Prochlorococcus contributes to new production in the Sargasso Sea deep chlorophyll maximum

Prochlorococcus is ubiquitous in tropical oceans, but its biogeochemical role is not well constrained. For example, cultured Prochlorococcus clones do not grow on NO3-, but these cultured clones may only represent 10 - 15% of the natural population variance resulting in a biased biogeochemical role. We report NO3-, NO2-, NH4+ and urea uptake rates for flow-cytometrically sorted Sargasso Sea Prochlorococcus populations. Reduced nitrogen substrates accounted for most, 90 - 95%, of the measured nitrogen uptake, but these populations also directly assimilate a significant fraction of NO3-, 5 - 10%; a finding in stark contrast to conclusions drawn from culture studies. The observed population-specific NO3- uptake rates compare favorably with both net Prochlorococcus population growth rates and diapycnal NO3- fluxes. We hypothesize that while reduced nitrogen supports overall high growth rates, balancing high grazing mortality, the net seasonal Prochlorococcus population growth is supported by NO3- assimilation and that Prochlorococcus contributes to new production in the oligotrophic ocean.