Which bacterium is the most abundant photosynthetic organism in the oceans?

Prochlorococcus is the most abundant photosynthetic organism in the oceans.

This tiny marine cyanobacterium was discovered in 1986 by Sallie Chisholm and colleagues. It is especially common in warm, nutrient-poor regions of the open ocean, where its small size and efficient light-harvesting systems help it compete successfully.

Prochlorococcus performs oxygen-producing photosynthesis and contributes substantially to marine primary production. Different ecotypes are adapted to different light levels, from bright surface waters to deeper parts of the sunlit zone.

It is sometimes confused with Synechococcus, another important marine cyanobacterium. Prochlorococcus is generally smaller and contains distinctive divinyl chlorophyll pigments. Its abundance makes it ecologically important even though an individual cell is invisible to the unaided eye.