Red algae, or Rhodophyta (, ; ), are one of the oldest groups of
eukaryotic
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority of species (6,793) are found in the
Florideophyceae
Florideophyceae is a class of red algae. They were once thought to be the only algae to bear pit connections, but these have since been found in the filamentous stage of the Bangiaceae. They were also thought only to exhibit apical growth, but t ...
(
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
), and mostly consist of
multicellular
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
seaweeds
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 ...
. Red algae are abundant in marine habitats but relatively rare in freshwaters. Approximately 5% of red algae species occur in freshwater environments, with greater concentrations found in warmer areas. Except for two coastal cave dwelling species in the asexual class
Cyanidiophyceae
Cyanidiophyceae is a class of unicellular red algae within subdivision Cyanidiophytina, and contain a single plastid, one to three mitochondria, a nucleus, a vacuole and floridean starch. Most are extremophiles inhabiting acid hot springs. The ...
, there are no terrestrial species, which may be due to an evolutionary bottleneck in which the last common ancestor lost about 25% of its core genes and much of its evolutionary plasticity.
The red algae form a distinct group characterized by having eukaryotic cells without
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have f ...
and
centriole
In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic cells, but are not present in conifers (Pinophyta), flowering plants (angiosperms) and most fungi, and a ...
s,
chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s that lack external
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
and contain unstacked (stroma)
thylakoid
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...
s, and use
phycobiliprotein
Phycobiliproteins are water-soluble proteins present in cyanobacteria and certain algae (rhodophytes, cryptomonads, glaucocystophytes). They capture light energy, which is then passed on to chlorophylls during photosynthesis. Phycobiliproteins are ...
s as
accessory pigment Accessory pigments are light-absorbing compounds, found in photosynthetic organisms, that work in conjunction with chlorophyll ''a''. They include other forms of this pigment, such as chlorophyll ''b'' in green algal and higher plant antennae, wh ...
s, which give them their red color. Red algae store sugars as
floridean starch
Floridean starch is a type of a storage glucan found in glaucophytes and in red algae (or rhodophytes), in which it is usually the primary sink for fixed carbon from photosynthesis. It is found in grains or granules in the cell's cytoplasm and is ...
, which is a type of starch that consists of highly branched
amylopectin Amylopectin is a water-insoluble polysaccharide and highly branched polymer of α-glucose units found in plants. It is one of the two components of starch, the other being amylose.
Plants store starch within specialized organelles called amylopl ...
without
amylose
Amylose is a polysaccharide made of α-D-glucose units, bonded to each other through α(1→4) glycosidic bonds. It is one of the two components of starch, making up approximately 20–30%. Because of its tightly packed helical structure, amylose ...
, as food reserves outside their plastids. Most red algae are also
multicellular
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
, macroscopic, marine, and
reproduce sexually
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
. The red algal life history is typically an
alternation of generations
Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...
that may have three generations rather than two.
The
coralline algae
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of re ...
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
Co ...
s, belong here. Red algae such as
dulse
''Palmaria palmata'', also called dulse, dillisk or dilsk (from Irish/Scottish Gaelic '/'), red dulse, sea lettuce flakes, or creathnach, is a red alga ( Rhodophyta) previously referred to as ''Rhodymenia palmata''. It grows on the northern coas ...
nori
Nori is a dried edible seaweed used in Japanese cuisine, made from species of the red algae genus ''Pyropia'', including ''P. yezonesis'' and '' P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls of sushi or '' ...
European
European, or Europeans, or Europeneans, may refer to:
In general
* ''European'', an adjective referring to something of, from, or related to Europe
** Ethnic groups in Europe
** Demographics of Europe
** European cuisine, the cuisines of Europe ...
and
Asian cuisine
Asian cuisine includes several major regional cuisines: Central Asian, East Asian, North Asian, South Asian, Southeast Asian, and West Asian. A cuisine is a characteristic style of cooking practices and traditions, usually associated with ...
s and are used to make other products such as
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
,
carrageenan
Carrageenans or carrageenins ( ; ) are a family of natural linear sulfated polysaccharides that are extracted from red edible seaweeds. Carrageenans are widely used in the food industry, for their gelling, thickening, and stabilizing properties. T ...
s and other
food additives
Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives have been used for centuries as part of an effort to preserve food, for example vinegar (pickling), salt (salt ...
