial plant, perennial flowering plants found in temperate and subtropical coastal waters, with the highest diversity located around Ko ...
) evolved from a single lineage of the
monocotyledonous Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. Th ...
flowering plants. Other plants that colonised the sea, such as
salt marsh A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open Seawater, saltwater or brackish water that is regularly flooded by the tides. It is ...
plants,
mangrove A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evoluti ...
s, and
marine algae Marine primary production is the chemical synthesis in the ocean of organic compounds from atmospheric or dissolved carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it al ...
, have more diverse evolutionary lineages. In spite of their low species diversity, seagrasses have succeeded in colonising the continental shelves of all continents except Antarctica. Recent
sequencing In genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinians ...
of the genomes of ''
Zostera marina ''Zostera marina'' is a flowering vascular plant species as one of many kinds of seagrass, with this species known primarily by the English name of eelgrass with seawrack much less used, and refers to the plant after breaking loose from the submer ...
'' and '' Zostera muelleri'' has given a better understanding of
angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
adaption In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a stat ...
to the sea. During the evolutionary step back to the ocean, different genes have been lost (e.g., stomatal genes) or have been reduced (e.g., genes involved in the synthesis of
terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeabl ...
s) and others have been regained, such as in genes involved in sulfation. Genome information has shown further that adaption to the marine habitat was accomplished by radical changes in
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
composition. However the cell walls of seagrasses are not well understood. In addition to the ancestral traits of
land plant The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all li ...
s one would expect habitat-driven adaption process to the new environment characterized by multiple
abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and Physical property, physical parts of the Natural environment, environment that affect living organisms and the functioning of ecosystems. Abiotic factors and ...
(high amounts of salt) and biotic (different seagrass grazers and bacterial colonization) stressors. The cell walls of seagrasses seem intricate combinations of features known from both angiosperm land plants and marine macroalgae with new structural elements.


Taxonomy

Today, seagrasses are a polyphyletic group of marine angiosperms with around 60 species in five families (
Zosteraceae Zosteraceae (one of the four seagrasses families, Kubitzki ed. 1998) is a family of Marine (ocean), marine Perennial plant, perennial flowering plants found in temperate and subtropical coastal waters, with the highest diversity located around Ko ...
,
Hydrocharitaceae Hydrocharitaceae is a flowering plant family including 16 known genera with a total of ca 135 known species (Christenhusz & Byng 2016), that including a number of species of aquatic plant, for instance the Vallisneria spiralis, tape-grasses, the ...
,
Posidoniaceae ''Posidonia'' is a genus of flowering plants. It contains nine species of marine plants ("seagrass"), found in the seas of the Mediterranean and around the south coast of Australia. The APG system (1998) and APG II system (2003) accept this gen ...
,
Cymodoceaceae Cymodoceaceae is a Family (biology), family of flowering plants, sometimes known as the "manatee-grass family", which includes only marine species. The 2016 APG IV system, APG IV does recognize Cymodoceaceae and places it in the order Alismatale ...
, and
Ruppiaceae ''Ruppia'', also known as the widgeonweeds, ditch grasses or widgeon grass, is the only extant genus in the family Ruppiaceae, with eight known species. These are aquatic plants widespread over much of the world. The genus name honours Heinrich ...
), which belong to the order Alismatales according to the
Angiosperm Phylogeny Group The Angiosperm Phylogeny Group (APG) is an informal international group of systematic botany, botanists who collaborate to establish a consensus on the Taxonomy (biology), taxonomy of flowering plants (angiosperms) that reflects new knowledge abo ...
IV System. The genus ''
Ruppia ''Ruppia'', also known as the widgeonweeds, ditch grasses or widgeon grass, is the only extant genus in the family Ruppiaceae, with eight known species. These are aquatic plants widespread over much of the world. The genus name honours Heinrich ...
'', which occurs in brackish water, is not regarded as a "real" seagrass by all authors and has been shifted to the Cymodoceaceae by some authors. The
APG IV system The APG IV system of flowering plant classification is the fourth version of a modern, mostly Molecular phylogenetics, molecular-based, list of systems of plant taxonomy, system of plant taxonomy for flowering plants (angiosperms) being develope ...
and The Plant List Webpage do not share this family assignment. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License


