Red algae, orRhodophyta (/roʊˈdɒfɪtə/,/ˌroʊdəˈfaɪtə/; from Ancient Greekῥόδον (rhódon)'rose' and φυτόν (phutón)'plant'), make up one of the oldest groups ofeukaryoticalgae.[3] The Rhodophyta comprises one of the largestphyla ofalgae, containing over 7,000 recognized species within over 900genera[4] amidst ongoing taxonomic revisions.[5] The majority of species (6,793) areFlorideophyceae, and mostly consist ofmulticellular,marine algae, including many notableseaweeds.[5][6] Red algae are abundant in marine habitats.[7] Approximately 5% of red algae species occur in freshwater environments, with greater concentrations in warmer areas.[8] Except for two coastal cave dwelling species in the asexual classCyanidiophyceae, no terrestrial species exist, 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.[9][10]
Chloroplasts probably evolved following anendosymbiotic event between an ancestral, photosyntheticcyanobacterium and an early eukaryoticphagotroph.[17] This event (termedprimary endosymbiosis) is at the origin of the red andgreen algae (including the land plants orEmbryophytes which emerged within them) and theglaucophytes, which together make up the oldest evolutionary lineages of photosynthetic eukaryotes, theArchaeplastida.[18] A secondary endosymbiosis event involving an ancestral red alga and aheterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages such asCryptophyta,Haptophyta,Stramenopiles (or Heterokontophyta), andAlveolata.[18] 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.[19]
Red algae are divided into theCyanidiophyceae, a class of unicellular andthermoacidophilicextremophiles found in sulphuric hot springs and other acidic environments,[20] an adaptation partly made possible byhorizontal gene transfers from prokaryotes,[21] with about 1% of their genome having this origin,[22] and two sister clades called SCRP (Stylonematophyceae,Compsopogonophyceae,Rhodellophyceae andPorphyridiophyceae) and BF (Bangiophyceae andFlorideophyceae), which are found in both marine and freshwater environments. 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.[23] In the SCRP clade the class Compsopogonophyceae is multicellular, with forms varying from microscopic filaments to macroalgae. Stylonematophyceae have both unicellular and small simple filamentous species, while Rhodellophyceae and Porphyridiophyceae are exclusively unicellular.[24][25] 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 asphycoerythrin) 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).[26] Some marine species are found on sandy shores, while most others can be found attached to rocky substrata.[27] Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats;[8] they generally prefer clean, high-flow streams with clear waters and rocky bottoms, but with some exceptions.[28] A few freshwater species are found in black waters with sandy bottoms[29] and even fewer are found in morelentic waters.[30] 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.[11] In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.[31][32]
In the classification system of Adlet al. 2005, the red algae are classified in theArchaeplastida, along with theglaucophytes and the green algae plus land plants (Viridiplantae 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."[33] Many subsequent studies provided evidence that is in agreement for monophyly in the Archaeplastida (including red algae).[34][35][36][37] However, other studies have suggested Archaeplastida isparaphyletic.[38][39] As of January 2020[update], the general consensus is that Archaeplastida is paraphyletic.[40]
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 oforder having received little scientific attention for most of the 20th century).[41]
If the kingdom Plantae is defined as the Archaeplastida, then red algae will be part of that group.
If Plantae are defined more narrowly, to be the Viridiplantae, then the red algae might be excluded.
A major research initiative to reconstruct the Red Algal Tree of Life (RedToL) usingphylogenetic andgenomic approach is funded by theNational Science Foundation as part of the Assembling the Tree of Life Program.
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.[3]: 107 )
Over 7,000 species are currently described for the red algae,[5] but the taxonomy is in constant flux with new species described each year.[41][42] The vast majority of these are marine with about 200 that live only infresh water.
Some examples of species and genera of red algae are:
WhileCyanidiophyceae is universally agreed to be the mostbasal, the remaining 6 classes in the subphylum Rhodophytina have uncertain relationships. The below cladogram follows the results of a 2016 study concerning diversification times among red algae.[44]
Red algal morphology is diverse ranging fromunicellular forms to complex parenchymatous and non- parenchymatous thallus.[45] Red algae have doublecell walls.[46] The outer layers contain the polysaccharidesagarose and agaropectin that can be extracted from the cell walls asagar by boiling.[46] The internal walls are mostly cellulose.[46] They also have the most gene-rich plastid genomes known.[47]
Red algae do not have flagella and centrioles during their entire life cycle. The distinguishing characters of red algal cell structure include the presence of normal spindle fibres, microtubules, un-stacked photosynthetic membranes, phycobilin pigment granules,[48] pit connection between cells, filamentous genera, and the absence of chloroplast endoplasmic reticulum.[49]
The presence of the water-soluble pigments calledphycobilins (phycocyanobilin,phycoerythrobilin,phycourobilin andphycobiliviolin), which are localized intophycobilisomes, gives red algae their distinctive color.[50] Theirchloroplasts contain evenly spaced and ungrouped thylakoids[51] and contain the pigments chlorophyll a, α- and β-carotene, lutein and zeaxanthin. Their chloroplasts are enclosed in a double membrane, lack grana and phycobilisomes on the stromal surface of the thylakoid membrane.[52]
The major photosynthetic products include floridoside (major product), D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc.[53] Floridean starch (similar to amylopectin in land plants), a long-term storage product, is deposited freely (scattered) in the cytoplasm.[54] 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.[55] 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 are unique and distinctive features of red algae that form during the process ofcytokinesis followingmitosis.[56][3] 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. Becauseapical 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 orderCeramiales.[3]
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.
