The shells contain minute depressions which form intricate patterns that are used to identify different species. Most occur as plankton and are found in both fresh and salt water. The dinoflagellates have thick cellulose plates that look like armor. This division has 1000 species. 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.