Sexual recruitment

Seagrass populations are currently threatened by a variety of
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human im ...
stressor A stressor is a chemical or biological agent, environment (biophysical), environmental condition, Stimulus (physiology) , external stimulus or an event seen as causing Stress (biology), stress to an biological organism, organism. Psychologically s ...
s. The ability of seagrasses to cope with environmental perturbations depends, to some extent, on genetic variability, which is obtained through sexual recruitment. By forming new individuals, seagrasses increase their
genetic diversity Genetic diversity is the total number of Genetics, genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species and can be attributed to the span of survival for a species. ...
and thus their ability to colonise new areas and to adapt to environmental changes. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Seagrasses have contrasting
colonisation Colonization, or colonisation, constitutes large-scale population movements wherein migrants maintain strong links with their, or their ancestors', former country – by such links, gain advantage over other inhabitants of the territory. When ...
strategies. Some seagrasses form
seed bank A seed bank (also seed banks or seeds bank) stores seeds to preserve genetic diversity; hence it is a type of gene bank. There are many reasons to store seeds. One is to preserve the genes that plant breeders need to increase yield, disease resi ...
s of small seeds with hard
pericarp Fruit anatomy is the plant anatomy of the internal structure of fruit. Fruits are the mature ovary (plants), ovary or ovaries of one or more flowers. They are found in three main anatomical categories: aggregate fruits, multiple fruits, and simp ...
s that can remain in the dormancy stage for several months. These seagrasses are generally short-lived and can recover quickly from disturbances by not germinating far away from parent meadows (e.g., '' Halophila'' sp., '' Halodule'' sp., '' Cymodocea'' sp., ''
Zostera ''Zostera'' is a small genus of widely distributed seagrasses, commonly called marine eelgrass, or simply seagrass or eelgrass, and also known as seaweed by some fishermen and recreational boaters including yachtsmen. The genus ''Zostera'' con ...
'' sp. and '' Heterozostera'' sp. In contrast, other seagrasses form dispersal
propagule In biology, a propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by Biological dispersal, dispersal. The propagule is usually distinct in form from the parent organism. Propagules are ...
s. This strategy is typical of long-lived seagrasses that can form buoyant fruits with inner large non-dormant seeds, such as the genera ''
Posidonia ''Posidonia'' is a genus of flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words (' ...
'' sp., '' Enhalus'' sp. and '' Thalassia'' sp. Accordingly, the seeds of long-lived seagrasses have a large dispersal capacity compared to the seeds of the short-lived type, which permits the evolution of species beyond unfavourable light conditions by the seedling development of parent meadows. The seagrass ''
Posidonia oceanica ''Posidonia oceanica'', commonly known as Neptune grass or Mediterranean tapeweed, is a seagrass Seagrasses are the only flowering plants which grow in marine (ocean), marine environments. There are about 60 species of fully marine seagra ...
'' (L.) Delile is one of the oldest and largest species on Earth. An individual can form
meadows A meadow ( ) is an open habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species ...
measuring nearly 15 km wide and can be hundreds to thousands of years old. ''P. oceanica''
meadows A meadow ( ) is an open habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species ...
play important roles in the maintenance of the
geomorphology Geomorphology (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following ...
of Mediterranean coasts, which, among others, makes this seagrass a priority habitat of conservation. Currently, the flowering and recruitment of ''P. oceanica'' seems to be more frequent than that expected in the past. Further, this seagrass has singular adaptations to increase its survival during recruitment. The large amounts of nutrient reserves contained in the seeds of this seagrass support shoot and root growth, even up to the first year of seedling development. In the first months of
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an flowering plant, angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spor ...
, when leaf development is scarce, ''P. oceanica'' seeds perform
photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
activity, which increases their photosynthetic rates and thus maximises seedling establishment success. Seedlings also show high morphology plasticity during their
root system In mathematics, a root system is a configuration of vector space, vectors in a Euclidean space satisfying certain geometrical properties. The concept is fundamental in the theory of Lie groups and Lie algebras, especially the classification and ...
development by forming adhesive
root hair Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root ...
s to help
anchor An anchor is a device, normally made of metal , used to secure a Watercraft, vessel to the Seabed, bed of a body of water to prevent the craft from drifting due to Leeway, wind or Ocean current, current. The word derives from Latin ''ancora' ...
themselves to rocky sediments. However, many factors about ''P. oceanica'' sexual recruitment remain unknown, such as when photosynthesis in seeds is active or how seeds can remain anchored to and persist on substrate until their root systems have completely developed.


