red algae
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Red algae, or Rhodophyta (, ; ), are one of the oldest groups of
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, 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 ...
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
. 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 ( class), and mostly consist of
multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
, marine algae, including many notable seaweeds. 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, 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#Cellular level, motility. Many protists with flagella are termed as flagellates. A m ...
and
centriole In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic Cell (biology), cells, but are not present in conifers (Pinophyta), flowering plants (Flowering plant, ang ...
s,
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, ...
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 cyanobacterium, cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen (an ...
s, and use phycobiliproteins as accessory pigments, which give them their red color. Red algae store sugars as floridean starch, 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 amyloplas ...
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 Helix, helical structure, a ...
, 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 (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
, macroscopic, marine, and reproduce sexually. 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 ...
, which secrete
calcium carbonate Calcium carbonate is a chemical compound with the Chemical formula, formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisti ...
and play a major role in building
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, 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 co ...
''(Palmaria palmata)'' and laver (
nori Nori is a dried edible seaweed used in Japanese cuisine, made from species of the red algae genus ''Pyropia'', including ''P. yezonesis'' and ''Pyropia tenera, P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls ...
/ gim) are a traditional part of European and
Asian cuisine Asian cuisine includes several major regional cuisines: #Central Asian cuisine, Central Asian, #East Asian cuisine, East Asian, #North Asia cuisine, North Asian, #South Asian cuisine, South Asian, #Southeast Asian cuisine, Southeast Asian, and ...
s and are used to make other products such as
agar Agar ( or ), or agar-agar, is a gelling agent, 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 n ...
,
carrageenan Carrageenans or carrageenins ( ; ) are a family of natural linear sulfation, sulfated polysaccharides that are extracted from red algae, red edible seaweeds. Carrageenans are widely used in the food industry, for their gelling, thickening, and stab ...
s and other
food additives Food additives are substances added to food to preserve Taste, 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), sal ...
.


Evolution

Chloroplasts evolved following an endosymbiotic event between an ancestral, photosynthetic cyanobacterium and an early eukaryotic phagotroph. This event (termed primary endosymbiosis) 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 (Embryophyte, Embryophytes) have emerged deep in the Charop ...
, 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 speci ...
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 chloroplast, plastids. About 220 species are known, and they are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 1 E-5 m, 10–50 μm in size an ...
,
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, a class of unicellular and thermoacidophilic
extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases Thriving, thrive) in extreme environments, i.e. environments that make survival challenging such as due to extre ...
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, Compsopogonophyceae, Rhodellophyceae and Porphyridiophyceae) and BF ( Bangiophyceae and Florideophyceae), 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) 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 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 Plant#Current definitions of Plantae, Plantae ''Sensu#Common qualifiers, sensu lato'' "in a broad sense"; pronounced Help:IPA/English, /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the autotrop ...
, 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 speci ...
s and green algae plus land plants (
Viridiplantae Viridiplantae (literally "green plants") are a clade of eukaryote, 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 alga ...
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, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
. , 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 Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukary ...
. A major research initiative to reconstruct the Red Algal Tree of Life ( RedToL) using
phylogenetic In biology, phylogenetics (; from Greek language, Greek wikt:φυλή, φυλή/wikt:φῦλον, φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary his ...
and genomic approach is funded by the [ ational Science Foundation 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, Rhodellophyceae and Stylonematophyceae. 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 salt (chemistry), salts and other total dissolved solids. Although the term specifically excludes seawater and brackish wate ...
. Some examples of species and genera of red algae are: *'' Cyanidioschyzon merolae'', a primitive red alga *'' Atractophora hypnoides'' *'' Gelidiella calcicola'' *'' Lemanea'', a freshwater genus *'' Palmaria palmata'', dulse *'' Schmitzia hiscockiana'' *''
Chondrus crispus ''Chondrus crispus''—commonly called Irish moss or carrageen moss (Irish language, Irish ''carraigín'', "little rock")—is a species of red algae which grows abundantly along the rocky parts of the Atlantic Ocean, Atlantic coast of Europe an ...
'', 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 Phyllophoraceae, Phyllophoracea family. ''M. stellatus'' is closely related to Ch ...
'' *'' Vanvoorstia bennettiana'', became extinct in the early 20th century *'' Acrochaetium efflorescens'' *''
Audouinella ''Audouinella'', also known as black algae, is a widespread genus of red algae, found in Marine (ocean), marine and freshwater environments. The form known as "''black brush algae''" (or "''black beard algae''", ''BBA'' for short) is a particula ...
'', with freshwater as well as marine species *'' Polysiphonia ceramiaeformis'', banded siphon weed *'' Vertebrata simulans''


