Date of Award

12-31-2017

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology/Environmental Biology

First Advisor

Jennifer L. Bowen

Second Advisor

Jarrett Byrnes

Third Advisor

Robert Chen

Abstract

Permeable reactive barriers (PRBs) remove nitrogen from groundwater by enhancing denitrification. Denitrification is important because

bacteria use nitrate (NO3-) to respire organic matter to create N2 gas, thereby mitigating pollution that results from excess NO3-. The PRBs promote denitrification because they consist of woodchips that provide a virtually unlimited carbon source for denitrifiers, but the woodchips also support the growth of other microorganisms. Although PRBs are designed to promote NO3- conversion to N2 gas, a byproduct is the greenhouse gas N2O. If PRBs promote N2O production instead of N2 production, they could promote global warming. Finally, PRBs support heterotrophic bacteria like sulfate reducers, some of which can methylate mercury in groundwater to create methylmercury (MeHg). MeHg is a neurotoxin that bioaccumulates in food webs. Thus, this dissertation had three goals. 1) to characterize the microbial community in PRBs, 2) determine whether PRBs are sources of N2O, and 3) assess whether PRBs have the capacity to promote mercury methylation. Understanding these goals will inform decisions on whether the PRBs should be used on a wider scale to stop nitrogen loading.

I tested three main hypotheses: first, that PRBs would increase abundances of other heterotrophs in addition to denitrifiers. I sequenced bacterial ribosomal genes and found that the PRBs promote the growth of bacteria with alternative metabolisms to nitrate reduction, including several that are sulfur cyclers and fermenters. I also hypothesized that due to high nitrate concentrations in groundwater and high denitrification rates inside the PRB, N2O production would be high. I sequenced two genes in the denitrification pathway and measured N2O in groundwater, and found that PRBs are a sink for N2O because they promote denitrifiers that are capable of reducing N2O to N2. Finally, I hypothesized that due to high abundances of mercury-methylating bacteria in the PRB, MeHg production would be high. I sequenced the bacterial community and measured MeHg coming out of PRB mesocosms, and found that, although Hg addition did alter bacterial communities, MeHg production did not increase. I therefore concluded that PRBs are an effective way to mitigate nitrogen loading without adding other pollutants to the environment.

Comments

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