THE EFFECT OF LONG-TERM MERCURY CONTAMINATION ON THE COMPOSITION AND DIVERSITY OF SOIL BACTERIAL COMMUNITIES IN RIVERINE ECOSYSTEMS The Effect of Long-Term Mercury Contamination on the Composition and Diversity of Soil Bacterial Communities in Riverine Ecosystems

Aspassia Chatziefthimiou, Aspassia Chatziefthimiou, Tamar Barkay
Hg contamination in riverine ecosystems is a persistent problem and clean-up efforts are a priority for EPA and local federal governments as potential methylation of Hg increases its toxicity due to its bioaccumulation and biomagnification in aquatic food chains. Understanding the microbial contribution to Hg contamination is of particular importance as microbial communities occupy the base of the food chain and the way they transform Hg has bottom-up effects to all trophic levels. The broad
more » ... evels. The broad objective of this dissertation was to investigate the role of abiotic factors in shaping the composition, diversity and distribution of bacterial communities inhabiting floodplain soils of the East Fork Poplar Creek (EFPC), TN, and South River (SR), VA, chronically contaminated with Hg as a result of industrial processes. Analysis of soil samples from the EFPC by direct cultivation and isolation, revealed a metabolic-dependent effect of Hg-stress on bacterial populations, with copiotrophs exhibiting higher mercury reduction potentials, as well as phylogenetic and functional diversity, than oligotrophs. As the great majority of the strains contained a merA gene in their genome, Hg-resistance in these isolates may have been conferred by the functions of the mercury resistance (mer) system. A total of 27 phylogenetic incongruencies were observed between this and the 16S rRNA genes of the isolates, suggesting that iii horizontal gene transfer may play a role in Hg adaptation. The culture-independent method of 16S rRNA-fingerprinting was used to assess spatial distribution and diversity of bacterial communities along the Hg-contamination gradient in SR. Higher levels of diversity were obtained in communities that experience low as compared to high soil Hg levels. The best predictors of community diversity were pH, moisture and soil texture, whereas THg and geography were poor predictors. In this study a new merA-based t-RFLP method was designed to assess distribution and diversity of merA genes. Results show high levels of diversity for this gene and clustering based on geographical proximity. These findings highlight the impact of long-term Hg-stress on microbial communities in riverine ecosystems and provide a micro-ecological framework for future remedial actions in Hg contaminated sites. iv Acknowledgements