Microbial Reduction, Precipitation, and Mobilization of Arsenic

Anaerobic microbes play a critical role in the biogeochemistry of arsenic. In this Dissertation, I utilize both classical microbiological techniques and cutting edge genomics to gain a greater understanding of how arsenic reducing bacteria from both freshwater aquifers and estuarine sediments interact with As(V) as a terminal electron acceptor. The novel As(V) reducing Strain MPA-C3 was isolated from arsenic contaminated estuarine sediments in Hong Kong, and when grown in the presence of sulfide with As(V) as a terminal electron acceptor, removes arsenic from solution by precipitating it as alacranite (As8 S9). Sequencing of both the 16S rRNA gene as well as the entire genome of Strain MPA-C3 places it among the Deferribacteres. Strain MPA-C3 is more metabolically versatile than any of the described Deferribacteres, and is able to utilize NO3 -, Se(VI), Se(IV), fumarate, Fe(III) and mixed oxidation state polysulfide as electron acceptors, and acetate, pyruvate, fructose and benzoate as sources of carbon and energy. The draft genome of Strain MPA-C3 has allowed for the elucidation of the diverse pathways this organism uses for the metabolism of carbon sources, as well as those used for the reduction of both nitrate and sulfur. The role of microbes in the mobilization of arsenic into groundwater was studied at three sites in New Jersey: Crosswicks Creek in Upper Freehold, and Six Mile Run and Pike Run along the Millstone River in Somerset County. At Crosswicks Creek, we determined that microbial As(V) reduction, driven by inputs of organic carbon, resulted in the mobilization of arsenic from iron-rich glauconitic sediments into the groundwater. The role of the redox status of the aquifer was studied by comparing the anoxic subsurface of Six Mile Run with the oxic subsurface at Pike Run. These two sites were found to have remarkably similar mineralogy and groundwater chemistry, and we demonstrated that the differing redox conditions resulted in the development of different microbial communities at each site. As(V) reducing organisms were found at Six Mile Run by both cultivation based and molecular methods, while these organisms were absent at Pike Run. These findings demonstrate that microbial As(V) reduction and mobilization is facilitated by the reducing conditions present at Six Mile Run, while it is inhibited by the oxidizing conditions present at Pike Run. Combined, these studies demonstrate how As(V) reducing microorganisms play a critical role in the biogeochemical cycling of arsenic.