.
Evolution
Chloroplasts evolved following an
endosymbiotic
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
event between an ancestral, photosynthetic cyanobacterium and an early eukaryotic
phagotroph
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is c ...
. This event (termed
primary endosymbiosis
Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possi ...
) resulted in the origin of the red and
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 (Embryophytes) have emerged deep in the Charophyte alga as ...
, and the
glaucophyte
The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of spe ...
s, which make up the oldest evolutionary lineages of photosynthetic eukaryotes. A secondary endosymbiosis event involving an ancestral red alga and a
heterotrophic
A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
eukaryote resulted in the evolution and diversification of several other photosynthetic lineages such as
Cryptophyta
The cryptophyceae are a class of algae, most of which have plastids. About 220 species are known, and they are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, ...
,
Haptophyta
The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for ''Prymnesium''), are a clade of algae.
The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at t ...
, Stramenopiles (or Heterokontophyta), and Alveolata. In addition to multicellular brown algae, it is estimated that more than half of all known species of microbial eukaryotes harbor red-alga-derived plastids.
Red algae are divided into the
Cyanidiophyceae
Cyanidiophyceae is a class of unicellular red algae within subdivision Cyanidiophytina, and contain a single plastid, one to three mitochondria, a nucleus, a vacuole and floridean starch. Most are extremophiles inhabiting acid hot springs. The ...
, a class of unicellular and
thermoacidophilic A thermoacidophile is an extremophilic microorganism that is both thermophilic and acidophilic; i.e., it can grow under conditions of high temperature and low pH. The large majority of thermoacidophiles are archaea (particularly the Thermoproteot ...
extremophile
An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
s found in sulphuric hot springs and other acidic environments, an adaptation partly made possible by
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...
s from prokaryotes, with about 1% of their genome having this origin, and two sister clades called SCRP (
Stylonematophyceae
Stylonematophyceae is a grouping of red algae
Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species ...
Rhodellophyceae
Rhodellophyceae is a grouping of red algae.
References
Red algae classes
{{Rhodophyta-stub ...
and
Porphyridiophyceae
Porphyridiophyceae is a class of red algae
Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species ...
) and BF (
Bangiophyceae
Bangiophyceae is a class of red algae. In some classifications it is merged with the Florideophyceae to form the Rhodophyceae. The Bangiophyceae, as defined traditionally, are paraphyletic. Their Taxonomy (biology), taxonomic identification h ...
and
Florideophyceae
Florideophyceae is a class of red algae. They were once thought to be the only algae to bear pit connections, but these have since been found in the filamentous stage of the Bangiaceae. They were also thought only to exhibit apical growth, but t ...
), which are found in both marine and freshwater environments. The SCRP clade are microalgae, consisting of both unicellular forms and multicellular microscopic filaments and blades. The BF are macroalgae, seaweed that usually do not grow to more than about 50 cm in length, but a few species can reach lengths of 2 m. Most rhodophytes are marine with a worldwide distribution, and are often found at greater depths compared to other seaweeds. While this was formerly attributed to the presence of pigments (such as
phycoerythrin
Phycoerythrin (PE) is a red protein-pigment complex from the light-harvesting phycobiliprotein family, present in cyanobacteria, red algae and cryptophytes, accessory to the main chlorophyll pigments responsible for photosynthesis.The red pigmen ...