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 ...
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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biolog ...
s. The outer layers contain the polysaccharides
agarose Agarose is a heteropolysaccharide, generally extracted from certain Red algae, red seaweed. It is a linear polymer made up of the repeating unit of agarobiose, which is a disaccharide made up of D-galactose, D-galactose and 3,6-anhydro-L-galacto ...
and agaropectin that can be extracted from the cell walls by boiling as
agar Agar ( or ), or agar-agar, is a gelling agent, 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 n ...
. 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 phycobilins ( phycocyanobilin, phycoerythrobilin, phycourobilin and phycobiliviolin), which are localized into phycobilisomes, 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 cell, plant and algae, algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, ...
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 biological process, 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 Mitosis, nuclear d ...
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 maintai ...
. 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.


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
sperm Sperm is the male reproductive Cell (biology), 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 ...
. Hence, they rely on water currents to transport their
gamete A gamete (; , ultimately ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also r ...
s to the female organs – although their sperm are capable of "gliding" to a carpogonium'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 (biology), order of crustaceans that includes Woodlouse, woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of Antenna (biology), antennae, s ...
Idotea balthica. The trichogyne will continue to grow until it encounters a spermatium; 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
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 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 biological life cycle, life cycle of a plant or alga which produces asexual Spore, spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyt ...
generations, the carposporophyte-producing carpospores, which germinate into a tetrasporophyte – this produces spore tetrads, which dissociate and germinate into gametophytes. The gametophyte is typically (but not always) identical to the tetrasporophyte. Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free-living) tetrasporophyte phase. Tetrasporangia may be arranged in a row ( zonate), in a cross (cruciate), or in a tetrad. The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp. The two following case studies may be helpful to understand some of the life histories algae may display: In a simple case, such as '' Rhodochorton investiens'': :In the carposporophyte: a spermatium merges with a trichogyne (a long hair on the female sexual organ), which then divides to form carposporangia – which produce carpospores. :Carpospores germinate into gametophytes, which produce sporophytes. Both of these are very similar; they produce monospores from monosporangia "just below a cross-wall in a filament" and their spores are "liberated through the apex of sporangial cell." :The spores of a sporophyte produce either tetrasporophytes. Monospores produced by this phase germinates immediately, with no resting phase, to form an identical copy of the parent. Tetrasporophytes may also produce a carpospore, which germinates to form another tetrasporophyte. :The gametophyte may replicate using monospores, but produces sperm in spermatangia, and "eggs"(?) in carpogonium. A rather different example is '' Porphyra gardneri'': :In its
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
phase, a carpospore can germinate to form a filamentous "conchocelis stage", which can also self-replicate using monospores. The conchocelis stage eventually produces conchosporangia. The resulting conchospore germinates to form a tiny
prothallus A prothallus, or prothallium, (from Latin ''pro'' = forwards and Greek language, Greek ''θαλλος'' (''thallos'') = twig) is usually the gametophyte stage in the life of a fern or other pteridophyte. Occasionally the term is also used to des ...
with
rhizoid Rhizoids are protuberances that extend from the lower epidermis (botany), epidermal cells of bryophytes and algae. They are similar in structure and function to the root hairs of vascular plant, vascular land plants. Similar structures are formed ...
s, which develops to a cm-scale leafy thallus. This too can reproduce via monospores, which are produced inside the thallus itself. They can also reproduce via spermatia, produced internally, which are released to meet a prospective carpogonium in its conceptacle.


Chemistry

The values of red algae reflect their lifestyles. The largest difference results from their photosynthetic
metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell (biology), cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence ...
: algae that use HCO3 as a carbon source have less negative values than those that only use . An additional difference of about 1.71‰ separates groups
intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of Marine habitat, habitats with var ...
from those below the lowest tide line, which are never exposed to atmospheric carbon. The latter group uses the more 13C-negative dissolved in sea water, whereas those with access to atmospheric carbon reflect the more positive signature of this reserve. Photosynthetic pigments of Rhodophyta are chlorophylls '' a'' and '' d''. Red algae are red due to phycoerythrin. They contain the sulfated polysaccharide
carrageenan Carrageenans or carrageenins ( ; ) are a family of natural linear sulfation, sulfated polysaccharides that are extracted from red algae, red edible seaweeds. Carrageenans are widely used in the food industry, for their gelling, thickening, and stab ...
in the amorphous sections of their cell walls, although red algae from the genus Porphyra contain porphyran. They also produce a specific type of tannin called
phlorotannin Phlorotannins are a type of tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and Precipitation (chemistry), precipitate proteins and various other organic compounds including amino acids and ...
s, but in a lower amount than brown algae do.


