Prochlorococcus, a marine photosynthetic prokaryote of global significance

The minute photosynthetic prokaryote Prochlorococus, which was discovered about 10 years ago, has proven exceptional from several standpoints. Its tiny size (0.5 to 07 mu m in diameter) makes it the smallest known photosynthetic organism. Its ubiquity within the 40 degrees S to 40 degrees N latitudinal band of oceans and its occurrence at high density from the surface down to depths of 200 m make it presumably the most abundant photosynthetic organism on Earth. Prochlorococcus typically divides once a day in the subsurface layer of oligotrophic areas, where it dominates the photosynthetic biomass. It also possesses a remarkable pigment complement which includes divinyl derivatives of chlorophyll a (Chl a) and Chl b, the so-called Chl a(2) and Chi b, and in some strains, small amounts of a new type of phycoerythrin. Phylogenetically Prochlorococcus has also proven fascinating. Recent studies suggest that it evolved from an ancestral cyanobacterium by reducing its cell and genome sizes and by recruiting a protein originally synthesized under conditions of iron depletion to build a reduced antenna system as a replacement for large phycobilisomes. Enviromental constraints clearly played a predominant role in Prochlorococcus evolution. Ifs tiny size is an advantage for ifs adaptation to nutrient-deprived environments. Furthermore, genetically distinct ecotypes, with different antenna systems rind ecophysiological characteristics, are present at depth and in surface waters. This vertical species variation has allowed Prochlorococcus to adapt to the natural light gradient occurring in the upper layer of oceans. The present review critically assesses the basic knowledge acquired about Prochlorococcus both in the ocean and in the laboratory.