The genus Prochlorococcus, phylum cyanobacteria

The genus Prochlorococcus consists of unicellular, oxygen-evolving photosynthetic organisms belonging to the phylum of cyanobacteria. Members of this genus contain an unusual pigment complement, consisting of divinyl-chlorophyll a and b as the major photosynthetic pigments rather than the chlorophyll a and phycobiliproteins found in most other cyanobacteria. Phylogenetically, Prochlorococcus forms a monophyletic group together with marine Synechococcus, to the exclusion of the other chlorophyll a/b containing genera, Prochlorothrix and Prochloron. Thus, the three genera of chlorophyll a/b containing cyanobacteria do not form a discrete, integral family within the cyanobacteria. The distribution of Prochlorococcus is limited to oceanic environments, predominantly in the latitudinal band between 40 °N and 40 °S where it contributes significantly to global primary production. Numerous strains have been cultured, their genomes described, and their physiological adaptations to environmentally relevant factors investigated. Five-to-six discrete ecotypes are currently recognized based on their phylogeny, light and temperature physiologies, and their ecological distribution patterns. Recent phylogenetic analyses of field populations suggest that additional lineages are present in the oceans. Members of this genus have the smallest genome size of any free-living photosynthetic organism. Core genes present in all members of this genus are organized in large syntenic genome regions and encode the genes for central metabolism. Non-core genes are those present in only a subset of strains and are primarily localized in genomic islands. Gene content appears to explain nutrient-linked differences in ecosystem niche specialization as well as differences in sensitivity to phages. In contrast, it remains unclear whether gene content can explain the light and temperature differences associated with the different ecotypes. The development of a robust genetic inactivation system for Prochlorococcus is essential if we are to test hypotheses relating to the role of specific genes in the adaptation of the members of this genus to different ecological niches.