Any of various photosynthetic protists belonging to the phylum Phaeophyta, almost all of which live in marine environments. Brown algae have chlorophylls a and c as well large quantities of the pigment fucoxanthin, which gives the group their characteristic brownish colors. This pigment absorbs the range of blue light frequencies available to brown algae when they are submerged and allows them to live in deeper waters than green algae. The brown algae store their food in a compound called laminarin and transport it throughout their bodies in a compound called mannitol (unlike plants and green algae). The cell walls of brown algae are made of cellulose and algin. Their bodies vary from small filaments to the immense leaflike thalli of kelp. Species of brown algae dominate shoreline and coastal ecosystems in cooler waters, and huge masses of the brown alga Sargassum cover the warm Sargasso sea in the Atlantic. See more at alga.