Intertidal and subtidal

Seagrasses occurring in the intertidal and subtidal zones are exposed to highly variable environmental conditions due to tidal changes. Subtidal seagrasses are more frequently exposed to lower light conditions, driven by plethora of natural and human-caused influences that reduce light penetration by increasing the density of suspended opaque materials. Subtidal light conditions can be estimated, with high accuracy, using artificial intelligence, enabling more rapid mitigation than was available using ''in situ'' techniques. Seagrasses in the
intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of Marine habitat, habitats with var ...
are regularly exposed to air and consequently experience extreme high and low temperatures, high photoinhibitory
irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The International System of Units, SI unit of irradiance is the watt per square metre (W⋅m−2). The Centimetre–gram–second system of units, CGS unit e ...
, and
desiccation Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container ...
stress relative to subtidal seagrass. Such extreme temperatures can lead to significant seagrass dieback when seagrasses are exposed to air during low tide. Desiccation stress during low tide has been considered the primary factor limiting seagrass distribution at the upper intertidal zone. Seagrasses residing the intertidal zone are usually smaller than those in the subtidal zone to minimize the effects of emergence stress. Intertidal seagrasses also show light-dependent responses, such as decreased photosynthetic efficiency and increased photoprotection during periods of high irradiance and air exposure. In contrast, seagrasses in the
subtidal zone The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...
adapt to reduced light conditions caused by light attenuation and scattering due to the overlaying water column and suspended particles. Seagrasses in the deep subtidal zone generally have longer leaves and wider leaf blades than those in the shallow subtidal or intertidal zone, which allows more photosynthesis, in turn resulting in greater growth. Seagrasses also respond to reduced light conditions by increasing
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf"). Chlorophyll al ...
content and decreasing the chlorophyll a/b ratio to enhance
light absorption In physics, absorption of electromagnetic radiation is how matter (typically electrons bound in atoms) takes up a photon's energy — and so transforms radiant energy, electromagnetic energy into internal energy of the absorber (for example, th ...
efficiency by using the abundant wavelengths efficiently. As seagrasses in the intertidal and subtidal zones are under highly different light conditions, they exhibit distinctly different photoacclimatory responses to maximize photosynthetic activity and photoprotection from excess irradiance. Seagrasses assimilate large amounts of
inorganic carbon Total inorganic carbon (''C''T or TIC) is the sum of the inorganic carbon species. Carbon Chemical compound, compounds can be distinguished as either Organic compound, organic or inorganic, and Dissolved organic carbon, dissolved or Particulate ...
to achieve high level production. Marine
macrophytes Aquatic plants are plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae ...
, including seagrass, use both and (
bicarbonate In inorganic chemistry, bicarbonate (International Union of Pure and Applied Chemistry, IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a Polyatomic ion, polyatomic anion wi ...
) for photosynthetic carbon reduction. Despite air exposure during low tide, seagrasses in the intertidal zone can continue to photosynthesize utilizing CO2 in the air. Thus, the composition of inorganic carbon sources for seagrass photosynthesis probably varies between intertidal and subtidal plants. Because stable carbon isotope ratios of plant tissues change based on the inorganic carbon sources for photosynthesis, seagrasses in the intertidal and subtidal zones may have different stable carbon isotope ratio ranges.


