Prochlorococcus

Prochlorococcus is a genus of extremely small, photosynthetic cyanobacteria that dominates picophytoplankton in oligotrophic regions of the world's oceans. Across vast areas of tropical and subtropical seas, Prochlorococcus is one of the most abundant photosynthetic organisms on Earth, contributing substantially to marine primary production and carbon cycling. Individual cells are on the order of 0.2–0.6 micrometers in diameter and have a high surface-area-to-volume ratio, enabling efficient nutrient uptake in dilute environments.

Physiology and genome: Prochlorococcus contains chlorophylls a and b and lacks the phycobilisome light-harvesting antennae typical of many cyanobacteria; its light-harvesting is accomplished by small, highly reduced antenna proteins. The genome is notably streamlined, with relatively few regulatory genes and a compact set of metabolic pathways. Genome sizes range roughly from 1.6 to 2.7 megabases, encoding about 1,500–2,000 protein-coding genes. Two main ecotypes predominate: high-light adapted and low-light adapted, which inhabit different depths in the photic zone.

Ecology and distribution: Prochlorococcus is most abundant in surface waters of oligotrophic gyres and can form millions of cells per milliliter under favorable conditions. It relies on dissolved inorganic nutrients, particularly phosphate; nitrogen limitation and nutrient speciation influence its distribution. It is prey to protists and is heavily affected by cyanophages, viruses that infect cyanobacteria; virus-encoded photosynthesis genes can be transferred to hosts.

Laboratory culture and significance: Only a subset of natural diversity has been cultivated; model strains have facilitated understanding of minimal genome and ecological strategies. Prochlorococcus is a key organism for studies of ocean biogeochemistry, microbial community interactions, and the evolution of photosynthesis; its abundance shapes regional primary production and local carbon flux.