Doctoral Dissertations

Date of Award

8-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Microbiology

Major Professor

Gary S. Sayler

Committee Members

Alison Buchan, Steven W. Wilhelm, Mark Radesovich

Abstract

The disseration presented below is the summation of research into the potential roles of microbial communities associated with aquifers of Bangladesh contaminated with naturally occuring arsenic. These investigations also included experimental microcosm experiments to assess the role of nutrients supplementation of complex carbon sources (molasses), and inorganic sulfate (MgSO4), on both the solubility of arsenic to determine the feasibility of this method for the goal of performing in situ bioremediation. Community structure and functional gene profiling was performed on all samples, as well as detection of community shifts following amendments predicted to encourage the growth of sulfate reducting microorganisms (SRM). This included community profiling via 16S analysis, as well as presence and quantification of a number of genes involved in respiratory sulfate reduction and genes involved in arsenic cycling. Investigation of samples gathered from contaminated aquifers seems to indicate that even in a community with relatively simple distribution of organisms, there is no distinct linkage between examined functional genes and concentrations of any detected elements in the aquifers.

Examination of the effects of nutrient supplementation on sediments gathered from one impacted aquifer shows that stimulation of the system with either nutrient tested is sufficient to stimulate growth of sulfate reducing microbes, as indicated by conserved genes in the respiratory sulfate reduction pathway. These shifts can be closely associated with an initial decrease in detectable soluble arsenic levels, as well as a commensurate decrease in soluble metals. However, only the addition of both a complex carbon source and magnesium sulfate in equal molar portions seemed to show prolonged removal of these elements from the soluble phase. Community shifts appear to have occurred by 14 days of incubation, and were coupled with expected changes in the color and consistency of sediment as black particulate can serve as an indicator of sulfidic minerals formed as a result of excess sulfides produced by SRM. Increased SRM numbers were maintained through 96 days of incubation.

Due to the ability of any perturbation of a microcosm system to produce increased density of SRM in the samples, a bioinformatic investigation of the identified subsystems encoded by all sequenced and finished bacteria capable of carrying out the most conserved steps in sulfate reduction was performed. These analyses indicated that there are a number of SRM capable of directly reducing complex carbon sources, both in syntrophic communities, as well as without additional aid from the environment.

These results indicate that sulfate reducing microbes are present, detectable and easily stimulated to grow in aquifer sediment, and that these communities of SRM are able to create conditions capable of removing arsenic from the soluble phase. The rate of growth and ability to maintain this immobilization supports the theory that SRM detected in the environment are capable of growth on complex nutrients, and require additional nutrients to successfully remediate arsenic for long periods of time.

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