Seagrass meadows

Seagrass bed A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish Brackish water, sometimes termed brack water, is water occurring ...
s/meadows can be either monospecific (made up of a single species) or in mixed beds. In
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
areas, usually one or a few species dominate (like the eelgrass ''
Zostera marina ''Zostera marina'' is a flowering vascular plant species as one of many kinds of seagrass, with this species known primarily by the English name of eelgrass with seawrack much less used, and refers to the plant after breaking loose from the submer ...
'' in the North Atlantic), whereas
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
beds usually are more diverse, with up to thirteen
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
recorded in the
Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...
. Seagrass beds are diverse and productive
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s, and can harbor hundreds of associated species from all phyla, for example juvenile and adult
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
,
epiphytic An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called Phorophy ...
and free-living
macroalgae Seaweed, or macroalgae, refers to thousands of species of Macroscopic scale, macroscopic, Multicellular organism, multicellular, ocean, marine algae. The term includes some types of ''Rhodophyta'' (red), ''Brown algae, Phaeophyta'' (brown) and ...
and
microalgae Microalgae or microphytes are microscopic scale, microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine life, marine systems, living in both the water column and sediment. They are unicellul ...
,
mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
s, bristle worms, and
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-Parasitism, parasitic nematodes also known as eelworms. They are a diverse animal phylum inhab ...
s. Few species were originally considered to feed directly on seagrass
leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
(partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass
herbivory A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
is an important link in the food chain, feeding hundreds of species, including
green turtle The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the Family (biology), family Cheloniidae. It is the only species in the genus ''Chelonia' ...
s,
dugong The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest m ...
s,
manatee Manatees (family (biology), family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivory, herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing thr ...
s,
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
,
geese A goose (plural, : geese) is a bird of any of several waterfowl species in the family (biology), family Anatidae. This group comprises the genera ''Anser (bird), Anser'' (the grey geese and white geese) and ''Branta'' (the black geese). Some o ...
,
swan Swans are birds of the family (biology), family Anatidae within the genus ''Cygnus''. The swans' closest relatives include the goose, geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form t ...
s,
sea urchin Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard s ...
s and
crab Crabs are Decapoda, decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen#Other animals, abdomen) ( el, :wikt:βραχύς, βραχύς , translit=brachys = short, / = tail), usually hid ...
s. Some fish species that visit/feed on seagrasses raise their young in adjacent
mangrove A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evoluti ...
s or
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s. Seagrasses trap sediment and slow down water movement, causing suspended sediment to settle out. Trapping sediment benefits
coral Corals are marine invertebrates within the class (biology), class Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important C ...
by reducing sediment loads, improving photosynthesis for both coral and seagrass. Although often overlooked, seagrasses provide a number of
ecosystem services Ecosystem services are the many and varied benefits to humans provided by the natural environment and healthy ecosystems. Such ecosystems include, for example, agroecosystems, forest ecosystem, grassland ecosystems, and aquatic ecosystems. Th ...
. Seagrasses are considered
ecosystem engineer An ecosystem engineer is any species that creates, significantly modifies, maintains or destroys a habitat. These organisms can have a large impact on species richness and landscape-level heterogeneity of an area. As a result, ecosystem enginee ...
s. This means that the plants alter the ecosystem around them. This adjusting occurs in both physical and chemical forms. Many seagrass species produce an extensive underground network of roots and
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary bud ...
which stabilizes sediment and reduces
coastal erosion Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of Wind wave, waves, Ocean current, currents, tides, wind-driven water, waterborne ice, or other impact ...
. This system also assists in oxygenating the sediment, providing a hospitable environment for
sediment-dwelling organism Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''Biota (ecology ...
s. Seagrasses also enhance
water quality Water quality refers to the Chemical property, chemical, Physical property, physical, and Biology, biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against wh ...
by stabilizing heavy metals, pollutants, and excess nutrients. The long blades of seagrasses slow the movement of water which reduces wave energy and offers further protection against coastal
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...
and
storm surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure area, low-pressure weather systems, such as cyclones. It is measured as the rise in water ...
. Furthermore, because seagrasses are underwater plants, they produce significant amounts of oxygen which oxygenate the water column. These meadows account for more than 10% of the ocean's total carbon storage. Per hectare, it holds twice as much carbon dioxide as rain forests and can sequester about 27.4 million tons of CO2 annually.
Seagrass meadows A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and ...
provide food for many marine herbivores. Sea turtles, manatees, parrotfish, surgeonfish, sea urchins and pinfish feed on seagrasses. Many other smaller animals feed on the epiphytes and invertebrates that live on and among seagrass blades. Seagrass meadows also provide physical habitat in areas that would otherwise be bare of any vegetation. Due to this three dimensional structure in the water column, many species occupy seagrass habitats for shelter and foraging. It is estimated that 17 species of coral reef fish spend their entire juvenile life stage solely on seagrass flats. These habitats also act as a nursery grounds for commercially and recreationally valued fishery species, including the gag grouper ('' Mycteroperca microlepis''), red drum,
common snook The common snook (''Centropomus undecimalis'') is a species of Seawater, marine fish in the family (biology), family Centropomidae of the order (biology), order Perciformes. The common snook is also known as the sergeant fish or robalo. It was o ...
, and many others. Some fish species utilize seagrass meadows and various stages of the life cycle. In a recent publication, Dr. Ross Boucek and colleagues discovered that two highly sought after flats fish, the common snook and spotted sea trout provide essential foraging habitat during reproduction. Sexual reproduction is extremely energetically expensive to be completed with stored energy; therefore, they require seagrass meadows in close proximity to complete reproduction. Furthermore, many commercially important
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordata, ...
s also reside in seagrass habitats including bay scallops ('' Argopecten irradians''),
horseshoe crab Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are Chelicerata, chelicerates, most closely rela ...
s, and
shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the Decapoda, decapod order, although some Shrimp#Non-decapods , crustaceans out ...
. Charismatic fauna can also be seen visiting the seagrass habitats. These species include
West Indian manatee The West Indian manatee (''Trichechus manatus''), also known as the North American manatee, is a large, aquatic mammal native to warm coastal areas of the Caribbean, from the eastern US to northern Brazil. Living alone or in herds, it feeds on un ...
,
green sea turtle The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the Family (biology), family Cheloniidae. It is the only species in the genus ''Chelonia' ...
s, and various species of sharks. The high diversity of marine organisms that can be found on seagrass habitats promotes them as a tourist attraction and a significant source of income for many coastal economies along the Gulf of Mexico and in the Caribbean. File:Floridian seagrass bed.jpg, ''
Thalassia testudinum ''Thalassia testudinum'', commonly known as turtlegrass, is a species of Marine (ocean), marine seagrass. It forms meadows in shallow sandy or muddy locations in the Caribbean Sea and the Gulf of Mexico. Turtle grass and other seagrasses form sea ...
''
seagrass bed A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish Brackish water, sometimes termed brack water, is water occurring ...
File:White-spotted puffer.jpg, White-spotted puffers, often found in seagrass areas File:Seagrass Meadow - Porthdinllaen.webm, Underwater footage of seagrass meadow, bull huss and conger eel


