Organic Content and Bacterial Metabolism in Amorphous Aggregations of the Northern Adriatic Sea

The "successional stage" of marine aggregates from formation to final decay was investigated in the northern Adriatic Sea in summer 1991. In marine snow, bacterial biomass and production rates increased exponentially and in gellike aggregates (GEA-considered as late-stage marine snow) reached values 2-3 orders of magnitude higher than in ambient water. Specific growth rates of marine snow-associated bacteria $(0.31-1.76 d^-1)$ were similar to ambient water bacteria; however, they were significantly lower in GEA $(0.08-0.31 d^-1)$. Concentration of dissolved free amino acids (DFAA) in aggregates were up to $80\mu M$. Despite these high concentrations of DFAA, the organic C:N ratio increased significantly over the 3 months of the investigation period, ranging from 6.7 to 17.3 and indicating that refractory substances accumulate in aging aggregates. This notion is also supported by the ratio of $\alpha-glucosidase$ activity to $\beta-glucosidase$ activity, which declined from ambient water $(\alpha-gluc:\beta-gluc=1.7)$ to marine snow and GEA $(\alpha-gluc: \beta-gluc =0.51)$. Extracellular enzymatic activity calculated on a per cell basis was similar for ambient water and marine snow-attached bacteria, but significantly lower rates were obtained for GEA. These data indicate that GEA become an increasingly poor habitat for bacterial growth. The relation between hydrolysis rate and substrate uptake does not support the hypothesis of a loose coupling between hydrolysis and uptake in marine snow-attached bacteria.