Half Earth’s photosynthesis rides on drifting microscopic plants
Plankton—from Greek planktos, “wanderer”—are water or air drifters too weak to swim against currents or wind. The cast spans bacteria and algae to jellyfish, split trophically into phytoplankton and zooplankton. Tiny phytoplankton alone drive roughly half of Earth’s photosynthesis.
Plankton takes its name from the Greek planktos, drifter or wanderer, and a single member is called a plankter. The category is ecological rather than genealogical: it covers anything in water or air that cannot push against the current or wind. That includes bacteria, archaea, protozoa and microscopic algae and fungi, but also jellyfish and comb jellies, which swim slowly yet not strongly enough to beat the flow. Strong swimmers are nekton, seafloor dwellers are benthos, and organisms at the surface are neuston, with the passively drifting ones counted as a subgroup of plankton.
The traditional split is by feeding. Phytoplankton such as diatoms and cyanobacteria make their own food; zooplankton such as radiolarians and copepods eat other organisms. Research has since shown that many single-celled plankton, including Mesodinium and many dinoflagellates, both photosynthesise and ingest prey, earning a third category, mixoplankton. A fourth group, the decomposers, includes microscopic fungi, bacteria and aquatic viruses that recycle organic nutrients back into the food supply.
Plankton smaller than about a millimetre hold aquatic ecosystems together. Phytoplankton produce roughly half of Earth's oxygen through photosynthesis and help lock away carbon. Marine microorganisms are estimated to make up 70 to 90 percent of ocean biomass, and they power the biological pump that pulls carbon dioxide from the air and carries carbon into deep water. Plankton form the base of the marine food web, supporting animals from forage fish to baleen whales. Some species spend part of their lives drifting through the atmosphere as aeroplankton.
Plankton live in salt oceans, brackish estuaries, freshwater lakes and rivers alike, and their numbers shift with depth, season and location depending on light, nutrients, the physical state of the water and the abundance of other plankton.
Source: Plankton
