Light intensity influences testate amoeba communities on floating Sphagnum peatlands
Testate amoebae are key microbial consumers on peatlands, feeding on a broad range of food sources including bacteria, algae, fungi, ciliates, and nematodes. Several common species are also mixotrophic, combining heterotrophy with autotrophy using endosymbiotic zoochlorellae. However, our knowledge of how resource variability influences the relative abundance of mixotrophic and heterotrophic species is extremely limited, although presumably factors such as shading, cloudiness, and the abundance of prey affect competitive outcomes. Given their importance to peatland microbial communities, changes in the abundance of testate amoeba species and functional groups have potentially broad implications for microbial community composition and carbon fluxes in carbon-rich peatlands. Testate amoebae are also commonly used as paleoecological indicators, so understanding the effects of light variability on community composition would aid the interpretation of paleorecords, and with appropriate calibration and site selection may provide a new tool to reconstruct past cloudiness. To assess the potential role of light variability in structuring testate amoeba communities, we used shade cloth mounted to wooden frames to experimentally shade 78 plots in northern Wisconsin during the 2014 growing season. We hypothesized that shading would result in a decrease in the relative abundance of mixotrophic species. Sites were established on floating Sphagnum peatlands because seasonal surface moisture is highly stable on these systems. Ambient light was variably reduced in the experimental plots (4-50% light reduction), and eighteen control plots with no light reduction were included for comparison. Testate amoeba community composition was quantified in May before the start of the experiment and in October after the growing season.
Moderate and high-shading treatments (i.e., 30-50% light reduction) resulted in significant changes in the relative abundance of mixotrophic and heterotrophic species (p<0.01), whereas no significant changes were observed in the controls or low-shade treatments (i.e., 4-22% light reduction). However, our hypothesis was not supported. Surprisingly, the relative abundance of mixotrophic species increased an average of 18% in the high-shading treatments. The common mixotrophic species Hyalosphenia papilio, Heleopera sphagni, and Amphitrema flavum collectively contributed to this increase. Causes of the patterns are unclear, but may be attributable to photoinhibition of mixotrophic species at higher light levels, or vertical migration of mixotrophic species to higher positions on the Sphagnum moss stems in shaded environments. More research is needed on microbial food web structure and the effects of light variability on species interactions before implications for peatland carbon fluxes or applications to paleoecology can be fully assessed.