Prochlorococcus and Synechococcus are single celled cyanobacteria. They are the smallest yet most abundant photosynthetic microbes in the ocean. They are so small that a hundred of these organisms can fit end-to-end across the width of a human hair and there are around 100 million Prochlorococcus cells per litre of seawater. Researchers estimate that Prochlorococcus and Synechococcus remove about 10 billion tons of carbon from the air each year; this is about two-thirds of the total carbon fixation that occurs in the oceans. Scientists have deciphered the genomes of these two microbes.