) that would permit red algae to inhabit greater depths than other macroalgae by chromatic adaption, recent evidence calls this into question (e.g. the discovery of green algae at great depth in the Bahamas). Some marine species are found on sandy shores, while most others can be found attached to rocky substrata. Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats; they generally prefer clean, high-flow streams with clear waters and rocky bottoms, but with some exceptions. A few freshwater species are found in black waters with sandy bottoms and even fewer are found in more
lentic
A lake ecosystem or lacustrine ecosystem includes biotic (living) plants, animals and micro-organisms, as well as abiotic (non-living) physical and chemical interactions. Lake ecosystems are a prime example of lentic ecosystems (''lentic'' re ...
waters. Both marine and freshwater taxa are represented by free-living macroalgal forms and smaller endo/epiphytic/zoic forms, meaning they live in or on other algae, plants, and animals. In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.
Taxonomy
In the system of Adl ''et al.'' 2005, the red algae are classified in the
Archaeplastida
The Archaeplastida (or kingdom Plantae ''sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group ...
, along with the
glaucophyte
The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments, less common today than they were during the Proterozoic. The stated number of spe ...
s and green algae plus land plants (
Viridiplantae
Viridiplantae (literally "green plants") are a clade of eukaryotic organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. They are made up of the green algae, which ...
or Chloroplastida). The authors use a hierarchical arrangement where the clade names do not signify rank; the class name Rhodophyceae is used for the red algae. No subdivisions are given; the authors say, "Traditional subgroups are artificial constructs, and no longer valid."
Many studies published since Adl ''et al.'' 2005 have provided evidence that is in agreement for monophyly in the Archaeplastida (including red algae). However, other studies have suggested Archaeplastida is
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
. , the situation appears unresolved.
Below are other published taxonomies of the red algae using molecular and traditional alpha taxonomic data; however, the taxonomy of the red algae is still in a state of flux (with classification above the level of order having received little scientific attention for most of the 20th century).
* If one defines the kingdom Plantae to mean the Archaeplastida, the red algae will be part of that kingdom.
* If Plantae are defined more narrowly, to be the Viridiplantae, then the red algae might be considered their own kingdom, or part of the kingdom
Protista
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...
.
A major research initiative to reconstruct the Red Algal Tree of Life (
RedToL
{{unsourced, date=July 2022
RedToL, or Red Algal Tree of Life, is part of the collaborative National Science Foundation Assembling the Tree of Life activity (AToL), funded through the Division of Environmental Biology, Directorate for Biological ...
) using
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
and
genomic
Genomics is an interdisciplinary field of biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, three-dim ...
approach is funded by the
National Science Foundation
The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National I ...
as part of the Assembling the Tree of Life Program.
Classification comparison
Some sources (such as Lee) place all red algae into the class "Rhodophyceae". (Lee's organization is not a comprehensive classification, but a selection of orders considered common or important.)
A subphylum - Proteorhodophytina - has been proposed to encompass the existing classes Compsopogonophyceae,
Porphyridiophyceae
Porphyridiophyceae is a class of red algae
Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species ...
,
Rhodellophyceae
Rhodellophyceae is a grouping of red algae.
References
Red algae classes
{{Rhodophyta-stub ...
and
Stylonematophyceae
Stylonematophyceae is a grouping of red algae
Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species ...
. This proposal was made on the basis of the analysis of the plastid genomes.
Species of red algae
Over 7,000 species are currently described for the red algae, but the taxonomy is in constant flux with new species described each year. The vast majority of these are marine with about 200 that live only in
fresh water
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
.
Some examples of species and genera of red algae are:
*''
Cyanidioschyzon merolae
''Cyanidioschyzon merolae'' is a small (2μm), club-shaped, unicellular haploid red alga adapted to high sulfur acidic hot spring environments (pH 1.5, 45 °C). The cellular architecture of ''C. merolae'' is extremely simple, containing only ...
'', a primitive red alga
*''
Atractophora hypnoides
''Atractophora hypnoides'' is a rare red alga (Rhodophyta) found in the British Isles, France and some Atlantic Islands and is the only species of the genus found in the British Isles. It is attached to the rock (as crustose) or other algae by a ...
Lemanea
''Lemanea'' is a genus of freshwater red algae, in the order Batrachospermales. Both species are considered to be widespread in the northern hemisphere. Although placed in the Rhodophyta (red algae) it in fact is green in colour.
Description
' ...
