Red algae

Red algae
Temporal range: Mesoproterozoic–present[1]
Chondrus crispus - Köhler–s Medizinal-Pflanzen-034.jpg
A-D : Chondrus crispus Stackhouse,
E-F : Mastocarpus stellatus J.Ag.
Scientific classification e
Domain:Eukaryota
(unranked):Diaphoretickes
(unranked):Archaeplastida
Division:Rhodophyta
Wettstein, 1922

Classification is currently disputed. See Taxonomy.

Red algae, or Rhodophyta (ə/ DOF-it-ə, ə/ FY-tə; from Ancient Greek ῥόδον (rhodon), meaning 'rose', and φυτόν (phyton), meaning 'plant'), are one of the oldest groups of eukaryotic algae.[2] The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing.[3] The majority of species (6,793) are found in the Florideophyceae (class), and mostly consist of multicellular, marine algae, including many notable seaweeds.[3][4] Approximately 5% of the red algae occur in freshwater environments with greater concentrations found in the warmer area.[5] There are no terrestrial species, which is assumed to be traced back to an evolutionary bottleneck where the last common ancestor lost about 25% of its core genes and much of its evolutionary plasticity.[6]

The red algae form a distinct group characterized by having eukaryotic cells without flagella and centrioles, chloroplasts that lack external endoplasmic reticulum and contain unstacked (stroma) thylakoids, and use phycobiliproteins as accessory pigments, which give them their red color.[7] Red algae store sugars as floridean starch, which is a type of starch that consists of highly branched amylopectin without amylose,[8] as food reserves outside their plastids. Most red algae are also multicellular, macroscopic, marine, and reproduce sexually. The red algal life history is typically an alternation of generations that may have three generations rather than two.[9]

Chloroplasts evolved following an endosymbiotic event between an ancestral, photosynthetic cyanobacterium and an early eukarytoic phagotroph.[10] This event (termed primary endosymbiosis) resulted in the origin of the red and green algae, and the glaucophytes, which make up the oldest evolutionary lineages of photosynthetic eukaryotes.[11] A secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages such as Cryptophyta, Haptophyta, Stramenopiles (or Heterokontophyta), Alveolata, Centrohelids, Katablepharids, and Telonemi.[11] In addition to multicellular brown algae, it is estimated that more than half of all known species of microbial eukaryotes harbor red-algal-derived plastids.[12]

The coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse (Palmaria palmata) and laver (nori/gim) are a traditional part of European and Asian cuisines and are used to make other products such as agar, carrageenans and other food additives.[13]

Red algae are divided into the Cyanidiophyceae, a class of unicellular and thermoacidophilic extremophiles found in sulphuric hot springs and other acidic environments,[14] an adaptation partly made possible by horizontal gene transfers from prokaryotes,[15] 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.[16][17] 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).[18] Some marine species are found on sandy shores, while most others can be found attached to rocky substrata.[19] Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats;[5] they generally prefer clean, high-flow streams with clear waters and rocky bottoms, but with some exceptions.[20] A few freshwater species are found in black waters with sandy bottoms [21] and even fewer are found in more lentic waters.[22] 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.[7] In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.[23][24]

Taxonomy

In the system of Adl et al. 2005, the red algae are classified in the Archaeplastida, along with the glaucophytes and 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."[25]

Many studies published since Adl et al. 2005 have provided evidence that is in agreement for monophyly in the Archaeplastida (including red algae).[26][27][28][29] However, other studies have suggested Archaeplastida is paraphyletic.[30][31] As of January 2011, 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).[32]

  • 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 major research initiative to reconstruct the Red Algal Tree of Life (RedToL) using phylogenetic and genomic approaches is funded by the National Science Foundation as part of the Assembling the Tree of Life Program.

Classification comparison

Classification system according to
Saunders and Hommersand 2004[32]
Classification system according to
Hwan Su Yoon et al. 2006[33]
Orders Multicelluar? Pit plugs? Example
Cyanidiales No No Cyanidioschyzon merolae
  • Subphylum Rhodophytina subphylum novus
Rhodellales No No Rhodella
Compsopogonales, Rhodochaetales, Erythropeltidales Yes No Compsopogon
Rufusiales, Stylonematales Yes No Stylonema

Bangiales

Yes Yes Bangia, "Porphyra"

Porphyridiales

No No Porphyridium cruentum
Hildenbrandiales Yes Yes Hildenbrandia
Batrachospermales, Balliales, Balbianiales, Nemaliales, Colaconematales, Acrochaetiales, Palmariales, Thoreales Yes Yes Nemalion
Rhodogorgonales, Corallinales Yes Yes Corallina officinalis
  • Subclass Ahnfeltiophycidae
Ahnfeltiales, Pihiellales Yes Yes Ahnfeltia
Bonnemaisoniales, Gigartinales, Gelidiales, Gracilariales, Halymeniales, Rhodymeniales, Nemastomatales, Plocamiales, Ceramiales Yes Yes Gelidium

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.[34])

A subphylum - Proteorhodophytina - has been proposed to encompass the existing classes Compsopogonophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae.[35] 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,[3] but the taxonomy is in constant flux with new species described each year.[32][33] The vast majority of these are marine with about 200 that live only in fresh water.

Some examples of species and genera of red algae are: