Red algae

The red algae are a large group of mostly multicellular, marine algae, including many notable seaweeds. Most of the coralline algae, which secrete calcium carbonate and play a major role in building reefs, belong here. Red algae are a traditional part of oriental cuisine—for instance nori—and are used to make certain other products like agar and food additives.

The chloroplasts of red algae are bound by a double membrane, so presumably were acquired by direct endosymbiosis of cyanobacteria. Like most cyanobacteria, they are pigmented with chlorophyll a and various proteins called phycobilins, which gives them a distinctive red colour. The only other groups with primary chloroplasts are the green plants (including green algae) and glaucophytes, and it has been proposed that these groups all share a common origin. Most other algae have chloroplasts taken secondarily from one of these forms, and in particular a number of groups appear to have acquired them from red algae: heterokonts, haptophytes, cryptomonads, and dinoflagellates. These chloroplasts typically have chlorophylls a and c lack phycobilins, but show genetic similarities to those of this group.

Red algae have mitochondria with flat cristae, and undergo closed mitosis. Unlike most other algae, no flagella are found in any member of the group. Unicellular forms typically live attached to surfaces rather than floating among the plankton, and both the larger female and smaller male gametes are non-motile, so that most have a low chance of fertilization. Cell walls are made out of cellulose and thick gelatinous polysaccharides, which are the basis for most of the industrial products made from red algae.

Taxonomy

The diverse eukaryotes that constitute the red algae have been the focus of numerous recent molecular surveys and remain a rich source of undescribed and little known species for the traditional taxonomist. Molecular studies place the red algae in the kingdom Plantae; however, supraordinal classification has been largely confined to debate on subclass vs. class level status for the two recognized subgroups, one of which is widely acknowledged as paraphyletic. This narrow focus has generally masked the extent to which red algal classification needs modification.

The most current classification system is below. Note that while this is a valid published taxonomy it does not necessary have to be used, the taxonomy of the algae is still in a state of flux.

Kingdom Plantae

Subkingdom Rhodoplantae

Phylum 1: Cyanidiophyta

Class Cyanidiophyceae

Phylum 2: Rhodophyta

Subphylum 1: Rhodellophytina

Class Rhodellophyceae

Subphylum 2: Metarhodophytina

Class Compsopogonophyceae

Subphylum 3: Eurhodophytina

Class 1: Bangiophyceae

Class 2: Florideophyceae

Subclass 1: Hildenbrandiophycidae

Subclass 2: Nemaliophycidae

Subclass 3: Ahnfeltiophycidae

Subclass 4: Rhodymeniophycidae

References

Saunders, Gary W., Hommersand, Max H. (2004) Assessing red algal supraordinal diversity and taxonomy in the context of contemporary systematic data. Am. J. Bot. 91: 1494-1507