Abstract

We performed a transplant experiment in eutrophic Sau reservoir to assess the factors that control bacterial abundance, activity, growth rate, and community composition. Samples from the lacustrine and the riverine ends of the reservoir were incubated in dialysis bags placed in situ and transplanted to the other side of the reservoir and also incubated after 1 µm filtration to measure predator effects. The bags were sampled at 12-h intervals to estimate bacterial abundance, whole community activity, activity structure (by flow cytometry), and phylogenetic composition (by in situ hybridization with group-specific phylogenetic probes). Bacterial production was always regulated by nutrient supply, but abundance and activity were differently regulated at both sites. The riverine bacteria were limited by predator activity, whereas the lacustrine were regulated by a combination of predation and nutrient supply. Therefore, even in the same environment, different modes of control can act simultaneously. Bacterial activity structure was also regulated in the same way. Abundance of high DNA bacteria and cells hybridizing with the universal EUB338 probe were well correlated. In the lacustrine sample, bacterial community structure did not change significantly, whereas in the riverine sample, Α- and Δ-Proteobacteria reduced their growth when transplanted, whereas Β-Proteobacteria were stimulated by the presence of predators. Members of the Cytophaga/Flavobacterium phylum grew only when incubated in situ in the absence of predators. This different behavior in the different bacterial groups resulted in strong changes in bacterial assemblage composition, evident already after 24 h. The experiment demonstrates that, together with the effect of predators, nutrient supply affects bacterial community properties and that a complex regulation involving both types of control can occur in a single heterogeneous planktonic system.

Josep M. Gasol

Staff scientist