Phytoplankton taxon-specific orthophosphate (Pi) and ATP utilization in the western subtropical North Atlantic

Utilization rates of inorganic and organic phosphorus by different picophytoplankton in the oligotrophic ocean are not well quantified. We used radioisotope tracers of orthophosphate (Pi) and the nucleic acid adenosine 5'triphosphate (ATP) to quantify P utilization into flow cytometrically sorted groups of picophytoplankton during the summer and fall of 2007 in the western Sargasso Sea. Dissolved organic phosphorus (DOP) dominated the dissolved P pool (mean +/- SD 71 +/- 56%), while soluble reactive phosphorus (SRP) concentrations were consistently <5 nmol l(-1). All of the groups studied assimilated Pi (rho(Pi)) and ATP (rho*(ATP)) at significant rates. In addition, rho(Pi) increased with ambient SRP concentrations, while ambient DOP concentrations had no apparent effect on either rho(Pi) or p*(ATP). Consistent with community composition and contributions to autotrophic biovolume, prokaryotes were primarily responsible for Pi and ATP turnover. In regions where SRP was depleted to <3 nmol l(-1), ATP accounted for > 70% of the total P utilized. Among the individual taxa, rho(Pi) and p*(ATP) increased in the order Prochlorococcus, Synechococcus, pico-, and nanoeukaryotes, when uptake was normalized to cell number, but the opposite relationship was observed when normalized to cell volume. This suggests that cyanobacteria are physiologically superior to the larger eukaryotes with respect to scavenging both Pi and ATP in the oligotrophic Sargasso Sea. A comparison of estimated C:P utilization rates with particulate C:P ratios suggests that different phytoplankton groups may be experiencing different degrees of P stress in the same ambient nutrient environment. Collectively, these data suggest that the labile DOP pool, assuming that ATP is a reasonable proxy for the labile DOP pool, in the Sargasso Sea may constrain primary productivity in the absence of sufficient SRP, and that cyanobacteria have a physiological advantage for P utilization under these conditions.