Red algae

[3] The Rhodophyta comprises one of the largest phyla of algae, containing over 7,000 recognized species within over 900 genera[4] amidst ongoing taxonomic revisions.[7] Approximately 5% of red algae species occur in freshwater environments, with greater concentrations in warmer areas.[15] 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.[17] This event (termed primary endosymbiosis) is at the origin of the red and green algae (including the land plants or Embryophytes which emerged within them) and the glaucophytes, which together make up the oldest evolutionary lineages of photosynthetic eukaryotes, the Archaeplastida.[18] 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), and Alveolata.[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 the Cyanidiophyceae, a class of unicellular and thermoacidophilic extremophiles found in sulphuric hot springs and other acidic environments,[20] an adaptation partly made possible by horizontal gene transfers from prokaryotes,[21] with about 1% of their genome having this origin,[22] 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.[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.[11] In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.Some examples of species and genera of red algae are: While Cyanidiophyceae is universally agreed to be the most basal, the remaining 6 classes in the subphylum Rhodophytina have uncertain relationships.[49] The presence of the water-soluble pigments called phycobilins (phycocyanobilin, phycoerythrobilin, phycourobilin and phycobiliviolin), which are localized into phycobilisomes, gives red algae their distinctive color.[53] Floridean starch (similar to amylopectin in land plants), a long-term storage product, is deposited freely (scattered) in the cytoplasm.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.Hence, they rely on water currents to transport their gametes to the female organs – although their sperm are capable of "gliding" to a carpogonium's trichogyne.They contain the sulfated polysaccharide carrageenan in the amorphous sections of their cell walls, although red algae from the genus Porphyra contain porphyran.Bangiomorpha pubescens, a multicellular fossil from arctic Canada, strongly resembles the modern red alga Bangia 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.[80] They are a source of antioxidants including polyphenols, and phycobiliproteins and contain proteins, minerals, trace elements, vitamins and essential fatty acids.Several species are food crops, in particular dulse (Palmaria palmata)[83] and members of the genus Porphyra, variously known as nori (Japan), gim (Korea), zicai 紫菜 (China), and laver (British Isles).