Effects of Protozoan Predators on Aquatic Microbial Assemblages in Sarracenia Purpurea Leaves

The importance of predators in influencing community structure is a well-studied area of ecology. However, few studies apply ecological hypotheses of predation when studying multi-predator systems, and even fewer apply these theories to microbial communities. The phytotelma community found within the leaves of the purple pitcher plant, Sarrecenia purpurea, exhibits a simple trophic structure that includes the micro-scale interactions of protozoan predators and microbial prey, making this microcosm ideal for studying how predator diversity and composition affects microbial community structure. Replicate inquiline microbial communities were maintained for seven days with five protozoan species (four ciliates and one flagellate) in a factorial design. We sought to determine how predator diversity (up to four species) affects prey community diversity, whether different predators target distinct microorganisms, and how interactions among protozoans affect resource (microorganism) use. In so doing, we sought to determine if predators followed ecological patterns of keystone predation or generalist predation. Microbial community structure was analyzed by 16S rRNA gene sequencing (iTag). The data revealed that Archaea were rare, with Proteobacteria and Bacteriodetes dominating regardless of treatment. Of the different protozoan species, two ciliates and one flagellate distinctly altered their communities as compared to one another (p<0.05). The ciliates followed patterns of keystone predation by increasing microbial evenness, while the flagellate left a community statistically similar to the control and followed generalist patterns. When in pairwise treatments, remaining prey communities more closely resembled those of the respective ciliate than that of the flagellate (as compared to these communities in monoculture). The relative abundance of one bacterial Operational Taxonomic Unit (OTU) classified as an Aquitalea sp. decreased from the control when in the presence these two ciliates. This OTU is 98% similar to an Aquitalea sp. associated with chitin degradation, an important function for this plant's fitness. Associated predicted metabolic functions revealed a positive correlation with the relative abundance of this OTU and chitinase, as well as an inverse correlation with community evenness. Taken together, the data demonstrated that specific predator behaviors may have a greater affect on prey diversity and relative abundances than overall predator diversity. Based on metabolic predictions, these trends could have an affect on microbial community function with consequences that reverberate throughout the trophic structure.