'', a freshwater genus
*''
Palmaria palmata
''Palmaria palmata'', also called dulse, dillisk or dilsk (from Irish/Scottish Gaelic '/'), red dulse, sea lettuce flakes, or creathnach, is a red alga (Rhodophyta) previously referred to as ''Rhodymenia palmata''. It grows on the northern coast ...
'', dulse
*''
Schmitzia hiscockiana
''Schmitzia hiscockiana'' is a small, rare, red seaweed or marine alga of the phylum Rhodophyta or red algae. It was discovered and named in 1985.
Distribution
This small red marine alga is known from most coasts of Ireland, Wales, England, a ...
''
*''
Chondrus crispus
''Chondrus crispus''—commonly called Irish moss or carrageen moss (Irish ''carraigín'', "little rock")—is a species of red algae which grows abundantly along the rocky parts of the Atlantic coast of Europe and North America. In its fresh ...
'', Irish moss
*''
Mastocarpus stellatus
''Mastocarpus stellatus,'' commonly known as carrageenan moss or false Irish moss, is a species in the Rhodophyceae division, a red algae seaweed division, and the Phyllophoracea family. ''M. stellatus'' is closely related to Irish Moss (''Ch ...
''
*''
Vanvoorstia bennettiana
''Vanvoorstia bennettiana'' (Bennett's seaweed) is an extinct red algae from Australia. It is named after naturalist George Bennett.William Henry Harvey: Phycologia Australica; or a History of Australian Seaweeds Volume 2. Lovel Reeve, Londo ...
'', became extinct in the early 20th century
*''
Acrochaetium efflorescens
''Acrochaetium efflorescens'' is a small marine red alga. It is a small alga with erect filaments and spiral shaped plastids. The base is multicellular. The species shows an alternation of generations with both tetrasporic and gametophytic stag ...
''
*''
Audouinella
''Audouinella'', also known as black algae, is a widespread genus of red algae, found in marine and freshwater environments.
The form known as "''black brush algae''" (or "''black beard algae''", ''BBA'' for short) is a particular nuisance in a ...
'', with freshwater as well as marine species
*''
Polysiphonia ceramiaeformis
''Polysiphonia ceramiaeformis'', also called banded siphon weed, is a small red algae ('' Rhodophyta''), in the genus '' Polysiphonia''. Individuals are irregularly branched with the branches extending up to from a central node and ending in d ...
'', banded siphon weed
*''
Vertebrata simulans
''Vertebrata simulans'' (''Polysiphonia simulans'') Harvey''Seaweeds of the British Isles. Volume 1V Rhodophyta, part 3A Ceramiales.'' The Natural History Museum, London. is a small densely branched alga in the Division Rhodophyta.
Description ...
''
Morphology
Red algal morphology is diverse ranging from
unicellular
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...
forms to complex parenchymatous and non- parenchymatous thallus. Red algae have double
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
s. The outer layers contain the polysaccharides
agarose
Agarose is a heteropolysaccharide, generally extracted from certain red seaweed. It is a linear polymer made up of the repeating unit of agarobiose, which is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agarose is o ...
and agaropectin that can be extracted from the cell walls by boiling as
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
. The internal walls are mostly cellulose. They also have the most gene-rich plastid genomes known.
Cell structure
Red algae do not have flagella and centrioles during their entire life cycle. Presence of normal spindle fibres, microtubules, un-stacked photosynthetic membranes, presence of phycobilin pigment granules, presence of pit connection between cells filamentous genera, absence of chloroplast endoplasmic reticulum are the distinguishing characters of red algal cell structure.
Chloroplasts
Presence of the water-soluble pigments called
phycobilin Phycobilins (from Greek: '' (phykos)'' meaning "alga", and from Latin: ''bilis'' meaning "bile") are light-capturing bilins found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae ...
s (
phycocyanobilin
Phycocyanobilin is a blue phycobilin, i.e., a tetrapyrrole chromophore found in cyanobacteria and in the chloroplasts of red algae, glaucophytes, and some cryptomonads. Phycocyanobilin is present only in the phycobiliproteins allophycocyanin a ...
phycourobilin
Phycourobilin is an orange tetrapyrrole involved in photosynthesis in cyanobacteria and red algae. This chromophore is bound to the phycobiliprotein phycoerythrin, the distal component of the light-harvesting system of cyanobacteria and red algae ( ...
phycobilisomes
Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. It was lost in the plastids of green algae / plants (chloroplasts).
