Red algae

Red algae Temporal range: Mesoproterozoic–present[1][2] Pha. Proterozoic Archean Had.
A-D : Chondrus crispus Stackhouse, E-F : Mastocarpus stellatus J.Ag.
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
(unranked):	Archaeplastida
Division:	Rhodophyta Wettstein, 1922
Clades
Eurhodophytina Bangiophyceae Florideophyceae Proteorhodophytina Compsopogonophyceae Porphyridiophyceae Rhodellophyceae Stylonematophyceae Cyanidiophytina Cyanidiophyceae

Red algae, or Rhodophyta (/roʊˈdɒfɪtə/, /ˌroʊdəˈfaɪtə/; from Ancient Greek ῥόδον (rhódon) 'rose', and φυτόν (phutón) 'plant'), make up one of the oldest groups of eukaryotic algae.^[3] The Rhodophyta comprises one of the largest phyla of algae, containing over 7,000 recognized species amidst ongoing taxonomic revisions.^[4] The majority of species (6,793) are Florideophyceae, and mostly consist of multicellular, marine algae, including many notable seaweeds.^[4][5] Red algae are abundant in marine habitats.^[6] Approximately 5% of red algae species occur in freshwater environments, with greater concentrations in warmer areas.^[7] Except for two coastal cave dwelling species in the asexual class Cyanidiophyceae, 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.^[8][9]

Red algae form a distinct group characterized by eukaryotic cells without flagella and centrioles, chloroplasts without external endoplasmic reticulum or unstacked (stroma) thylakoids, and use phycobiliproteins as accessory pigments, which give them their red color.^[10] Despite their name, red algae can vary in color from bright green, soft pink, resembling brown algae, to shades of red and purple, and may be almost black at greater depths.^[11][12] Unlike green algae, red algae store sugars as food reserves outside the chloroplasts as floridean starch, a type of starch that consists of highly branched amylopectin without amylose.^[13] Most red algae are multicellular, macroscopic, and reproduce sexually. The life history of red algae is typically an alternation of generations that may have three generations rather than two.^[14] Coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong there.

Red algae such as Palmaria palmata (dulse) and Porphyra species (laver/nori/gim) are a traditional part of European and Asian cuisines and are used to make products such as agar, carrageenans, and other food additives.^[15]