Differential effect of algal- and soil-derived dissolved organic matter on alpine lake bacterial community composition and activity

We studied the effects of algal- and soil-derived dissolved organic matter (DOM) enrichments on the activity and community composition of bacterioplankton in an alpine lake. The effects of the DOM source on bacteria were tested by establishing dilution cultures amended with either an algal lysate or with a soil extract. Cultures were incubated for 6 days under close to in situ conditions, and changes in bacterial community composition and activity were tracked by micro-autoradiography ([(3)H]-L-leucine) combined with fluorescent in situ hybridization and signal amplification by catalyzed reporter deposition. Heterotrophic bacterial production increased by 25-fold after the algal lysate addition. After 3 days, up to 90% of the bacteria were active in this treatment, and beta-Proteobacteria, particularly the subgroup R-BT, represented 86% and 67% of the total bacterial counts, respectively. From day 3 onward, the percentage of active cells in this treatment decreased dramatically as did the relative abundance of subgroup R-BT. By the end of the experiment, beta-Proteobacteria still dominated bacterial abundance (80%), but active cells were only 25% of total bacterial counts. In contrast, the addition of soil-derived DOM led only to a two- to three-fold increase in bacterial production. beta-Proteobacteria was still the dominant group (50-60% of total bacterial counts and 50% of the cells positive for leucine incorporation), but Actinobacteria made a substantial contribution (15-23%). This pattern contrasted with that observed in the treatment receiving the algal lysate, where the relative abundance of the latter group rapidly decreased. In the treatment amended with algal-derived DOM, bacterial carbon production matched the observed decrease in dissolved organic carbon concentration. However, bacterial carbon produced on soil-derived DOM accounted only for 30% of the decrease in dissolved organic carbon concentration, suggesting a more inefficient utilization of this material. The expected climate-driven changes in DOM supply to alpine lakes will affect their bacterial community structure and activity.