Prochlorococcus cyanobacteria are the smallest known photosynthetic organisms (0.5–0.7 mm in diameter) and contain a unique photosynthetic apparatus. Along with the related Synechococcus, they are the numerically dominant photoautotrophs at latitudes from 40°S to 40°N in surface waters (upper 200 m) of the open ocean and as a consequence are among the most abundant organisms on Earth, contributing significantly to global primary productivity. Despite their abundance, broad distribution, and ecological importance, Prochlorococcus were only described in 1992 (the history of their discovery is reviewed in Partensky et al. 1999). Synechococcus are distributed more ubiquitously throughout the ocean, ranging from polar through temperate to tropical waters, but are generally more abundant in nutrient-rich surface waters; Prochlorococcus are generally absent from brackish or well-mixed waters, typically dominating in subtropical oligotrophic areas. Prochlorococcus also generally extend deeper in the water column than do Synechococcus. (Partensky et al. 1999 and references therein; Scanlan and West 2002 and references therein; Coleman and Chisholm 2007 and references therein; Partensky and Garczarek 2010)
Partensky and Garczarek (2010) consider the biological explanations for the tiny cells and genome of Prochlorococcus and the implications of this evolutionary downsizing for the ecological success of these organisms in warm, oligotrophic regions of the ocean.