Phytoplankton - (n.)

Microscopic photosynthesizing organisms adrift in the sunlit surface waters of the ocean. The most important types of phytoplankton are "diatoms" and "dinoflagellates." Diatoms have shells made of silica (precipitated from silicate in seawater) and built on the pillbox principle. Diatoms are ubiquitous, but are most abundant in upwelling regions. Dinoflagellates have cell walls made of organic matter, and a tail, called a "flagella," which is used to move the organism through the water. Both diatoms and dinoflagellates are unicellular organisms.