Impact of toxic species on alternative accepting reductive processes in near to deep subsurface environments

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Zhou, Lang, Ph. D.

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Subsurface microorganisms can use many different electron acceptors to yield energy for metabolism and growth through dissimilatory pathways. The reductive processes can be impacted by toxic species introduced by human activities. The research mainly focused on two conditions: the co-occurrence of nitrate and antibiotics in soil and groundwater; the co-occurrence of sulfate and biocides in hydraulically fractured shale. The first condition causes the activities of nitrate reducing bacteria to be severely impacted by excessive antibiotics discharged into soil/groundwater by human. Prior research mostly evaluated the impact of antibiotics on nitrate biotransformation in batch microcosms, resulting in a knowledge gap between spatial heterogeneity and concentration gradients of antibiotics present in soil/groundwater and bacteria response. To address this gap, a microfluidic gradient chamber was used to create concentration gradients of the antibiotic polymyxin b, and the growth, migration and nitrate reduction activity of Shewanella oneidensis was monitored and analyzed; complementary batch experiments were also performed to support the microfluidic experiments. Results show that S. oneidensis migrated to bactericidal regions for nutrients without developing antibiotic resistance, with motility being the key factor in a spatially heterogenous environment. The migration was inhibited by acutely lethal concentration of antibiotics. The second condition has been extensively studied in batch microcosms and inferred from limited field observations, primarily motivated from the concerns that sulfate reducing bacteria promote bioclogging and biosouring in oil and gas reservoirs. However, few studies have quantified biomass growth and activity in shale fractures due to the complexity of studying processes at this scale, resulting in a knowledge gap regarding the ability of microbes to colonize shale fracture surfaces and clog these oil and gas flow conduits. To address this gap, a shale-based microfluidic flow cell reactor was inoculated with a field culture dominated by dissimilatory sulfate-reducing bacteria (DSRB), followed by incubation and biocide injection. Microscopic images and effluent samples were analyzed to assess biofilm growth and biocide inhibition, with a numerical model developed to quantify sulfate reduction rates. The results indicate that biomass grows as biofilms on shale surfaces with little mass transfer limitations, and that accurate quantification of sulfate reduction rates depends on quantifying this biomass. The results also indicate that while biocides eliminated microbial activity in shale fractures, they do little to remove biomass and open up shale fractures. The findings of the study are an important contribution to our understanding of the impacts of toxic species on microbial accepting reductive processes in subsurface environments and will help decisions on the use and control of toxic species including antibiotics and biocides for human health and environmental protection.


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