Prochlorococcus, a marine photosynthetic prokaryote of global significance

The minute photosynthetic prokaryote Prochlorococus, which was discovered a bout 10 years ago, has proven exceptional from several standpoints. Its tin y size (0.5 to 07 mu m in diameter) makes it the smallest known photosynthe tic organism. Its ubiquity within the 40 degrees S to 40 degrees N latitudi nal 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 org anism on Earth. Prochlorococcus typically divides once a day in the subsurf ace layer of oligotrophic areas, where it dominates the photosynthetic biom ass. It also possesses a remarkable pigment complement which includes divin yl derivatives of chlorophyll a (Chl a) and Chl b, the so-called Chl a(2) a nd Chi b, and in some strains, small amounts of a new type of phycoerythrin . Phylogenetically Prochlorococcus has also proven fascinating. Recent stud ies suggest that it evolved from an ancestral cyanobacterium by reducing it s cell and genome sizes and by recruiting a protein originally synthesized under conditions of iron depletion to build a reduced antenna system as a r eplacement for large phycobilisomes. Enviromental constraints clearly playe d a predominant role in Prochlorococcus evolution. Ifs tiny size is an adva ntage for ifs adaptation to nutrient-deprived environments. Furthermore, ge netically distinct ecotypes, with different antenna systems rind ecophysiol ogical characteristics, are present at depth and in surface waters. This ve rtical species variation has allowed Prochlorococcus to adapt to the natura l light gradient occurring in the upper layer of oceans. The present review critically assesses the basic knowledge acquired about Prochlorococcus bot h in the ocean and in the laboratory.