Nutrient dynamics in the North Pacific subtropical gyre : phosphorus fluxes in the upper oligotrophic ocean

Abstract: In the contemporary North Pacific subtropical gyre (NPSG) the extant microbial community is based on prokaryotic cells with the cyanobacterium Prochlorococcus dominating the phototrophic component. The prevalence of dinitrogen (N2) fixing organisms, such as Trichodesmium spp., appears to have relieved the ecosystem from its historically perceived nitrogen (N) limited state. It is thus conceivable that phosphorus (P) or trace elements such as iron (Fe) may be important factors in the regulation of primary and secondary productivity in these nutrient impoverished habitats. In a system under P control, determining the underlying mechanisms driving the P flux among its different compartments will be important in predicting ecosystem productivity and food web interactions.The papers presented herein were aimed at addressing dissolved organic matter (DOM) dynamics with specific objectives to investigate the P pool fluxes in the NPSG. These included assessment of P uptake and turnover rates, estimation of the bioavailable P (BAP) pool concentrations, dissolved organic phosphorus (DOP) and dissolved organic carbon (DOC) production, bioavailability of selected P compounds to natural assemblages of microorganisms and the role of dissolved nucleotides in the oligotrophic marine environment.At Station ALOHA the production of DOC could be nearly 50% of the net CO2 fixed into biomass. DOP production ranged from 0.6-2.5 nM d-1, equivalent to 10-40% of net P incorporation. Uptake rates and turnover times of P were approximately an order of magnitude higher in the coastal region compared to the open ocean habitats at about 40-80 nM P d-1, and 0.8-2.5 d respectively. The size of the BAP pool frequently exceeded the soluble reactive phosphorus (SRP) pool sometimes by a factor of two. There was a pronounced difference in microbial preference for the known P substrates tested. Although orthophosphate (Pi) was always the most readily available substrate nucleotides had the highest bioavailability of the P compounds tested. A large but variable percentage of P from the tested compounds accumulated outside the cells as SRP, indicating an efficient regeneration of Pi from some substrates.Dissolved and particulate adenosine triphosphate (ATP) and guanosine triphosphate (GTP) pools were dynamic and varied on temporal and spatial scales. The dissolved pools were frequently larger than their corresponding particulate fractions. Dissolved ATP uptake and production rates appeared to be in balance and the estimated turnover time for this DOP pool constituent, 1-2 days. Considering the high bioavailability and Pi regeneration from nucleotides, the potentially large dissolved nucleotide and nucleic acid pool, and a high P flux through ATP relative to bulk DOP, it is plausible that a substantial part of the P flux in the upper oligotrophic ocean revolves around the nucleic acid pool and its derivatives.