JGOFS: Bacterial Cycling Of Dissolved And Particulate Organic Carbon In Oligotrophic And Eutrophic Production Regimes Of The Arabian Sea

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9312695 Azam This is a joint research proposal (F. Azam and H. Ducklow, co-PIs) 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 US JGOFS Arabian Sea Process Study. We propose a program of core measurements to quantify carbon and nitrogen pools and fluxes as well as mechanistic studies to understand the response of the bacteria-mediated processes to dramatic changes in many primary production that occur regularly due to monsoonal reversals in the Arabian Sea. The JGOFS core measurements will include bacterial abundance and biomass, bacterial production (by both 3H thymidine and 3H leucine incorporation methods), DOC, POC and PN. Our research is design, further, 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? 2) Does the decomposition of sinking particles slow down in the suboxic zone and result in enhanced vertical flux though this layer? We 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. We predict that the strong dominance of the microbial loop renders the oligotrophic period net- heterotrphic 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 hyothesis we wish to test is that sinking particles in the surface waters are rapidly solubilized by hydrolytic ectoenzymes of colonizing bacteria but t he enzymatic solubilization is greatly reduced in the suboxic layer resulting in much slower depth-dissipation of POC.