Multicellular marine macroalgae including some members of red (RHODOPHYTA), green (CHLOROPHYTA), and brown (PHAEOPHYTA) algae. They are widely distributed in the ocean, occurring from the tide level to considerable depths, free-floating (planktonic) or anchored to the substratum (benthic). They lack a specialized vascular system but take up fluids, nutrients, and gases directly from the water. They contain CHLOROPHYLL and are photosynthetic, but some also contain other light-absorbing pigments. Many are of economic importance as FOOD, fertilizer, AGAR, potash, or source of IODINE.