Blooms of dinoflagellates may pose health and environmental risks. Brown algae is a multicellular algae that grows primarily in salt water. The division contains 1500 species. Red algae is usually found attached to a substrate. The green algae are very diverse.
Most green algae occur in fresh water, although a few groups are marine.
Some green algae is found in snow, soil, on tree trunks and in symbiosis with other organisms. Algae of this class are either unicellular, few- celled, or filamentous in nature. Most green algae belong to this class. All euglenoids but one, Colacium , are unicellular. Most euglenoids are freshwater organisms.