DOP Utilization in the Sargasso Sea: Quantifying Taxon-specific Rates of Hydrolysis and Uptake

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Photosynthetic uptake of CO2 by oceanic phytoplankton and the export of the resulting organic carbon to the deep sea comprise a "biological pump" capable of extracting globally significant amounts of CO2 from the atmosphere. Mounting evidence suggests that primary production in two of the larger subtropical ocean gyres, the Western Tropical/Subtropical Atlantic (Sargasso Sea) and the North Pacific Subtropical Gyre, may be controlled by phosphorus availability. There are vanishingly low inorganic phosphorus (SRP) concentrations, sub-nanomolar in some locales, and ratios of inorganic N:P greatly exceed the canonical Redfield Ratio in these environments. In these low SRP regions dissolved organic phosphorus (DOP) may help meet biological phosphorus demand: data collected in the Sargasso Sea shows a 30% decline in DOP inventories during summer stratification. The hydrolysis and assimilation of DOP by primary producers is likely dependent on phytoplankton physiology, and highly variable between taxa, and through space and time. The investigators hypothesize that despite rapid turnover times, chronically low and seasonally invariant SRP concentrations at BATS cannot support measured rates of primary production without utilization of additional P from the DOP pool. Furthermore, inherent physiological differences among microbial taxa represent a significant source of temporal and spatial variability in DOP utilization rates that is yet neither understood nor constrained. The PIs propose to use proven taxon-specific methodologies to: quantify temporal and spatial variability in DOP hydrolysis in the Sargasso Sea; quantify temporal and spatial variability in taxon-specific SRP and DOP uptake rates; quantify whole-community total P uptake rates as well as SRP and model compound DOP uptake and regeneration rates; identify factors regulating rates of DOP hydrolysis and assimilation; and evaluate the role of DOP in supporting primary production in the Sargasso Sea. An understanding of ocean ecosystem function is important on a broad scale. Photosynthetic uptake of CO2 by oceanic phytoplankton and the export of the resulting organic carbon to the deep ocean are a "biological pump" that removes globally significant amounts of CO2 from the atmosphere. This project will help to constrain predictions of the strength of the oceanic biological pump in the Sargasso Sea. If dissolved organic phosphorus supports a significant fraction of primary production in the Sargasso Sea, then diversity in biological metabolic processes in the central oceans plays a greater role in the global carbon cycle than is presently recognized. The investigators will sponsor a minimum of three undergraduate researchers each year, as well as two graduate students. Data generated from the project will be used in problem-based learning modules for courses taught by the investigators, and the curriculum will be submitted to an appropriate digital repository such as www.dlese.org.