General structure
Phycobilisomes are protein complexes (up to 60 ...
, gives red algae their distinctive color.
Chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
contains evenly spaced and ungrouped thylakoids. Other pigments include chlorophyll a, α- and β-carotene, lutein and zeaxanthin. Double membrane of chloroplast envelope surrounds the chloroplast. Absence of grana and attachment of phycobilisomes on the stromal surface of the thylakoid membrane are other distinguishing characters of red algal chloroplast.
Storage products
The major photosynthetic products include floridoside (major product), D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc. Floridean starch (similar to amylopectin in land plants), a long term storage product, is deposited freely (scattered) in the cytoplasm. The concentration of photosynthetic products are altered by the environmental conditions like change in pH, the salinity of medium, change in light intensity, nutrient limitation etc. When the salinity of the medium increases the production of floridoside is increased in order to prevent water from leaving the algal cells.
Pit connections and pit plugs
Pit connections
Pit connections and pit plugs are unique and distinctive features of red algae that form during the process of
cytokinesis
Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and meios ...
following
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
. In red algae, cytokinesis is incomplete. Typically, a small pore is left in the middle of the newly formed partition. The pit connection is formed where the daughter cells remain in contact.
Shortly after the pit connection is formed, cytoplasmic continuity is blocked by the generation of a pit plug, which is deposited in the wall gap that connects the cells.
Connections between cells having a common parent cell are called primary pit connections. Because apical growth is the norm in red algae, most cells have two primary pit connections, one to each adjacent cell.
Connections that exist between cells not sharing a common parent cell are labelled secondary pit connections. These connections are formed when an unequal cell division produced a nucleated daughter cell that then fuses to an adjacent cell. Patterns of secondary pit connections can be seen in the order
Ceramiales
Ceramiales is an order of red algae. It was established by Friedrich Oltmanns in 1904.
Families
* Callithamniaceae Kützing, 1843
* Ceramiaceae Dumortier, 1822
* Choreocolacaceae Sturch
* Dasyaceae Kützing, 1843
* Delesseriaceae Bory, ...
.
Pit plugs
After a pit connection is formed, tubular membranes appear. A granular protein called the plug core then forms around the membranes. The tubular membranes eventually disappear. While some orders of red algae simply have a plug core, others have an associated membrane at each side of the protein mass, called cap membranes. The pit plug continues to exist between the cells until one of the cells dies. When this happens, the living cell produces a layer of wall material that seals off the plug.
Function
The pit connections have been suggested to function as structural reinforcement, or as avenues for cell-to-cell communication and transport in red algae, however little data supports this hypothesis.
Reproduction
The reproductive cycle of red algae may be triggered by factors such as day length. Red algae reproduce sexually as well as asexually. Asexual reproduction can occur through the production of spores and by vegetative means (fragmentation, cell division or propagules production).
Fertilization
Red algae lack
motile
Motility is the ability of an organism to move independently, using metabolic energy.
Definitions
Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
. Hence, they rely on water currents to transport their
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s to the female organs – although their sperm are capable of "gliding" to a
carpogonium The carpogonium (plural ''carpogonia'') is the female organ in the Red Algae (Rhodophyta) which have a highly specialized type of reproduction. It contains the reproductive nucleus. It may contain a number of cells usually without chloroplasts. It s ...
's trichogyne. Also animals help with the dispersal and fertilization of the gametes. The first species discovered to do so is the
isopod
Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, an ...
Idotea balthica.
The trichogyne will continue to grow until it encounters a
spermatium
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, wh ...
; once it has been fertilized, the cell wall at its base progressively thickens, separating it from the rest of the carpogonium at its base.
