This research will seek to elucidate the pathway for thioarsenate decomposition by microbial assemblages, using alkaline, sulfidic and arsenic-enriched Mono Lake bottom waters as a study site. Enrichment cultures and manipulation experiments will be performed to test the effects of different electron acceptors on thioarsenate decomposition. The PI will analyze the microbial communities to determine which organisms and which enzyme systems respond to enrichments with thioarsenates to aid in identifying pathways. These experiments are designed to address the following questions: 1) What is the behavior of thioarsenate with arsenate, nitrate or oxygen as electron acceptors? Are there specific groups of microbes associated with each pair of reactants? 2) What are the products of the reaction? 3) How does gene expression change during thioarsenate decomposition? What are the pathways involved in thioarsenate oxidation?
The results of this study will be relevant, not only for a better understanding arsenic cycling in Mono Lake, but for modeling arsenic in other neutral or alkaline environments where arsenite and sulfide are present and exposed to changing redox conditions. Thioarsenate transformation may be of particular importance in alkaline hot springs, industrial waste sites, or aquifers with bedrock containing arsenic- and sulfur-bearing minerals. One outcome will be a better understanding of arsenic transformations and mobility under fluctuating redox conditions in anoxic groundwater. The information may also be useful in designing bioremediation strategies. The proposed studies are also relevant to understanding and modeling the role of arsenic in the early stages of the evolution of life on Earth.
14.00 Normal 0 false false false EN-US X-NONE X-NONE Microbial Transformations of Arsenic and Antimony Thioarsenates can be a dominant form of the toxic element arsenic in anoxic water when sulfide is present. Anoxic, sulfidic conditions are common in well water in many areas, including areas where groundwater is naturally rich in arsenic. The goal of this project was to understand the complex chemistry, biochemistry and microbiology that leads to the transformation of these compounds into other arsenic-containing compounds. These transformations affect arsenic bioavailability, toxicity and environmental mobility. It is our belief that this knowledge will contribute to improved strategies for the remediation of arsenic-contaminated sites and for improved purification of drinking water. Our work has led us to conclude that the transformations of thioarsenic compounds are intricately tied to sulfur chemistry, biochemistry and microbiology. The same organisms that make a living by oxidizing sulfur compounds are important in thioarsenic transformations, and these transformations appear to be mediated by some of the same enzyme systems that are used to metabolize sulfur compounds. The organisms responsible for the transformations are well-known for their abilities to use reduced sulfur compounds for growth. Although this link between the metabolism of sulfur and arsenic has been suggested by previous work in our laboratory, the studies performed on this project add a new dimension to our understanding of the link. We have shown that thioarsenic compounds can be used to support a form of photosynthesis that does not liberate oxygen, adding to the very short list of compounds that can be used this way. Since this is an ancient metabolism, older than the oxygen-generating photosynthesis with which we are more familiar, this metabolism may have been important very early in the evolution of life on Earth. It is interesting to speculate that this and similar metabolisms might be a significant mechanism for trapping light energy in organic matter on other planets where biochemistry has not evolved as far as it has on Earth. We have also studied the transformation of other toxic metals that are chemically similar to arsenic. One in particular, antimony, is widely used in industry for the production of solar cells, LEDs, fire retardants, as an alloy with other metals and for many other products. It is a metal of strategic importance and most of the commercially important deposits are located outside of the US. It is also as toxic as arsenic and acid mine drainage from the few mines in the US are significant environmental hazards. We have isolated a microorganism that is capable of growing by converting a soluble, oxidized form of antimony called antimonate to an insoluble, reduced form called antimonite, or antimony trioxide, while oxidizing organic matter. Cultures of this organism produce pure crystals of antimony trioxide in the form of a mineral called valentinite, shown in the attached photo. Our discovery offers the possibility of mining antimony from waste waters, allowing us to recover significant quantities of the mineral from these waters or from deposits that are too dilute for standard processing methods. Because the process is mediated by bacteria, it produces fewer toxic by-products than standard commercial smelting techniques. We are working to scale what is now a lab process up to one that may be commercially viable. We are also analyzing the genomes of the microorganisms involved in arsenic and antimony transformations to identify genes encoding the biochemical pathways that enable these organisms to metabolize these compounds. Our goal is to understand how cells derive energy for growth from these compounds, how they tolerate their toxicity, and how these biochemical pathways are related to the pathways by which other, structurally similar, compounds are metabolized. These enzyme systems are members of a large and ancient group of proteins that are crucial to central metabolism, so the knowledge we gain from studying them in these cells using odd compounds will improve our ability to understand their function in higher organisms. We may also be able to use the knowledge re-design or improve the enzymes to work better in specific, commercial applications. This project has contributed to the development of US workforce by training of two Ph.D. students, one of whom is a member of an ethnic group that is underrepresented in the science and technology work force. Several undergraduate students and a high school student were also involved in the project. These students have all gone on to get degrees in majors that contribute to the science and technology work force. The discovery of the antimonate reducing organism described above garnered significant coverage by the media and resulted in inquiries by commercial mining companies interested in the potential for using it in their plants.
