The role of physiology in the formation of Prochlorococcus sub-surface maxima in the Sargasso Sea

Prochlorococcus marinus is a unicellular cyanobacterium that can account for a large share of primary production in oligotrophic environments. Using the contrast between shallow and deep Prochlorococcus concentrations in the summertime Sargasso Sea as a model system, we tested the importance of physiological factors in controlling Prochlorococcus abundance in this environment. Prochlorococcus growth rates were measured using a cell cycle-based approach. We found that growth rate did not vary consistently with depth, nor was it related to ambient Prochlorococcus concentration. Thus in some cases growth rate was highest at the subsurface Prochlorococcus maximum, while in others it was highest in shallower water, where Prochlorococcus concentrations were relatively low. We conclude that physiological factors alone are insufficient to explain depth-related differences in Prochlorococcus abundance, and that top-down factors such as grazing and viral lysis likely exert a strong influence on Prochlorococcus dynamics.