Consumption of dissolved amino acids and carbohydrates by limnetic bacterioplankton according to molecular weight fractions and proportions bound to humic matter

We investigated the bacterial decomposition of dissolved amino acids (DAA) and carbohydrates (DCHO) bound to humic substances and in the nonhumic fraction, and of low (< 3 kDa) and high (> 3 kDa) molecular weight (MW). Experiments were conducted in mesotrophic Lake Constance, Germany, in October and November 1997 and June 1998 during periods of low phytoplankton biomass. Dilution cultures, inoculated with bacterioplankton of the < 1 uri size fraction from 3 in, were run over 5 to 8 days to determine bacterial consumption of the various fractions of DAA and DCHO. In all experiments consumption rates of DCHO were higher than that of DAA. On average, 70% of the DAA and DCHO consumed were associated with the humic fraction, which, however, also consisted of a substantial recalcitrant component. In contrast, there was no preferential consumption of DAA and DCHO in either of the two MW fractions. Overall, we found a highly significant positive correlation between the consumption of the various fractions of DAA and DCHO and their initial concentrations. The consumed DAA, i.e, their labile pool, was dominated by serine and glutamate and that of DCHO by arabinose. In contrast, the residual DAA, i.e., the recalcitrant pool, comprised highest proportions of glycine + threonine and that of DCHO of mannose + xylose. These results indicate that the bacterial consumption of DAA and DCHO was mainly a function of their concentrations, irrespective of the molecular weight and whether they are bound to humic substances or not and despite the fact that they consist of a more labile and a more refractory pool.