Which cyanobacterium is considered the most abundant photosynthetic organism on Earth?

Prochlorococcus is considered the most abundant photosynthetic organism on Earth. This tiny marine cyanobacterium is found mainly in warm, sunlit ocean waters, where its enormous population contributes substantially to global photosynthesis.

The cells are exceptionally small, commonly around 0.5 to 0.7 micrometres in diameter. They contain chlorophyll a and chlorophyll b, pigments more commonly associated with green plants, and are adapted to different light conditions at varying ocean depths. Different ecotypes specialize in brighter surface water or dimmer deeper water.

Prochlorococcus was discovered in 1986 by Sallie W. Chisholm and colleagues using flow cytometry. It is not a eukaryotic alga: it is a bacterium, specifically a cyanobacterium. Synechococcus is another abundant marine cyanobacterium, but it is generally larger and is not the organism identified by the “most abundant photosynthetic organism” distinction.