Algae were a group of plant-like organisms, they ranged from single-celled varieties to larger multicellular growths.
In 2369 the water supply of the Dracon monastery on the planet Riat was infected by an algae which had been irradiated by solar flare activity. When the monks at the monastery consumed the water they underwent a mutation which made them violent and cannibalistic. Doctor Beverly Crusher of the USS Enterprise-D was able to cure the monks of the infection. (TNG comic: "Light of the Day")