Bacterioplankton responses to bottom-up and top-down controls in a West African reservoir (Selingue, Mali)

We conducted experiments to determine whether bacterial growth, with or without predators, is limited by inorganic (N and P) and organic (C) substrates in Selingue, a mesotrophic reservoir located in Mali, West Africa. Significant increases (relative to controls) in bacterial cell volumes and thymidine incorporation rates were observed after 24 h incubation only for samples amended with the combination +PC and +CNP. The data revealed a colimitation of bacterioplankton growth by organic carbon during the dry season. Flow cytometry discriminated 3 groups of bacteria (Bact I, Bact II, Bact III) differing in increases in nucleic acid content and cell size. The Bact I group, comprising cells with low nucleic acid content and of small size, was the dominant population in all experiments. In the absence of bacterial predators, only the Bact II group showed significant differences between the control and the +PC and +CNP treatments, indicating that this bacterial group was the most sensitive to nutrient additions. The Bact II group, corresponding to cells with high DNA content, are active members of the bacterioplankton community. The Bact III group did not increase in any treatment but the proportions of these cells always increased in the presence of bacterial predators. These cells may be, at least partly, grazing - resistant bacteria.