 Phytoplankton are photosynthesizing microscopic biotic organisms that inhabit the upper sunlit layer of almost all oceans and bodies of fresh water on Earth.They are agents for primary production, the creation of organic compounds from carbon dioxide dissolved in the water, a process that sustains the aquatic food web.Phytoplankton are extremely diverse, varying from photosynthesising bacteria (cyanobacteria), to plant-like diatoms, to armour-plated coccolithophores.