Microbial degradation of organic carbon and nitrogen on diatom aggregates

The major pathways of transformation of particulate organic matter by heterotrophic bacteria are respiration and production of new biomass. Until today only a limited number of studies have measured simultaneously respiration and production by aggregate-associated bacteria. To study their role in the carbon cycle of aquatic systems we have formed model particles from diatoms (Skeletonema costatum, Thalasiosira weissflogii, Chaetoceros debilis) in roller tanks filled with natural seawater from Øresund, Denmark. Changes in bacterial community structure were analyzed by in situ hybridization and revealed members of the Cytophaga/Flavobacterium cluster and of the g subclass of Proteobacteria to be the main actors. The combination of radiotracer and microsensor techniques allowed determination of bacterial protein production and community respiration on the same aggregate and hence the apparent growth efficiency. Apparent growth efficiency (bacterial production/[bacterial production 1 community respiration]) was 0.50 6 0.03 (se) on 1.5–2.5 d old aggregates and independent of bacterial growth rate. The initial carbonspecific bacterial production and community respiration was 0.082 d21 and 0.084 d21, respectively. Thereafter, the carbon-specific bacterial production decreased to 0.020 d21, whereas specific community respiration decreased to 0.057 d21. Hence, the apparent net growth efficiency decreased, partly as a result of grazing by protozoa, and it was much lower (0.23 6 0.04) at the end of incubation. Bacterial production was best correlated to particulate amino acids, whereas community respiration was best correlated to particulate organic carbon (POC). Protease activity was correlated to bacterial production and particulate combined amino acid content, whereas b-glucosidase activity was better correlated to POC and community respiration than to particulate combined amino acid content. Turnover times of radiolabeled amino acids increased from 17.8 to 1,190 h during incubation and were tightly coupled to particulate combined amino acids and POC. Eighty-seven percent of the decrease in particulate organic nitrogen (PON) over time could be explained by turnover of particulate combined amino acids by aggregateassociated food web. Thus, transformation and remineralization of freshly produced particulate organic matter by aggregate-attached food web is significant and the vertical flux of particulate organic matter in the ocean is highly reduced during sedimentation. Marine and lake snow (larger than 0.5 and 0.3 mm in diameter, respectively) are characterized as microzones highly enriched with nutrients (Shanks and Trent 1980; Grossart and Simon 1993). High concentrations of nutrients and heterotrophic flagellates and ciliates on these aggregates (Silver and Alldredge 1981; Caron et al. 1982) suggest high activities of aggregate-associated microbes. Whereas measured enzymatic hydrolysis rates are high, those of bacterial production are relatively low (Simon et al. 1990; Smith et al. 1992). This notion implies that large fractions of particulate organic matter are released into the surrounding water and supply free-living bacteria with dissolved organic matter and nutrients (Cho and Azam 1988; Smith et al. 1992; Grossart 1 Corresponding author ([email protected]).