Bacterial stimulation in mixed cultures of bacteria and organic carbon from river and lake waters

Interactions between natural bacterial assemblages and dissolved organic ca rbon (DOC) were investigated in two complementary batch experiments. In the first, a positive relationship was found between the proportion of electro n transport system (ETS) active bacteria and the diversity of DOC in microc osms enriched with an increasing number of organic substrates. In a second experiment, bacterial and nutrient dynamics were measured in microcosms wit h natural bacterial populations and organic matter from rivers and lakes of different trophic levels. The interactions between the bacterial assemblag es and DOC from different sources was investigated using source systems (ri vers or lakes) and blended (different proportions of river and lake water) batch cultures. In each experiment, the number of total and ETS-active bact eria, the fluorescein diacetate (FDA)-hydrolytic activity, and the total (D OC), biodegradable (BDOC) and refractory (RDOC) dissolved organic carbon we re measured four times during 5 days. The results suggested that the temper ature, more than trophic level, controlled planktonic bacterial production. Furthermore, bacterial activity was stimulated in micrososms where river a nd lake waters were mixed. For the oligotrophic microcosms, this observatio n can be explained by a greater diversity of the organic nutrients ("qualit ative" stimulation of bacteria), whereas for the meso-eutrophic microcosms, the production of new pools of dissolved organic carbon (both biodegradabl e and total) could account for the observed "quantitative" stimulation of t he bacteria. These experiments suggest that the mixing of bacteria and orga nic matter from two different systems can give rise to novel nutrient and b acterial dynamics that are likely similar to those that occur in river-lake ecotones.