Phytoplankton Diversity
Phytoplankton are microscopic photosynthetic organisms that drift in the sunlit surface waters and produce roughly half of Earth's oxygen. Major groups include: diatoms (class Bacillariophyceae)—single-celled algae enclosed in intricate silica frustules (glass shells), dominating temperate and polar waters and responsible for about 20% of global photosynthesis; dinoflagellates—biflagellated protists, some photosynthetic, some heterotrophic, some causing harmful algal blooms (red tides); coccolithophores—tiny cells covered in calcium carbonate plates (coccoliths) that reflect sunlight, affecting Earth's albedo; and cyanobacteria (Prochlorococcus and Synechococcus)—the smallest and most abundant photosynthetic organisms on Earth, with Prochlorococcus alone numbering approximately 3 × 10²⁷ cells globally. Despite being invisible individually, phytoplankton collectively contain as much carbon as all terrestrial rainforests.