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.[57]
The reproductive cycle of red algae may be triggered by factors such as day length.[3] 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).[58]
Red algae lackmotilesperm. Hence, they rely on water currents to transport theirgametes to the female organs – although their sperm are capable of "gliding" to acarpogonium'strichogyne.[3] Animals also help with the dispersal and fertilization of the gametes. The first species discovered to do so is theisopod Idotea balthica.[59]
The trichogyne will continue to grow until it encounters aspermatium; once it has been fertilized, the cell wall at its base progressively thickens, separating it from the rest of the carpogonium at its base.[3]
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.[3]
Carpospores may also germinate directly intothalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free-living) tetrasporophyte phase.[60] Tetrasporangia may be arranged in a row (zonate), in a cross (cruciate), or in a tetrad.[3]
The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form acystocarp.[60]
The two following case studies may be helpful to understand some of the life histories algae may display:
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"[3] and their spores are "liberated through the apex of sporangial cell."[3]
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.[3]
The gametophyte may replicate asexually using monospores, but also produces nonmotile sperm in spermatangia, and a lower, nucleus-containing "egg" region of the carpogonium.[3][61]
In itsdiploid 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 tinyprothallus withrhizoids, which develops to a cm-scale leafy thallus. This too can reproduce via monospores, which are produced inside the thallus itself.[3] They can also reproduce via spermatia, produced internally, which are released to meet a prospective carpogonium in itsconceptacle.[3]
Theδ13C values of red algae reflect their lifestyles. The largest difference results from their photosyntheticmetabolic pathway: algae that useHCO3 as a carbon source have less negativeδ13C values than those that only useCO2.[63] An additional difference of about 1.71‰ separates groupsintertidal from those below the lowest tide line, which are never exposed to atmospheric carbon. The latter group uses the more13C-negative CO2 dissolved in sea water, whereas those with access to atmospheric carbon reflect the more positive signature of this reserve.
Photosynthetic pigments of Rhodophyta are chlorophyllsa andd. Red algae are red due tophycoerythrin. They contain the sulfated polysaccharidecarrageenan in the amorphous sections of their cell walls, although red algae from the genusPorphyra containporphyran. They also produce a specific type of tannin calledphlorotannins, but in a lower amount than brown algae do.
As enlisted inrealDB,[64] 27 complete transcriptomes and 10 complete genomes sequences of red algae are available. Listed below are the 10 complete genomes of red algae.
One of the oldest fossils identified as a red alga is also the oldest fossileukaryote that belongs to a specific moderntaxon.Bangiomorpha pubescens, a multicellular fossil from arcticCanada, strongly resembles the modern red algaBangia and occurs in rocks dating to 1.05 billion years ago.[2]
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.[76]
Red algae are important builders oflimestone reefs. The earliest such coralline algae, thesolenopores, are known from theCambrian period. Other algae of different origins filled a similar role in the latePaleozoic, and in more recent reefs.
Chromista andAlveolata algae (e.g., chrysophytes, diatoms, phaeophytes, dinophytes) seem to have evolved frombikonts that have acquired red algae asendosymbionts. According to this theory, over time these endosymbiont red algae have evolved to become chloroplasts. This part ofendosymbiotic theory is supported by various structural andgenetic similarities.[79]
Red algae have a long history of use as a source of nutritional, functional food ingredients and pharmaceutical substances.[80] They are a source of antioxidants including polyphenols, and phycobiliproteins and contain proteins, minerals, trace elements, vitamins and essential fatty acids.[81][82]
Traditionally, red algae are eaten raw, in salads, soups, meal and condiments. Several species are food crops, in particulardulse (Palmaria palmata)[83] and members of the genusPorphyra, variously known asnori (Japan),gim (Korea),zicai紫菜 (China), andlaver (British Isles).[84]
Red algal species such asGracilaria andLaurencia are rich inpolyunsaturated fatty acids (eicopentaenoic acid, docohexaenoic acid,arachidonic acid)[85] and have protein content up to 47% of total biomass.[80] 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.[86] Red algae, likeGracilaria,Gelidium,Euchema,Porphyra,Acanthophora, andPalmaria are primarily known for their industrial use for phycocolloids (agar, algin, furcellaran and carrageenan) as thickening agent, textiles, food, anticoagulants, water-binding agents, etc.[87] Dulse (Palmaria palmata) is one of the most consumed red algae and is a source of iodine, protein, magnesium and calcium.[88] Red algae's nutritional value is used for the dietary supplement ofalgas calcareas.[89]
China, Japan, Republic of Korea are the top producers of seaweeds.[90] In East and Southeast Asia,agar is most commonly produced fromGelidium amansii. These rhodophytes are easily grown and, for example,nori cultivation in Japan goes back more than three centuries.[91]
Researchers in Australia discovered that limu kohu (Asparagopsis taxiformis) can reducemethane emissions incattle. In oneHawaii experiment, the reduction reached 77%. TheWorld Bank predicted the industry could be worth ~$1.1 billion by 2030. As of 2024, preparation included three stages of cultivation and drying. Australia's first commercial harvest was in 2022. Agriculture accounts for 37% of the world’s anthropogenic methane emissions. One cow produces between 154 and 264 pounds of methane/yr.[92]
Other algae-based markets include construction materials, fertilizers and other agricultural inputs, bioplastics, biofuels and fabric. Red algae also provides ecosystem services such as filtering water and carbon sequestration.[92]
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