Research

Sargasso Sea Phosphorus Cycle

The Sargasso Sea seasonal cycle is characterized by an increase in suspended particulate P, as part of the winter/spring bloom, and a commensurate decrease in SRP inventories. There is a similar increase in export fluxes of particulate P that can only be balanced by the consumption of DOP. This seasonal pattern is different than that of the HOT time-series program in the subtropical Pacific Ocean.

(LEFT) Seasonal cycle of particulate phosphorus, SRP, and DOP inventories and TPP fluxes (Lomas et al. unpubl. data.).

Project Rationale:

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, indeed 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. We 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. Moreover, we hypothesize that inherent physiological differences among microbial taxa represents a significant source of temporal and spatial variability in DOP utilization rates that is yet neither understood nor constrained.

Making use of a new suite of proven taxon-specific methodologies our specific research objectives are as follows:

1. To quantify temporal and spatial variability in DOP hydrolysis in the Sargasso Sea with measures of whole-community and taxon-specific alkaline phosphatase activity.

2. To quantify temporal and spatial variability in taxon-specific SRP and DOP uptake rates by combining flow cytometry and radioisotope methodologies.

3. To quantify whole-community total P uptake rates through BAP (biologically available phosphorus) assays, as well as SRP and model compound DOP uptake and regeneration rates.

4. To identify factors regulating rates of DOP hydrolysis and assimilation using experimental nutrient manipulations, and to evaluate the role of DOP in supporting primary production in the Sargasso Sea.

Dissolved Organic Phosphorus Cycling in the Sargasso Sea.

As mentioned above, there is evidence that the dissolved organic phosphorus pool is a significant player in the Sargasso Sea biogeochemical system. In the past year, we have begun intensive studies of the into the cycling of the DOP pool. These studies include analysis of bulk Alkaline Phosphatase Activity (APA), single-cell APA labeling using ELF and quantification by flow cytometry, genetic studies conducted by Sonya Dyhrman's group, and single-cell uptake of radiolabelled organic phosphorus compounds. Below are some of the results from those activities.

(LEFT) Images (A-D) and results (E,F) of single-cell AP measurements. A) comparison of Trichodesmium under P-replete and P-deplete culture conditions with the P-deplete trichome displaying AP activity (green fluorescence); B) Trichodesmium collected from the Sargasso Sea displaying AP activity; C) mixed assemblage of Trichodesmium and Plectonema sp. with only Plectonema sp.displaying AP activity; D) Ceratium sp. displaying AP activity; E) depth profile of AP activity (expressed as % of total cells of each type enumerated) in several phytoplankton groups during the summer period (June/July); F) same as in E, but for the fall period (October/November). Panels A and C reproduced from Dyhrman et al. 2002 and panel B is from Dyhrman unpubl. data. Panels D-F reproduced from Lomas et al. 2004.

Consistent with the above observations of a 'shortage' of available phosphorus in the euphotic zone of the Sargasso Sea, we have tested the hypothesis that net phytoplankton growth rates are phosphorus limited. During 2006, we conducted phosphorus addition bioassays on cruises in May (left panel below) and November (right panel below). Experiments in May suggested the phytoplankton at BATS were co-limited by the availailability of nitrogen and phosphorus, as that was the only treatment where a biomass response was observed. In November, the resident phytoplankton were definitely limited by phosphorus as all of the phosphorus treatments (phosphorus alone, nitrogen and phosphorus, and ATP) all resulted in substantial increases in chlorophyll biomass. These are the first results to suggest a seasonality in the limiting nutrient in the Sargasso Sea.

PI's on this research:

Dr. Sonya Dyhrman at Woods Hole Oceanographic Institution

Dr. Jim Ammerman at the Institute of Marine and Coastal Sciences at Rutgers University

Peer Reviewed Publications Resulting from this Research:

Lomas, M.W., Swain, A. Shelton, R, and J.W. Ammerman. 2004. Taxonomic diversity of phosphorus limitation in the Sargasso Sea. Limnology and Oceanography, 49:2303-2310 (download PDF)

Salihoglu, B., Garcon, V., Oschlies, A., and Lomas. M.W. 2007. Influence of nutrient utilization and remineralization stoichiometry on phytolankton species and carbon export: a modeling study at BATS. Deep-Sea Research I, doi:10.1016/j.dsr.2007.09.010 (download PDF)

Abstracts at Aquatic Sciences Meeting, Santa Fe 2007:

Burke, A.L., Lomas, M.W., Ammerman, J.W., Dyhrman, S.D. DOES ASSIMILATION OF DISSOLVED ORGANIC PHOSPHORUS SUPPORT PARTICULATE PHOSPHORUS EXPORT FLUES IN THE SARGASSO SEA? (download Word document)

        Orchard, E.D., Ammerman, J.W., Benitez-Nelson, C.R., Lomas, M.W., Dyhrman, S.D. POLYPHOSPHATE                                  METABOLISM IN THE MARINE CYANOBACTERIA TRICHODESMIUM AND CROCOSPHAERA.
    (download Word document)

'Popular' Articles Resulting from this Research:

Research Awards Supporting this Research:

   This research is funded by NSF Award OCE - 0453023.

Other Relevant Research Links:

    Cycling of Phosphorus in the Mediterranean Project

We have begun quantifying rates of ELF-97 activity using flow cytometry, where we can track the green intensity of individual cells over time (Right Figure). This data is from a culture of the marine diatom Chaetoceros neo-gracile. The Blue Curve is the 'green-ness' after ~1minute, the Red Curve is ~5minutes, the Green Curve is ~10minutes, and the Purple Curve is ~20minutes. By plotting the mean fluorescent intensity over time, and keeping the flow cytomter conditions the same, we can directly compare phosphatase activities between different phytoplankton species and between different sample locations.

This is very new work, please stop back in the future to see some new results.

(ABOVE) Monthly time-series of bulk APA at the BATS site in the Sargasso Sea. Note the particularly bulk APA in the late summer of 2006 when a significant eddy feature was impinging upon the BATS site.

(RIGHT) During May 2006 we conducted a transect cruise between the BATS site and Puerto Rico to determine biogeochemical properties and rates of alkaline phosphatase activity. Note the coincidence of the deep chlorophyll maximum (upper panel), the phosphacline (middle panel) and the depth of maximum alkaline phosphatase activity (lower panel).