Any of various photosynthetic protists belonging to the phylum Rhodophyta. Most red algae are marine seaweeds, with bladelike or filamentous bodies. Others deposit calcium carbonate in their cell walls and grow as crusty layers on rocks, and these species are essential reef-building organisms. Unicellular forms are also known, and a few red algae live in fresh water. Red algae have chlorophyll a as well as pigments called phycobilins, which impart the reddish colors characteristic of the group. Phycobilins absorb the blue and green wavelengths of light that reach deeper coastal waters where red algae are abundant. Red algae often show complicated life cycles involving an alternation of a haploid generation with two distinct diploid generations, both of which produce a different kind of spore. Their cell walls are made of cellulose and gelatinous polysaccharide compounds, some of which have commercial value. Agar and carrageenan are extracted from red algae, and the genus Porphyra is cultivated and dried to make nori, the edible seaweed used in sushi. See more at alga.