Genomes and transcriptomes of red algae

As enlisted in realDB, 27 complete transcriptomes and 10 complete genomes sequences of red algae are available. Listed below are the 10 complete genomes of red algae. *'' Cyanidioschyzon merolae'', Cyanidiophyceae *'' Galdieria sulphuraria'', Cyanidiophyceae *'' Pyropia yezoensis'', Bangiophyceae *''
Chondrus crispus ''Chondrus crispus''—commonly called Irish moss or carrageen moss (Irish language, Irish ''carraigín'', "little rock")—is a species of red algae which grows abundantly along the rocky parts of the Atlantic Ocean, Atlantic coast of Europe an ...
'', Florideophyceae *'' Porphyridium purpureum'', Porphyridiophyceae *'' Porphyra umbilicalis'', Bangiophyceae * '' Gracilaria changii'', Gracilariales * '' Galdieria phlegrea'', Cyanidiophytina * '' Gracilariopsis lemaneiformis'', Gracilariales * '' Gracilariopsis chorda'', Gracilariales


Fossil record

One of the oldest fossils identified as a red alga is also the oldest fossil
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, 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 ...
that belongs to a specific modern
taxon In biology, a taxon (back-formation from ''Taxonomy (biology), 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 ...
. '' Bangiomorpha pubescens'', a multicellular fossil from arctic
Canada Canada is a country in North America. Its Provinces and territories of Canada, 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 ...
, 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 rock, carbonate sedimentary rock which is the main source of the material Lime_(material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphis ...
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 era (geology), geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Ancient Greek, Greek words ' ...
, 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, Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian, Cambrian Period 538.8 Mya. It marks the end of the Pro ...
Period. Thallophytes resembling coralline red algae are known from the late Proterozoic Doushantuo formation.


Relationship to other algae

Chromista and Alveolata algae (e.g., chrysophytes, diatoms, phaeophytes, dinophytes) seem to have evolved from bikonts that have acquired red algae as endosymbionts. 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 possibly ...
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'', 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 ''Pyropia tenera, P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls ...
(Japan), '' gim'' (Korea), 紫菜 (China). Laver and
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 co ...
('' Palmaria palmata'') are consumed in Britain. Some of the red algal species like '' Gracilaria'' and '' Laurencia'' are rich in
polyunsaturated fatty acids Polyunsaturated fatty acids (PUFAs) are fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsa ...
(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, cupuaçu butter. Its name derives from the New Latin word ...
) 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'', '' Gelidium'', '' Euchema'', '' Porphyra'', '' Acanthophora'', 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 gelling agent, 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 n ...
is most commonly produced from '' Gelidium amansii''. 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 ''Pyropia tenera, P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls ...
cultivation in Japan goes back more than three centuries.


Gallery

File:Cyanidium O5A.jpg, '' Cyanidium'' sp. ( Cyanidiophyceae) File:Porphyra (haploide y diploide).jpg, '' Porphyra'' sp., haploid and diploid ( Bangiophyceae) Image:Chondrus crispus.jpg, ''
Chondrus crispus ''Chondrus crispus''—commonly called Irish moss or carrageen moss (Irish language, Irish ''carraigín'', "little rock")—is a species of red algae which grows abundantly along the rocky parts of the Atlantic Ocean, Atlantic coast of Europe an ...
'' ( Florideophyceae: Gigartinales) Image:Gracilaria2.JPG, '' Gracilaria'' sp. (Florideophyceae: Gracilariales) File:Corallina officinalis Helgoland.JPG, '' Corallina officinalis'' sp. (Florideophyceae: Corallinales) File:Laurencia.jpg, '' Laurencia'' sp. (Florideophyceae: Ceramiales) 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 Gradient, gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, ...
when not covered with water


See also

*
Brown algae Brown algae (singular: alga), comprising the class (biology), 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 ...
*
Green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
* History of phycology


References


External links


AlgaeBase: RhodophytaTree of Life: Rhodophyta
{{Authority control Proterozoic first appearances