Red algae come in green, pink, purple and almost black
The sushi wrapper nori, the jelly agar and the thickener carrageenan all come from red algae, one of the oldest and largest groups of seaweeds. Despite the name, many are not red at all. Some look bright green, others brown, and the deepest-growing species can appear nearly black.
The phylum Rhodophyta, from the Greek for rose and plant, holds more than 7,000 species in over 900 genera, most of them multicellular marine seaweeds. Only about 5 percent live in freshwater. One explanation is an ancient bottleneck: their last common ancestor appears to have lost about a quarter of its core genes, and with them much of its capacity to adapt.
Their colour comes from water-soluble pigments called phycobilins, packed into structures called phycobilisomes alongside chlorophyll a and carotenoids. These accessory pigments tint the reddish members and give the whole group its name. Red algae never have flagella or centrioles at any stage of life. They store food as floridean starch, a highly branched form kept outside the chloroplasts, and when the surrounding water grows saltier they make more of a compound called floridoside to stop water leaking out of their cells. Oddly, they carry the most gene-rich plastid genomes known.
Their cell walls come in two layers. The inner is mostly cellulose, while the outer holds agarose and agaropectin, which boiling extracts as agar. Cell division is never quite finished, leaving a small pore between daughter cells that is then sealed by a protein pit plug, a feature found nowhere else. Whether these connections help cells communicate or simply add strength is still unclear.
Many red algae have life cycles with three alternating generations instead of the usual two. The coralline algae secrete calcium carbonate and play a major part in building coral reefs. On the plate, dulse and laver have long been eaten in Europe, and Porphyra, sold as nori in Japan and gim in Korea, is a staple of Asian cooking.
Source: Red algae
