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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 words ('container, vessel') and ('seed'), and refers to those plants t ...
s which grow in
marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...
environments. There are about 60 species of fully marine seagrasses which belong to four families ( Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), 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 or aquatic. Some grow in fresh water, some in marine habitats. Description The Alismatales compr ...
(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 embryonic leaf, or cotyledon. They constitute one ...
s). Seagrasses evolved from terrestrial plants which recolonised the ocean 70 to 100 million years ago. The name ''seagrass'' stems from the many species with long and narrow leaves, which grow by rhizome extension and often spread across large " meadows" resembling grassland; many species superficially resemble terrestrial grasses of the family Poaceae. Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone, 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'', which carries out a mixed biotic-abiotic strategy. Crustaceans (such as crabs, '' Majidae zoae'', '' Thalassinidea zoea'') and syllid polychaete 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 an ...
s which are among the most productive ecosystems in the world. They function as important carbon sinks and provide habitats and food for a diversity of marine life 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 angiosperms which evolved
in parallel Two-terminal components and electrical networks can be connected in series or parallel. The resulting electrical network will have two terminals, and itself can participate in a series or parallel topology. Whether a two-terminal "object" is a ...
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 or in the marine environment, 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, an epidermis which lacks stomata and is the main photosynthetic tissue. The rhizome 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 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. ...
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. Seagrass loss and degradation of seagrass biodiversity 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 embryonic leaf, or cotyledon. They constitute one ...
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 words ('container, vessel') and ('seed'), and refers to those plants t ...
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 germinating seed." The numb ...
. Terrestrial plants evolved perhaps as early as 450 million years ago from a group of green algae. 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, Cymodoceaceae complex, and Zosteraceae) evolved from a single lineage of the monocotyledonous flowering plants. Other plants that colonised the sea, such as salt marsh plants, mangroves, and marine algae, 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 of the genomes of '' Zostera marina'' and ''
Zostera muelleri ''Zostera muelleri '' is a southern hemisphere temperate species of seagrass native to the seacoasts of South Australia, Victoria and Tasmania.Paul Friedrich August Ascherson. 1867. Sitzungsberichte der Gesellschaft Naturforschender Freunde zu ...
'' has given a better understanding of angiosperm adaption to the sea. During the evolutionary step back to the ocean, different genes have been lost (e.g.,
stomatal In botany, a stoma (from 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. The pore is bor ...
genes) or have been reduced (e.g., genes involved in the synthesis of terpenoids) 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 composition. However the cell walls of seagrasses are not well understood. In addition to the
ancestral trait In phylogenetics, a primitive (or ancestral) character, trait, or feature of a lineage or taxon is one that is inherited from the common ancestor of a clade (or clade group) and has undergone little change since. Conversely, a trait that appea ...
s of land plants one would expect habitat-driven adaption process to the new environment characterized by multiple abiotic (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, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae), which belong to the order Alismatales according to the Angiosperm Phylogeny Group IV System. The genus '' Ruppia'', 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-based, system of plant taxonomy for flowering plants (angiosperms) being developed by the Angiosperm Phylogeny Group (APG). It was published ...
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 stressors. 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 and thus their ability to
colonise 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 ...
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 strategies. Some seagrasses form seed banks of small seeds with hard pericarps 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 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 angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fern ...
far away from parent meadows (e.g., '' Halophila'' sp., '' Halodule'' sp., ''
Cymodocea ''Cymodocea'' is a genus in the family Cymodoceaceae described as a genus in 1805. It includes four species of sea grass distributed in warm oceans. Habitat ''Cymodocea'' can be found in clear water and in the high intertidal areas. It is a h ...
'' 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'' co ...
'' sp. and '' Heterozostera'' sp. In contrast, other seagrasses form dispersal propagules. This strategy is typical of long-lived seagrasses that can form buoyant fruits with inner large non-dormant seeds, such as the genera '' Posidonia'' sp., ''
Enhalus ''Enhalus'' is a monotypic genus of marine flowering plants. The sole species is ''Enhalus acoroides''. ''Enhalus'' is a large seagrass native to coastal waters of the tropical Indian and Western Pacific Oceans. It is the only species of seag ...
'' 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'' (L.) Delile is one of the oldest and largest species on Earth. An individual can form meadows measuring nearly 15 km wide and can be hundreds to thousands of years old. ''P. oceanica'' meadows play important roles in the maintenance of the geomorphology 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, when leaf development is scarce, ''P. oceanica'' seeds perform photosynthetic activity, which increases their photosynthetic rates and thus maximises seedling establishment success. Seedlings also show high morphology plasticity during their root system development by forming adhesive root hairs to help
anchor An anchor is a device, normally made of metal , used to secure a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current. The word derives from Latin ''ancora'', which itself comes from the Greek � ...
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 are regularly exposed to air and consequently experience extreme high and low temperatures, high photoinhibitory irradiance, and desiccation 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 content and decreasing the chlorophyll a/b ratio to enhance light absorption 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 to achieve high level production. Marine macrophytes, including seagrass, use both and (
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemi ...
) 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 ratio An isotopic signature (also isotopic fingerprint) is a ratio of non-radiogenic ' stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample ...
s 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 waters of estuaries. Seagrasses are flowering plants with stems and ...
s/meadows can be either monospecific (made up of a single species) or in mixed beds. In temperate areas, usually one or a few species dominate (like the eelgrass '' Zostera marina'' in the North Atlantic), whereas tropical beds usually are more diverse, with up to thirteen
species In biology, a species is the basic unit of 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 the appropriat ...