Seagrass microbiome


Seagrass holobiont

The concept of the
holobiont A holobiont is an assemblage of a Host (biology), host and the many other species living in or around it, which together form a discrete ecological unit through symbiosis, though there is controversy over this discreteness. The components of a ...
, which emphasizes the importance and interactions of a microbial host with associated microorganisms and viruses and describes their functioning as a single biological unit, has been investigated and discussed for many model systems, although there is substantial criticism of a concept that defines diverse host-microbe symbioses as a single biological unit. The holobiont and hologenome concepts have evolved since the original definition, and there is no doubt that symbiotic microorganisms are pivotal for the biology and ecology of the host by providing vitamins, energy and inorganic or organic nutrients, participating in defense mechanisms, or by driving the evolution of the host. Although most work on host-microbe interactions has been focused on animal systems such as corals, sponges, or humans, there is a substantial body of literature on plant holobionts. Plant-associated microbial communities impact both key components of the fitness of plants, growth and survival, and are shaped by nutrient availability and plant defense mechanisms. Several habitats have been described to harbor plant-associated microbes, including the rhizoplane (surface of root tissue), the
rhizosphere The rhizosphere is the narrow region of soil or Substrate (biology), substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Pore space in soil, Soil pores in the rhizosphere can ...
(periphery of the roots), the endosphere (inside plant tissue), and the phyllosphere (total above-ground surface area). The microbial community in the ''P. oceanica'' rhizosphere shows similar complexity as terrestrial habitats that contain thousands of taxa per gram of soil. In contrast, the chemistry in the rhizosphere of ''P. oceanica'' was dominated by the presence of sugars like sucrose and phenolics.


