CONICET | Buscador de Institutos y Recursos Humanos

We examined the effect of light attenuation, due to floating plants, on the community structure of the main phagotrophic protists and their grazing rates in a wetland in the Lower Paraná Basin. Ingestion experiments (winter and summer) were conducted at 2 sites in the same shallow lake that had contrasting light scenarios: open waters (light) and under profuse macro- phyte coverage (dark: light attenuation ~97%). We compared the rates at which protists ingested 3 types of tracer prey: fluorescently labelled heterotrophic bacteria (FLB), picocyanobacteria (FLC) and picoeukaryotic algae (FLA). Light influenced both the structure of the microbial com- munities and the protistan grazing rates. Heterotrophic flagellates (HF) were more abundant under the macrophytes, whereas mixotrophic algae (cryptophytes) and autotrophic and heterotro- phic picoplankton populations attained higher abundances in open waters. Specific grazing rates (SGRs) of mixotrophs on heterotrophic bacteria (HB) were higher in the light (7.9 to 15.5 prey cells grazer−1h−1), than in darkness (0.1 to 5.1 prey cells grazer−1h−1); the same trend was observed on picocyanobacteria (Pcy) (1.1 and 0.2 prey cells grazer−1h−1, light and dark). SGRs of HF were 1.0 to 7.3 cells grazer−1h−1(on HB) and 0.01 to 1.8 prey cells grazer−1h−1(on Pcy), with highest values in summer and no pattern in relation to light. SGRs of ciliates were higher in summer and in dark- ness. Clearance rates (CR) on Pcy were higher than on HB, for both HF and mixotrophic algae. In winter, cryptophytes contributed up to 93% of the microbial grazing in the light, whereas HF were more important in darkness; in summer, bacterivory was dominated by heterotrophs in both light scenarios. Our experimental results highlight the importance of light conditions in structuring bacterial grazing by protists.