Upon their collision, the walls of the spermatium and carpogonium dissolve. The male nucleus divides and moves into the carpogonium; one half of the nucleus merges with the carpogonium's nucleus.
The
polyamine
A polyamine is an organic compound having more than two amino groups. Alkyl polyamines occur naturally, but some are synthetic. Alkylpolyamines are colorless, hygroscopic, and water soluble. Near neutral pH, they exist as the ammonium derivatives. ...
spermine
Spermine is a polyamine involved in cellular metabolism that is found in all Eukaryote, eukaryotic cells. The precursor for synthesis of spermine is the amino acid ornithine. It is an essential growth factor in some Bacterium, bacteria as well. ...
is produced, which triggers carpospore production.
Spermatangia
A gametangium (plural: gametangia) is an organ or cell in which gametes are produced that is found in many multicellular protists, algae, fungi, and the gametophytes of plants. In contrast to gametogenesis in animals, a gametangium is a haploid ...
may have long, delicate appendages, which increase their chances of "hooking up".
Life cycle
They display
alternation of generations
Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...
. In addition to a
gametophyte
A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...
generation, many have two
sporophyte
A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the zygote pr ...
generations, the
carposporophyte
{{Short pages monitor
One of the oldest fossils identified as a red alga is also the oldest fossil
eukaryote
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
that belongs to a specific modern
taxon
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
. ''
Bangiomorpha pubescens
''Bangiomorpha pubescens'' is a red alga.
It is the first known sexually reproducing organism. A multicellular fossil of ''Bangiomorpha pubescens'' was recovered from the Hunting Formation in Somerset Island (Nunavut), Somerset Island, Canada tha ...
'', a multicellular fossil from arctic
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, strongly resembles the modern red alga ''Bangia'' and occurs in rocks dating to 1.05 billion years ago.
Two kinds of fossils resembling red algae were found sometime between 2006 and 2011 in well-preserved sedimentary rocks in Chitrakoot, central India. The presumed red algae lie embedded in fossil mats of cyanobacteria, called stromatolites, in 1.6 billion-year-old Indian phosphorite – making them the oldest plant-like fossils ever found by about 400 million years.
Red algae are important builders of
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
reefs. The earliest such coralline algae, the solenopores, are known from the
Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
period. Other algae of different origins filled a similar role in the late
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
, and in more recent reefs.
Calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
crusts that have been interpreted as the remains of coralline red algae, date to the
Ediacaran
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
Period.
Thallophyte
Thallophytes (Thallophyta, Thallophyto or Thallobionta) are a polyphyletic group of non-motile organisms traditionally described as "thalloid plants", "relatively simple plants" or "lower plants". They form a division of kingdom Plantae that inc ...
s resembling coralline red algae are known from the late Proterozoic
Doushantuo formation
The Doushantuo Formation (formerly transcribed as Toushantuo or Toushantou, from ) is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerst ...
.
Relationship to other algae
Chromista
Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', such ...
and Alveolata algae (e.g., chrysophytes, diatoms, phaeophytes, dinophytes) seem to have evolved from
bikont
A bikont ("two flagella") is any of the eukaryotic organisms classified in the group Bikonta. Many single-celled members of the group, and the presumed ancestor, have two flagella.
Enzymes
Another shared trait of bikonts is the fusion of two ge ...
s that have acquired red algae as
endosymbionts
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within" ...
. According to this theory, over time these endosymbiont red algae have evolved to become chloroplasts. This part of
endosymbiotic theory
Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...
is supported by various structural and genetic similarities.
Human consumption
Red algae have a long history of use as a source of nutritional, functional food ingredients and pharmaceutical substances.Wang, T., Jónsdóttir, R., Kristinsson, H. G., Hreggvidsson, G. O., Jónsson, J. Ó., Thorkelsson, G., & Ólafsdóttir, G. (2010). "Enzyme-enhanced extraction of antioxidant ingredients from red algae Palmaria palmata". ''LWT – Food Science and Technology'', 43(9), 1387–1393. They are a source of antioxidants including polyphenols, and phycobiliproteins and contain proteins, minerals, trace elements, vitamins and essential fatty acids. Traditionally red algae are eaten raw, in salads, soups, meal and condiments. Several species are food crops, in particular members of the genus ''
Porphyra
''Porphyra'' is a genus of coldwater seaweeds that grow in cold, shallow seawater. More specifically, it belongs to red algae phylum of laver species (from which comes laverbread), comprising approximately 70 species.Brodie, J.A. and Irvine, ...
