Grazing rates of protists in wetlands under contrasting light conditions due to floating plants

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 Parana 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 macrophyte coverage (dark: light attenuation similar to 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 communities and the protistan grazing rates. Heterotrophic flagellates (HF) were more abundant under the macrophytes, whereas mixotrophic algae (cryptophytes) and autotrophic and heterotrophic 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 super(-1 h) super(-)1), than in darkness (0.1 to 5.1 prey cells grazer super(-1 h) super(-)1); the same trend was observed on picocyanobacteria (Pcy) (1.1 and 0.2 prey cells grazer super(-1 h) super(-)1, light and dark). SGRs of HF were 1.0 to 7.3 cells grazer super(-1 h) super(-)1 (on HB) and 0.01 to 1.8 prey cells grazer super(-1 h) super(-)1 (on Pcy), with highest values in summer and no pattern in relation to light. SGRs of ciliates were higher in summer and in darkness. 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.