Collaborative Research: Assessment of Rates and Microbial Pathways for Arsenate Reduction in Sediments

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EAR-0525387/EAR-0525392 Microbial processes have been widely implicated in the mobilization of arsenic (As) from geologic materials into groundwater. However, a number of unresolved questions regarding the biogeochemical redox cycling of As and microbial pathways for arsenate, As(V), reduction remain to be answered. The proposed project will address the following unresolved questions:Question 1: In a given sediment environment, is in situ As(V) reduction related to detoxification or energy acquisition pathways? What factors induce these pathways?Question 2: Under environmental conditions and at environmentally-relevant As concentrations, what are in situ rates of As(V) reduction and what factors limit these rates?Question 3: How do the varying microbial capacities (i.e., iron (Fe)(III), and/or As(V) reducing activity) influence the mobilization of As from solid phases? The proposed project will combine molecular biological and geochemical approaches to interrogate microbial redox cycling of As and its coupling with Fe cycling at field sites with natural, geothermal As inputs. Factors influencing in situ rates of As(V) reduction will be examined and the conditions that trigger this process will be identified. Molecular tools will be employed to track abundance and expression of genes involved in As(V) reduction in situ. Specifically, Question 1 will be addressed by determining the environmental conditions that affect the gene expression for the two main pathways for As(V) reduction: respiratory (arrA) and detoxification (arsC). The bacterium, Shewanella sp. str. ANA-3, which is amenable to genetic manipulation will serve as our model organism for these studies. Field studies will focus on the development and application of molecular biological techniques based on the polymerase chain reaction (PCR) to track and monitor the expression of arrA and arsC in environmental samples. Question 2 will be addressed by introducing hydrous ferric oxide (HFO) pre-equilibrated with As(V) into the ambient sediment environment using a novel gel probe in which the HFO suspension is immobilized in a highly porous gel. Withdrawal of the gels over time and analysis of the oxidation state of As associated with the HFO suspension will allow us to determine the timescale for As(V) reduction in situ. Sediment incubation experiments will also be performed to determine how As(V) reduction rates respond to changes in, for example, organic carbon substrates. We will address Question 3 by examining As mobilization from natural sediments and model solids by mutants of ANA-3 that are deficient in one or more of the genes for Fe(III) reduction and/or As(V) reduction by respiratory and/or detoxification pathways. The results of the proposed project will substantially increase our understanding of the fundamental biological and geochemical processes that influence the occurrence and mobility of As in the subsurface environment. The project will contribute to development of the science base for prediction and mitigation of human exposure to As, which is a major health issue world-wide. The proposed project will also support the training of graduate and undergraduate students, who will be exposed to a wide range of experimental methods and who will develop the skills needed to pursue interdisciplinary environmental research.