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 syst ...
s, and can harbor hundreds of associated species from all
phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...
, 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 and free-living
macroalgae Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...
and microalgae,
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 species of molluscs are recognized. The number of fossil species is e ...
s, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is an important link in the food chain, feeding hundreds of species, including green turtles, dugongs, manatees,
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, swans, sea urchins and crabs. Some fish species that visit/feed on seagrasses raise their young in adjacent mangroves 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 by reducing sediment loads, improving photosynthesis for both coral and seagrass. Although often overlooked, seagrasses provide a number of ecosystem services. Seagrasses are considered ecosystem engineers. 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 which stabilizes sediment and reduces coastal erosion. This system also assists in oxygenating the sediment, providing a hospitable environment for sediment-dwelling organisms. Seagrasses also enhance water quality 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 water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
and storm surge. 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 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 marine fish in the family Centropomidae of the order Perciformes. The common snook is also known as the sergeant fish or robalo. It was originally assigned to the sciaenid genus ...
, 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 chorda ...
s also reside in seagrass habitats including bay scallops ('' Argopecten irradians''), horseshoe crabs, and shrimp. Charismatic fauna can also be seen visiting the seagrass habitats. These species include West Indian manatee, green sea turtles, 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''
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 waters of estuaries. Seagrasses are flowering plants with stems and ...
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, 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 holobiont Since the colonization of land by ancestral plant lineages 450 million years ago, plants and their associated microbes have been interacting with each other, forming an assemblage of species that is often referred to as a holobiont. Selective p ...
s. 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 (periphery of the roots), the endosphere (inside plant tissue), and the
phyllosphere In microbiology, the phyllosphere is the total above-ground surface of a plant when viewed as a habitat for microorganisms. The phyllosphere can be further subdivided into the caulosphere (stems), phylloplane (leaves), anthosphere (flowers), an ...
(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 walls contain the same polysaccharides found in angiosperm land plants, such as
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
However, the cell walls of some seagrasses are characterised by sulfated polysaccharides, which is a common attribute of
macroalgae Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...
from the groups of red,
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 and black. In the RGB color model ...
and also green algae. 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, glycoproteins of the
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 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 hydroxyproline from hydrolyzed gelatin ...
-rich glycoprotein family, are important components of cell walls of land plants. The highly glycosylated
arabinogalactan protein Arabinogalactan-proteins (AGPs) are highly glycosylated proteins (glycoproteins) found in the cell walls of plants. Each one consists of a protein with sugar molecules attached (which can account for more than 90% of the total mass). They are mem ...
s 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 ferns, lycophytes and mosses. They are structurally characterised by large polysaccharide moieties composed of arabinogalactans (normally over 90% of the molecule) which are covalently linked via
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 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 hydroxyproline from hydrolyzed gelatin ...
to relatively small protein/peptide backbones (normally less than 10% of the molecule). Distinct glycan 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. Further components of secondary walls of plants are cross-linked phenolic polymers called lignin, 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 fungi) which are vulnerable to endangerment in the near future. Species that are threatened are sometimes characterised by the population dynamics measure of '' critical depe ...
or
Near Threatened A near-threatened species is a species which has been categorized as "Near Threatened" (NT) by the International Union for Conservation of Nature as that may be vulnerable to endangerment in the near future, but it does not currently qualify fo ...
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 either physically destroy seagrass beds or increase turbidity in the water, causing seagrass die-off. Since seagrasses have some of the highest light requirements of angiosperm 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 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 Ocean and Cr ...
, and higher temperatures as a result of global warming 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, the majority (64%) have been documented to infer negative effects on the ecosystem. Another major cause of seagrass disappearance is
coastal eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyto ...
. 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 and
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
, 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, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...
and epiphytes in shallow water, and phytoplankton 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 seagrasses. Algal blooms 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 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 ...
. 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 w ...
, 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") , ...
the main reason for regression was due to human activity such as illegal trawling and aquaculture 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'' in the Mediterranean by 2050. Scientists suggested that the trends they identified appear to be part of a large-scale trend worldwide.
Conservation Conservation is the preservation or efficient use of resources, or the conservation of various quantities under physical laws. Conservation may also refer to: Environment and natural resources * Nature conservation, the protection and manageme ...
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 The Sustainable Development Goals (SDGs) or Global Goals are a collection of 17 interlinked objectives designed to serve as a "shared blueprint for peace and prosperity for people and the planet, now and into the future".United Nations (2017) R ...
. 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 and socio-cultural practices to evolve conservation policies.
World Seagrass Day World Seagrass Day is an annual event held on March 1 to raise awareness about seagrass and its important functions in the marine ecosystem. Background On 27 May 2022, United Nations General Assembly adopted a resolution on World Seagrass Day in ...
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 or aquatic. Some grow in fresh water, some in marine habitats. Description The Alismatales compr ...
*
Blue carbon Blue Carbon refers to organic carbon that is captured and stored by the world's oceanic and coastal ecosystems, mostly by algae, seagrasses, macroalgae, mangroves, salt marshes and other plants in coastal wetlands. The term Blue Carbon was coine ...
* Salt marsh * Mangrove * Sea rewilding *
Nursery habitat In marine environments, a nursery habitat is a subset of all habitats where juveniles of a species occur, having a greater level of productivity per unit area than other juvenile habitats (Beck et al. 2001). Mangroves, salt marshes and seagrass ar ...
* 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