1. Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from Ancient Greek φυτόν (phutón), meaning "plant", and πλαγκτός (planktós), meaning "drifter, wanderer, roamer", and thus, "plant drifter". Phytoplankton obtain their energy through photosynthesis, as trees and other plants do on land. This means phytoplankton must receive sunlight, so they live in the well-lit surface layers (euphotic zone) of oceans and lakes. Compared with terrestrial plants, phytoplankton are distributed over a larger surface area, experience less seasonal variation, and have markedly faster turnover rates (days versus decades). As a result, phytoplankton respond rapidly on a global scale to climate variations. Phytoplankton form the base of marine and freshwater food webs and are key players in the global carbon cycle. They account for about half of global photosynthetic activity and at least half of the oxygen... from wikipedia.org