Local Adaptation of Microbial Communities along Geochemical Spatial Gradients in Sediments of the Lake Erie Region

Lake Erie demonstrates the greatest productivity of the Laurentian Great Lakes, yet has been critically impacted by anthropogenic activities throughout the Lake Erie watershed. Lake Erie is comprised of three major basins, with east-to-west gradients of increasing drainage areas, increasing riverine inputs of nutrients and xenobiotics, as well as decreasing depth. These large-scale geochemical gradients may be expected to result in spatial patterns of microbial community composition, nutrient cycling, and xenobiotic transformation. As such, Lake Erie provides an excellent system to examine the local adaptation of microbial communities throughout a large freshwater ecosystem. Spatial patterns of microbial community composition, as well as functional diversity, across the three basins of Lake Erie were assessed to examine the potential adaptation of microbial communities to local selective pressures. Community composition was investigated through the generation of 16S rDNA libraries, while functional diversity was evaluated with substrate-induced respiration (SIR) and extracellular enzyme activities (EEA) profiles. EEA profiles were subsequently measured to examine microbial community resilience to metal inoculations in sediments contaminated with heavy metals compared to relatively pristine sediments. Bioinformatic studies of bacterial genes involved in the efflux of heavy metals from the cell were performed to provide a conceptual framework of how horizontal gene transfer may expedite the adaptation of bacterial communities to heavy metal stress. Finally, the local adaptation of bacterial communities to PCBs and PAHs was assessed by comparing the diversity of bphA, a gene that initiates PCB metabolism, in polluted and relatively unpolluted sediments within the Lake Erie watershed. Collectively, results suggested large-scale spatial patterns of microbial community composition, functional diversity, and metabolic resilience consistent with the local adaptation of sediment bacterial communities to allochthonous inputs of organic matter and heavy metal pollutants into Lake Erie. Furthermore, estimates of diversity from bphA environmental gene libraries suggest that PCB and PAH contamination represents a driving force in the adaptation of microbial communities in polluted sediments. Results from this study suggest that microbial communities are highly integrated assemblages of multiple taxa locally adapted to differential inputs of nutrients and xenobiotics across geochemical gradients within freshwater ecosystems