緑藻
Not all algae are microscopic. Green, red and brown algae all have multicellular macroscopic forms that make up the familiar seaweeds. Green algae, an informal group, contains about 8,000 recognised species. Many species live most of their lives as single cells or are filamentous, while others form colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds. Red algae, a (disputed) phylum contains about 7,000 recognised species, mostly multicellular and including many notable seaweeds. Brown algae form a class containing about 2,000 recognised species, mostly multicellular and including many seaweeds such as kelp.