Collaborative Research: JGOFS: Bacterial Cycling of Dissolved and Particulate Carbon in the Oligotrophic and Eutrophic Production Regimes of the Arabian Sea

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9500601 Ducklow This is a joint research project by Drs. F Azam H. Ducklow to perform quantitative studies on the significance of bacteria in mediating and regulating biogeochemical fluxes of carbon and nitrogen in the Arabian Sea, as a component of the USJGOFS Arabian Sea Process Study. These scientists will make core measurements used by bacteria to respond to dramatic changes in primary production that occurs regularly due to monsoonal reversals in the Arabian Sea. The JGOFS core measurement s will include bacterial abundance and biomass, bacteria production (by both 3H thymidine and 3H leucine incorporation methods), DOC, POC and PON. The research is designed to address two key questions concerning microbial heterotrophic processes identified by US JGOFS: 1) How do the community structures which occur during the oligotrophic vs. eutrophic periods differentially impact carbon storage and export fluxes? and 2) Does the decomposition of sinking particles slow down in the suboxic zone and result in enhanced vertical flux through this layer? The investigators hypothesize that sequential occurrence of highly eutrophic and oligotrophic productivity regimes creates a uniquely robust microbial loop during the oligotrophic period at the expense of slow- to-degrade DOM which accumulates during the eutrophic period. They predict that the strong dominance of the microbial loop renders the oligotrophic period net- heterotrophic and this has implications for spatial and temporal patterns of carbon oxidation and exchange. The 'stored' DOC may also support high bacterial carbon demand (higher than would be supported by particle flux) below the mixed layer following shoaling of the pycnocline. A second hypothesis they wish to test is that sinking particles in the surface waters are rapidly solubilized by hydrolytic exoenzymes of colonizing bacteria but the enzymatic solubilization is greatly reduced in the suboxic layer resulting in much slower depth dissipat ion of POC.