Prochlorococcus is a genus of very small (0.6 µm) marine cyanobacteria with an unusual pigmentation (chlorophyll b). These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth. Microbes of the genus Prochlorococcus are among the major primary producers in the ocean, responsible for at least 20% of atmospheric oxygen. Analysis of the genome sequences of 12 Prochlorococcus strains show that 1100 genes are common to all strains, and the average genome size is about 2000 genes. In contrast, eukaryotic algae have over 10,000 genes.
Marine cyanobacteria are to date the smallest known photosynthetic organisms; Prochlorococcus is the smallest at just 0.5 to 0.8 micrometres across. The coccoid shaped cells are non-motile and free-living. Their small size, and therefore large surface-to-volume ratio, gives them an advantage in nutrient poor water. Still, it is assumed that Prochlorococcus have a very small nutrient requirement.  Typically, Prochlorococcus divide once a day in the subsurface layer or oligotrophic waters. 
Prochlorococcus has been found to be abundant in the euphotic zone of the world’s tropical oceans.  It is possibly the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be several octillion (~1027) individuals. Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans. Prochlorococcus is mostly found in a temperature range of 10-33°C and some strains can grow at depths with low light (<1% surface light). The bacterium accounts for an estimated 20% of the oxygen in the Earth’s atmosphere, and forms part of the base of the ocean food chain.
(From Wikipedia, May 31st, 2012)