Cell walls

Seagrass
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
s contain the same
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic bond, glycosidic linkages. This carbohydrate c ...
s found in
angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
land plants, such as
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...
However, the cell walls of some seagrasses are characterised by
sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
d polysaccharides, which is a common attribute of
macroalgae Seaweed, or macroalgae, refers to thousands of species of Macroscopic scale, macroscopic, Multicellular organism, multicellular, ocean, marine algae. The term includes some types of ''Rhodophyta'' (red), ''Brown algae, Phaeophyta'' (brown) and ...
from the groups of
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometre 330px, Different lengths as in respect to the molecula ...
,
brown Brown is a color. It can be considered a composite color, but it is mainly a darker shade of orange. In the CMYK color model used in printing or painting, brown is usually made by combining the colors Orange (colour), orange and black. In the RGB ...
and also
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
. It was proposed in 2005 that the ability to synthesise sulfated polysaccharides was regained by marine angiosperms. Another unique feature of cell walls of seagrasses is the occurrence of unusual pectic polysaccharides called apiogalacturonans. In addition to polysaccharides,
glycoprotein Glycoproteins are proteins which contain oligosaccharide chains Covalent bond, covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a translation (genetics), cotranslational or posttranslational modifica ...
s of the
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline (Carbon, C5Hydrogen, H9Oxygen, O3Nitrogen, N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank. Structure and discovery In 1902, Hermann Emil Fischer isolated hydroxy ...
-rich glycoprotein family, are important components of cell walls of land plants. The highly glycosylated arabinogalactan proteins are of interest because of their involvement in both wall architecture and cellular regulatory processes. Arabinogalactan proteins are ubiquitous in seed land plants and have also been found in
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The Polypodiophyta, polypodiophytes include all living pteridop ...
s,
lycophyte The lycophytes, when broadly Circumscription (taxonomy), circumscribed, are a vascular plant (tracheophyte) subgroup of the kingdom Plantae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina ...
s and
moss Mosses are small, non-vascular flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to ...
es. They are structurally characterised by large polysaccharide moieties composed of
arabinogalactan Arabinogalactan, also known as galactoarabinan, larch arabinogalactan, and larch gum, is a biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Like other polymers, biopolymers consist of monomeric uni ...
s (normally over 90% of the molecule) which are covalently linked via
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline (Carbon, C5Hydrogen, H9Oxygen, O3Nitrogen, N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank. Structure and discovery In 1902, Hermann Emil Fischer isolated hydroxy ...
to relatively small protein/peptide backbones (normally less than 10% of the molecule). Distinct
glycan The terms glycans and polysaccharides are defined by IUPAC as synonyms meaning "compounds consisting of a large number of monosaccharides linked glycosidically". However, in practice the term glycan may also be used to refer to the carbohydrate p ...
modifications have been identified in different species and tissues and it has been suggested these influence physical properties and function. In 2020, AGPs were isolated and structurally characterised for the first time from a seagrass. Although the common backbone structure of land plant arabinogalactan proteins is conserved, the glycan structures exhibit unique features suggesting a role of seagrass arabinogalactan proteins in
osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration of ...
. Further components of secondary walls of plants are cross-linked phenolic polymers called
lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and Bark (botany), bark, because they l ...
, which are responsible for mechanical strengthening of the wall. In seagrasses, this polymer has also been detected, but often in lower amounts compared to angiosperm land plants. Thus, the cell walls of seagrasses seem to contain combinations of features known from both angiosperm land plants and marine macroalgae together with new structural elements. Dried seagrass leaves might be useful for papermaking or as insulating materials, so knowledge of cell wall composition has some technological relevance.


