"Discovered only in the last 2 decades, Prochlorococcus is now known to be the most abundant photosynthetic cell in the oceans, often reaching 105 cells ml-1 and accounting for a significant fraction of the chlorophyll. The cells are less than 1 micron in diameter, and are quite unusual for prokaryotes in that they contain divinyl chlorophyll a and b as their primary light-harvesting pigments. Their global success may be due in part to the existence of physiologically and genetically distinct “ecotypes” which have different minimum, maximum, and optimal light intensities for growth, and are thus differentially suited for growth at different depths in the oceans."
excerpt from Sallie W. Chisolm, MIT
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Photograph from DOE Joint Genome Institute.