'', variously known as
nori
Nori is a dried edible seaweed used in Japanese cuisine, made from species of the red algae genus ''Pyropia'', including ''P. yezonesis'' and '' P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls of sushi or '' ...
dulse
''Palmaria palmata'', also called dulse, dillisk or dilsk (from Irish/Scottish Gaelic '/'), red dulse, sea lettuce flakes, or creathnach, is a red alga ( Rhodophyta) previously referred to as ''Rhodymenia palmata''. It grows on the northern coas ...
(''
Palmaria palmata
''Palmaria palmata'', also called dulse, dillisk or dilsk (from Irish/Scottish Gaelic '/'), red dulse, sea lettuce flakes, or creathnach, is a red alga (Rhodophyta) previously referred to as ''Rhodymenia palmata''. It grows on the northern coast ...
'') are consumed in Britain. Some of the red algal species like ''
Gracilaria
''Gracilaria'' is a genus of red algae (Rhodophyta) notable for its economic importance as an agarophyte, as well as its use as a food for humans and various species of shellfish. Various species within the genus are cultivated among Asia, South ...
'' and ''
Laurencia
''Laurencia'' is a genus of red algae that grow in temperate and tropical shore areas, in littoral to sublittoral habitats, at depths up to .
Description
''Laurencia'' species have a thallus that is erect or decumbent with distichous, whorle ...
'' are rich in
polyunsaturated fatty acids
Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic proper ...
(eicopentaenoic acid, docohexaenoic acid,
arachidonic acid
Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6), or 20:4(5,8,11,14). It is structurally related to the saturated arachidic acid found in cupuaçu butter. Its name derives from the New Latin word ''arachi ...
) and have protein content up to 47% of total biomass. Where a big portion of world population is getting insufficient daily iodine intake, a 150 ug/day requirement of iodine is obtained from a single gram of red algae. Red algae, like ''
Gracilaria
''Gracilaria'' is a genus of red algae (Rhodophyta) notable for its economic importance as an agarophyte, as well as its use as a food for humans and various species of shellfish. Various species within the genus are cultivated among Asia, South ...
'', ''
Gelidium
''Gelidium'' is a genus of thalloid red algae comprising 134 species. Its members are known by a number of common names.Recorded common names are tengusa, makusa, genso, niu mau tsai, japansche scheleiachtige mos, steen-or klipbloem, hay tsay, ol ...
Porphyra
''Porphyra'' is a genus of coldwater seaweeds that grow in cold, shallow seawater. More specifically, it belongs to red algae phylum of laver species (from which comes laverbread), comprising approximately 70 species.Brodie, J.A. and Irvine, ...
'', ''
Acanthophora
:''Acanthophora'' is also a synonym for ''Aralia''. For the section of ''Solanum
''Solanum'' is a large and diverse genus of flowering plants, which include three food crops of high economic importance: the potato, the tomato and the eggplant ...
'', and '' Palmaria''
are primarily known for their industrial use for phycocolloids (agar, algin, furcellaran and carrageenan) as thickening agent, textiles, food, anticoagulants, water-binding agents etc. Dulse (''Palmaria palmata'') is one of the most consumed red algae and is a source of iodine, protein, magnesium and calcium. China, Japan, Republic of Korea are the top producers of seaweeds.Manivannan, K., Thirumaran, G., Karthikai, D.G., Anantharaman. P., Balasubramanian, P. (2009). "Proximate Composition of Different Group of Seaweeds from Vedalai Coastal Waters (Gulf of Mannar): Southeast Coast of India". ''Middle-East J. Scientific Res.'', 4: 72–77. In East and Southeast Asia,
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
is most commonly produced from ''
Gelidium amansii
''Gelidium amansii'', also known as ''umutgasari'', is an economically important species of red algae commonly found and harvested in the shallow coast (3 to 10 m or 10 to 33 ft of depth below the water) of many East Asian countries includin ...
