Microbial and geochemical mechanisms controlling dissolution processes of particulate organic matter in the oceans

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Microbes play the major role in the regulation of material cycling in the oceans, but the mechanisms by which material fluxes mediated by bacteria are controlled have yet to be clarified fully. Especially our knowledge is limited regarding large scale patterns and controls of organic matter - bacteria interactions in oceanic water columns. In order to better understand dynamic features of the oceanic biogeochemical cycles at a large scale, this study examined patters in distributions of microbes and organic matter transformations and decomposition in the surface and deep water columns of the South and North Pacific. Our data revealed that bacterial production is largely coupled with the flux of sinking particulate organic carbon in the mesopelagic zone (100 - 1,000 m), supporting the hypothesis that particulate organic carbon -> dissolved organic carbon -> free-living prokaryote pathway is important in the regulation of the biological pump. However the coupling between the sinking fluxes of particulate organic carbon and prokaryotic activity was weak in the bathypelagic layer (>1,000 m), suggesting that the lateral transport or resuspension of organic matter from the sediment is important in that layer. We also investigated the dynamics of bacterial communities and microbial activities during the algal blooms. On the basis of the results obtained by mesocosm experiments, we concluded that filamentous bacteria affiliated with the Cytophaga-Flavobacter cluster play an important role in particulate organic matter decomposition during the bloom. Finally, we constructed a new mathematical model to examine dynamic behavior of bacterial communities during the decomposition of particulate and dissolved organic matter in the oceans. The above studies collectively revealed novel features in the organic matter- microbe interactions in the oceans.