Threats and conservation

Despite only covering 0.1 - 0.2% of the ocean’s surface, seagrasses form critically important ecosystems. Much like many other regions of the ocean, seagrasses have been faced with an accelerating global decline. Since the late 19th century, over 20% of the global seagrass area has been lost, with seagrass bed loss occurring at a rate of 1.5% each year. Of the 72 global seagrass species, approximately one quarter (15 species) could be considered at a
Threatened Threatened species are any species (including animals, plants and fungus, fungi) which are vulnerable to Species endangerment, endangerment in the near future. Species that are threatened are sometimes characterised by the population dynamics ...
or
Near Threatened A near-threatened species is a species which has been Conservation status, categorized as "Near Threatened" (NT) by the International Union for Conservation of Nature as that may be vulnerable to Endangered species, endangerment in the near futu ...
status on the IUCN’s Red List of Threatened Species. Threats include a combination of natural factors, such as storms and disease, and anthropogenic in origin, including habitat destruction, pollution, and climate change. By far the most common threat to seagrass is human activity. Up to 67 species (93%) of seagrasses are affected by human activity along coastal regions. Activities such as coastal land development, motorboating, and fishing practices like
trawling Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different speci ...
either physically destroy seagrass beds or increase
turbidity Turbidity is the cloudiness or haze, haziness of a fluid caused by large numbers of individual Particle (ecology), particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of wat ...
in the water, causing seagrass die-off. Since seagrasses have some of the highest light requirements of
angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
plant species, they are highly affected by environmental conditions that change water clarity and block light. Seagrasses are also negatively affected by changing global climatic conditions. Increased weather events,
sea level rise Globally, Sea level, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Oce ...
, and higher temperatures as a result of
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate variability and change, Climate change in a broader sense also includes ...
all have the potential to induce widespread seagrass loss. An additional threat to seagrass beds is the introduction of non-native species. For seagrass beds worldwide, at least 28 non-native species have become established. Of these
invasive species An invasive species otherwise known as an alien is an introduced species, introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, i ...
, the majority (64%) have been documented to infer negative effects on the ecosystem. Another major cause of seagrass disappearance is coastal eutrophication. Rapidly developing human population density along coastlines has led to high nutrient loads in coastal waters from sewage and other impacts of development. Increased nutrient loads create an accelerating cascade of direct and indirect effects that lead to seagrass decline. While some exposure to high concentrations of nutrients, especially
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the pnictogens. It is a common element in the ...
and
phosphorus Phosphorus is a chemical element with the Symbol (chemistry), symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly Reactivity (chemistry), reactive, phosphor ...
, can result in increased seagrass productivity, high nutrient levels can also stimulate the rapid overgrowth of
macroalgae Seaweed, or macroalgae, refers to thousands of species of Macroscopic scale, macroscopic, Multicellular organism, multicellular, ocean, marine algae. The term includes some types of ''Rhodophyta'' (red), ''Brown algae, Phaeophyta'' (brown) and ...
and
epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called Phorophy ...
s in shallow water, and
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), m ...
in deeper water. In response to high nutrient levels, macroalgae form dense canopies on the surface of the water, limiting the light able to reach the
benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
seagrasses.
Algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in Fresh water, freshwater or Ocean, marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ...
s caused by eutrophication also lead to hypoxic conditions, which seagrasses are also highly susceptible to. Since coastal sediment is generally
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...
, seagrass must supply oxygen to their below-ground roots either through
photosynthesis Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
or by the diffusion of oxygen in the water column. When the water surrounding seagrass becomes hypoxic, so too do seagrass tissues. Hypoxic conditions negatively affect seagrass growth and survival with seagrasses exposed to hypoxic conditions shown to have reduced rates of photosynthesis, increased respiration, and smaller growth. Hypoxic conditions can eventually lead to seagrass die-off which creates a positive feedback cycle, where the decomposition of organic matter further decreases the amount of oxygen present in the water column. Possible seagrass population trajectories have been studied in the
Mediterranean sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
. These studies suggest that the presence of seagrass depends on physical factors such as temperature, salinity, depth and turbidity, along with natural phenomena like climate change and anthropogenic pressure. While there are exceptions, regression was a general trend in many areas of the Mediterranean Sea. There is an estimated 27.7% reduction along the southern coast of
Latium Latium ( , ; ) is the region of central western Italy in which the city of Rome was founded and grew to be the capital city of the Roman Empire. Definition Latium was originally a small triangle of fertile, volcanic soil (Old Latium) on whi ...
, 18%-38% reduction in the Northern Mediterranean basin, 19%-30% reduction on Ligurian coasts since the 1960s and 23% reduction in
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
in the past 50 years. In
Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = ''Plus ultra'' (Latin)(English: "Further Beyond") , national_anthem = (English: "Royal March") , i ...
the main reason for regression was due to human activity such as illegal
trawling Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different speci ...
and
aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelumb ...
farming. It was found that areas with medium to high human impact suffered more severe reduction. Overall, it was suggested that 29% of known areal seagrass populations have disappeared since 1879. The reduction in these areas suggests that should warming in the Mediterranean basin continue, it may lead to a functional extinction of ''
Posidonia oceanica ''Posidonia oceanica'', commonly known as Neptune grass or Mediterranean tapeweed, is a seagrass Seagrasses are the only flowering plants which grow in marine (ocean), marine environments. There are about 60 species of fully marine seagra ...
'' in the Mediterranean by 2050. Scientists suggested that the trends they identified appear to be part of a large-scale trend worldwide. Conservation efforts are imperative to the survival of seagrass species. While there are many challenges to overcome with respect to seagrass conservation there are some major ones that can be addressed. Societal awareness of what seagrasses are and their importance to human well-being is incredibly important. As the majority of people become more urbanized they are increasingly more disconnected from the natural world. This allows for misconceptions and a lack of understanding of seagrass ecology and its importance. Additionally, it is a challenge to obtain and maintain information on the status and condition of seagrass populations. With many populations across the globe, it is difficult to map the current populations. Another challenge faced in seagrass conservation is the ability to identify threatening activities on a local scale. Also, in an ever growing human population, there is a need to balance the needs of the people while also balancing the needs of the planet. Lastly, it is challenging to generate scientific research to support conservation of seagrass. Limited efforts and resources are dedicated to the study of seagrasses. This is seen in areas such as
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
and
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
where there is little to no plan in place to conserve seagrass populations. However, the conservation and restoration of seagrass may contribute to 16 of the 17 UN Sustainable Development Goals. In a study of seagrass conservation in China, several suggestions were made by scientists on how to better conserve seagrass. They suggested that seagrass beds should be included in the Chinese conservation agenda as done in other countries. They called for the Chinese government to forbid land reclamation in areas near or in seagrass beds, to reduce the number and size of culture ponds, to control raft aquaculture and improve sediment quality, to establish seagrass reserves, to increase awareness of seagrass beds to fishermen and policy makers and to carry out seagrass restoration. Similar suggestions were made in India where scientists suggested that public engagement was important. Also, scientists, the public, and government officials should work in tandem to integrate
traditional ecological knowledge Traditional ecological knowledge (TEK) describes indigenous and other traditional knowledge of local resources. As a field of study in Northern American anthropology, TEK refers to "a cumulative body of knowledge, belief, and practice, evolving by ...
and socio-cultural practices to evolve conservation policies. World Seagrass Day is an annual event held on March 1 to raise awareness about seagrass and its important functions in the marine ecosystem.