''. These rhodophytes are easily grown and, for example,
nori
Nori is a dried edible seaweed used in Japanese cuisine, made from species of the red algae genus ''Pyropia'', including ''P. yezonesis'' and '' P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls of sushi or '' ...
cultivation in Japan goes back more than three centuries.
Cyanidiophyceae
Cyanidiophyceae is a class of unicellular red algae within subdivision Cyanidiophytina, and contain a single plastid, one to three mitochondria, a nucleus, a vacuole and floridean starch. Most are extremophiles inhabiting acid hot springs. The ...
)
File:Porphyra (haploide y diploide).jpg, ''
Porphyra
''Porphyra'' is a genus of coldwater seaweeds that grow in cold, shallow seawater. More specifically, it belongs to red algae phylum of laver species (from which comes laverbread), comprising approximately 70 species.Brodie, J.A. and Irvine, ...
'' sp., haploid and diploid (
Bangiophyceae
Bangiophyceae is a class of red algae. In some classifications it is merged with the Florideophyceae to form the Rhodophyceae. The Bangiophyceae, as defined traditionally, are paraphyletic. Their Taxonomy (biology), taxonomic identification h ...
)
Image:Chondrus crispus.jpg, ''
Chondrus crispus
''Chondrus crispus''—commonly called Irish moss or carrageen moss (Irish ''carraigín'', "little rock")—is a species of red algae which grows abundantly along the rocky parts of the Atlantic coast of Europe and North America. In its fresh ...
'' (
Florideophyceae
Florideophyceae is a class of red algae. They were once thought to be the only algae to bear pit connections, but these have since been found in the filamentous stage of the Bangiaceae. They were also thought only to exhibit apical growth, but t ...
:
Gigartinales
Gigartinales is a red algae order in the class Florideophyceae
Florideophyceae is a class of red algae. They were once thought to be the only algae to bear pit connections, but these have since been found in the filamentous stage of the Bang ...
)
Image:Gracilaria2.JPG, ''
Gracilaria
''Gracilaria'' is a genus of red algae (Rhodophyta) notable for its economic importance as an agarophyte, as well as its use as a food for humans and various species of shellfish. Various species within the genus are cultivated among Asia, South ...
'' sp. (Florideophyceae:
Gracilariales
Gracilariales is an order of red algae.
References
Red algae orders
{{Rhodophyta-stub ...
)
File:Corallina officinalis Helgoland.JPG, ''
Corallina officinalis
''Corallina officinalis'' is a calcareous red seaweed which grows in the lower and mid-littoral zones on rocky shores.
It is primarily found growing around the rims of tide pools, but can be found in shallow crevices anywhere on the rocky shor ...
Laurencia
''Laurencia'' is a genus of red algae that grow in temperate and tropical shore areas, in littoral to sublittoral habitats, at depths up to .
Description
''Laurencia'' species have a thallus that is erect or decumbent with distichous, whorle ...
'' sp. (Florideophyceae:
Ceramiales
Ceramiales is an order of red algae. It was established by Friedrich Oltmanns in 1904.
Families
* Callithamniaceae Kützing, 1843
* Ceramiaceae Dumortier, 1822
* Choreocolacaceae Sturch
* Dasyaceae Kützing, 1843
* Delesseriaceae Bory, ...
)
File:Seagrass at california tide pools.jpg, Some red algae are
iridescent
Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...
when not covered with water
See also
*
Brown algae
Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and po ...
*
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 (Embryophytes) have emerged deep in the Charophyte alga as ...
*
History of phycology
The history of phycology is the history of the scientific study of algae. Human interest in plants as food goes back into the origins of the species, and knowledge of algae can be traced back more than two thousand years. However, only in the last ...