Investigating exudate- and habitat-mediated effects of phytoplankton on lake bacterial community dynamics

Correlated patterns of abundance, activity, and composition are commonly observed between phytoplankton and bacterial communities. There are a number of potential explanations for correlated dynamics. Recent observations of temporal succession in lake microbial communities provide evidence for phytoplankton populations acting as biological drivers structuring bacterial communities. The following thesis addresses two potentially important connections between phytoplankton and bacteria: resource-mediated linkage through production of labile carbon exudates and habitat-mediated linkage. To investigate phytoplankton exudates as a potential linkage, the first investigation focused on bacteria able to use the photorespiration-specific exudate glycolate. Diversity and dynamics of glycolate-utilizing bacteria were characterized in six lakes using functional gene glycolate oxidase subunit D (glcD). Freshwater glycolate-utilizing populations exhibited a range of taxonomic diversity and contained many sequences clustering with a glcD sequence from the cosmopolitan freshwater Polynucleobacter genus. Glycolate-utilizing and total bacterial community-level variation was largely explained by dynamics of phytoplankton populations (35-40%) and the interaction between these phytoplankton populations and the environment (17-18%). Population-level correlations between specific phytoplankton and glycolate-utilizing bacteria were also detected. These observations support the hypothesis that algal exudates are resource-based drivers of bacterial community composition and identify bacterial taxonomic groups with members capable of responding directly to a specific exudate. Contribution of epiphytic bacteria inhabiting algal cells to correlated community dynamics was investigated by comparing temporal community patterns in the particle-associated bacteria to those of whole bacteria and phytoplankton in Crystal and South Sparkling Bogs. Regular patterns of succession, in addition to correlations between phytoplankton and whole bacterial communities (ρ=0.514, p=0.001) and phytoplankton and particle-associated bacterial communities (ρ=0.739, p=0.001), were detected in Crystal, but not South Sparkling Bog (ρ=0.265, p=0.038; ρ=0.167, p=0.103, respectively). Attached and free-living bacterial assemblages were compared to classify bacterial taxa based on presence in attached and/ or free-living fractions. Despite mixed results for community-level correlations, bacterial populations were positively correlated to abundance of specific phytoplankton in both lakes. Algal-correlated bacteria ranged from primarily particle-associated bacteria in Crystal Bog to a mix of particle-associated and free-living bacteria in South Sparkling Bog. These observations provide support for habitat-mediated linkages, while also indicating the importance of other mechanisms that affect free-living and habitat generalist populations. Taken together, these investigations contribute to a growing body of research demonstrating the importance of biological interactions in shaping microbial community structure. Microbial community composition and function have implications for energy flow, carbon flux, and biogeochemical transformations with ecosystem-level consequences. Understanding biological interactions that structure bacterial communities may facilitate the building of a predictive framework for understanding compositional and functional responses of bacteria and microbially-mediated processes to changing environmental conditions