Microbial degradation of organic carbon and nitrogen on diatom aggregates

The major pathways of transformation of particulate organic matter by heter otrophic bacteria are respiration and production of new biomass. Until toda y only a limited number of studies have measured simultaneously respiration and production by aggregate-associated bacteria. To study their role in th e carbon cycle of aquatic systems we have formed model particles from diato ms (Skeletonema costatum, Thalasiosira weissflogii, Chaetoceros debilis) in roller tanks filled with natural seawater from Oresund, Denmark. Changes i n bacterial community structure were analyzed by in situ hybridization and revealed members of the Cytophaga/Flavobacterium cluster and of the gamma s ubclass of Proteobacteria to be the main actors. The combination of radiotr acer and microsensor techniques allowed determination of bacterial protein production and community respiration on the same aggregate and hence the ap parent growth efficiency. Apparent growth efficiency (bacterial production/ [bacterial production + community respiration]) was 0.50 +/- 0.03 (se) on 1 .5-2.5 d old aggregates and independent of bacterial growth rate. The initi al carbon-specific bacterial production and community respiration was 0.082 d(-1) and 0.084 d(-1), respectively. Thereafter, the carbon-specific bacte rial production decreased to 0.020 d(-1), whereas specific community respir ation decreased to 0.057 d(-1). Hence, the apparent net growth efficiency d ecreased, partly as a result of grazing by protozoa, and it was much lower (0.23 +/- 0.04) at the end of incubation. Bacterial production was best cor related to particulate amino acids, whereas community respiration was best correlated to particulate organic carbon (POC). Protease activity was corre lated to bacterial production and particulate combined amino acid content, whereas P-glucosidase activity was better correlated to POC and community r espiration than to particulate combined amino acid content. Turnover times of radiolabeled amino acids increased from 17.8 to 1,190 h during incubatio n and were tightly coupled to particulate combined amino acids and POC. Eig hty-seven percent of the decrease in particulate organic nitrogen (PON) ove r time could be explained by turnover of particulate combined amino acids b y aggregate associated food web. Thus, transformation and remineralization of freshly produced particulate organic matter by aggregate-attached food w eb is significant and the vertical flux of particulate organic matter in th e ocean is highly reduced during sedimentation.