Habitat
Both bacillariophyta (diatoms) and dinoflagellates, which are "photosynthetic plantlike protists [that] are called algae," live in marine and freshwater ecosystems. However, "some species can be found in moist soil or on mosses."
http://globalclimaticchange.blogspot.com/ http://www.energyinsight.info/ocean_algae_biomass.jpg http://www.oceanchampions.org/blog/
Niche: Providing Carbon, Oxygen, and Food Sources
Diatoms and dinoflagellates, which are single-cellular, are primary producers, meaning that they are significant food sources at the base of the food chain. These organisms are responsible for 20-25% of all the carbon fixation on Earth. "Carbon fixation is a term describing the photosynthetic process of removing atmospheric carbon in the form of carbon dioxide and converting it to organic carbon in the form of sugar." By doing so, diatoms and dinoflagellates provide food to organisms such as mollusks and aquatic animal larvae. These organisms also provide a large portion of the earth's oxygen. Phytoplankton, which is classified under both dinoflagellata and bacillariophyta, is said to have produced over half the oxygen we breathe.
Sources: Holt McDougal Biology by Stephen Nowicki
http://www.encyclopedia.com/topic/Dinoflagellata.aspx
http://www.encyclopedia.com/topic/Dinoflagellata.aspx