See also

*
Alismatales The Alismatales (alismatids) are an order of flowering plants including about 4,500 species. Plants assigned to this order are mostly Tropical vegetation, tropical or Aquatic plant, aquatic. Some grow in fresh water, some in marine habitats. ...
*
Blue carbon Blue Carbon refers to organic carbon that is captured and stored by the world's marine ecosystem, oceanic and Marine coastal ecosystem, coastal ecosystems, mostly by algae, seagrasses, macroalgae, mangroves, salt marshes and other plants in coast ...
*
Salt marsh A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open Seawater, saltwater or brackish water that is regularly flooded by the tides. It is ...
*
Mangrove A mangrove is a shrub or tree that grows in coastal saline water, saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evoluti ...
* Sea rewilding * Nursery habitat * Ocean Data Viewer: contains the global distribution of seagrasses dataset


References


Further references

* den Hartog, C. 1970. ''The Sea-grasses of the World''. ''Verhandl. der Koninklijke Nederlandse Akademie van Wetenschappen, Afd. Natuurkunde'', No. 59(1). * Duarte, Carlos M. and Carina L. Chiscano “Seagrass biomass and production: a reassessment” Aquatic Botany Volume 65, Issues 1–4, November 1999, Pages 159–174. * Green, E.P. & Short, F.T.(eds). 2003. ''World Atlas of Seagrasses''. University of California Press, Berkeley, CA. 298 pp. * Hemminga, M.A. & Duarte, C. 2000. ''Seagrass Ecology''. Cambridge University Press, Cambridge. 298 pp. * Hogarth, Peter ''The Biology of Mangroves and Seagrasses'' (Oxford University Press, 2007) * Larkum, Anthony W.D., Robert J. Orth, and Carlos M. Duarte (Editors) ''Seagrasses: Biology, Ecology and Conservation'' (Springer, 2006) * Orth, Robert J. et al. "A Global Crisis for Seagrass Ecosystems" ''BioScience'' December 2006 / Vol. 56 No. 12, Pages 987–996. * Short, F.T. & Coles, R.G.(eds). 2001. ''Global Seagrass Research Methods''. Elsevier Science, Amsterdam. 473 pp. * A.W.D. Larkum, R.J. Orth, and C.M. Duarte (eds). Seagrass Biology: A Treatise. CRC Press, Boca Raton, FL, in press. * A. Schwartz; M. Morrison; I. Hawes; J. Halliday. 2006. Physical and biological characteristics of a rare marine habitat: sub-tidal seagrass beds of offshore islands. ''Science for Conservation 269.'' 39 pp

* Waycott, M, McMahon, K, & Lavery, P 2014, A guide to southern temperate seagrasses, CSIRO Publishing, Melbourne


External links

*
Project Seagrass - Charity advancing the conservation of seagrass through community, research and actionSeagrasses
Project Regeneration.
SeagrassSpotter - Citizen Science project raising awaress for seagrass meadows and mapping their locationsSeagrass and Seagrass Beds
overview from the Smithsonian Ocean Portal

* ttp://www.seagrasswatch.org/ Seagrass-Watch - the largest scientific, non-destructive, seagrass assessment and monitoring program in the worldbr>Restore-A-Scar - a non-profit campaign to restore seagrass meadows damaged by boat propsSeagrassNet - global seagrass monitoring programThe Seagrass Fund at The Ocean FoundationWorld Seagrass AssociationSeagrassLISeagrass Science and Management in the South China Sea and Gulf of Thailand''Marine Ecology'' (December 2006)
- special issue on seagrasses
Seagrass Productivity - COST Action ES0906 Fisheries Western Australia - Seagrass Fact Sheet
{{Authority control Blue carbon Plant common names Roofing materials de:Seegras