| Summary: | © 2016 Dr. Caitlin Marissa Gionfriddo Microorganisms strongly influence the environmental form and fate of mercury. Recent advancements in culture-independent molecular techniques, such as high-throughput sequencing, allow us to delve deeply into the functional and phylogenetic composition of environmental microbial communities. Metagenomic techniques can be used to link the distribution of microbes equipped with mercury cycling genes to environmental processes, such as the production of the toxin, methylmercury. Presented in this thesis are metagenomic-sequencing data (Illumina Hiseq 2 x 100-bp paired-end technology) from two mercury-enriched environments: Antarctic sea ice and brine, and acidic geothermal springs. Metagenomic analyses are paired with geochemical analyses to examine how various physicochemical parameters may impact microbial community structure and function as it relates to mercury cycling. Both environments exhibit microbial communities equipped with various adaptive mechanisms, including the mercury detoxifying mer-operon amongst other metal and stress resistance pathways. Furthermore, close analogues of mercury methylation genes (hgcAB) are detected alongside the mer detoxification pathway in both environments, indicating an active methylation-demethylation-reduction cycling of mercury by these microbial communities. In polar marine environments, the long-range atmospheric transport of inorganic mercury, and an active atmospheric cycling of mercury following polar sunrise, results in the deposition of mercury onto sea-ice covered waters of the Southern Ocean. Sea-ice covered waters and the fauna that inhabit them tend to have inflated concentrations of the bioaccumulative neurotoxin, methylmercury, compared to open ocean waters. However, the role of sea ice in biogeochemical mercury cycling is poorly understood, particularly the production of methylmercury. In this thesis, deep metagenomic sequencing is used to identify potential microbial transformations of mercury in first-year sea ice and brine sampled from East Antarctica. Measured concentrations of total and methylated mercury in the sea-ice environment ranged from 1.01 to 895 pM, and <0.1 to 0.80 pM, respectively. Heterotrophic bacteria and photoautotrophic eukaryotes dominated the metagenomic datasets; however, chemoautotrophic archaea and bacteria were also present at lower abundance within the sea-ice and brine communities. Mercuric reductase genes (merA) closely related to those of Proteobacteria were identified in brine and ice communities. Metagenomic screening for mercury methylation genes (hgcAB) showed similarity (>50%) to highly conserved sites of hgcA. The putative methylation gene belongs to Nitrospina, and is predicted to encode an HgcA-like pterin-binding enzyme in the family of cobalamin-dependent methyltransferase. This study shows that the toxin, methylmercury, may be microbially produced within sea ice, constituting a source of the toxin to the Southern Ocean ecosystem. This work suggests, for the first time, that microbial mercury methylation can occur in a low oxygen environment, and implicates a common marine nitrite-oxidizer in the production of methylmercury. This thesis presents new information on how the structure and functionality of sea-ice microbial communities impact mercury transformations in polar environments. Few studies have explored the microbial underpinnings of mercury transformations in geothermal settings. Yet, hot springs and fumaroles release significant quantities of aqueous and gaseous mercury into the environment. Presented here are results from metagenomic sequencing of geothermal microbial communities cycling mercury, focusing on the connections between putative metabolisms and mercury methylation, and mercury demethylation and reduction in contaminated sediments. Presented are data from two adjacent, acidic (pH<3), mesothermal (22-40.5 °C) hot springs of the Ngawha Geothermal Field (Northland, New Zealand), extremely enriched in total mercury (>1000 ng L-1), and varying MeHg concentrations (1-10 ng L-1). Acidophilic, thermophilic, sulfur-cycling, and iron-cycling bacteria and archaea dominate microbial communities of both springs. Although genes of the mer operon were detected in both springs (at high abundance), mercury methylation genes (hgcAB) were only detected in the cooler spring (ΔT~10°C), with an order of magnitude greater methylmercury. The hgcAB genes have no known closest relative, but most likely belong to an uncultured acidophilic, sulfate-reducing bacterium. This study shows that geothermal microbial communities are capable of net production of methylmercury, alongside active demethylation-reduction by mer-capable microbes, despite selective pressures from low pH and high mercury levels. However, temperature may be a major limiting factor on the presence and activity of mercury methylating microorganisms in the environment.
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