Ecology and biogeochemical cycling of photosynthetic arsenite-oxidizing bacteria

该项目将研究复杂的砷生物地球化学循环中的一个新的分支点，即无氧光合亚砷酸氧化。在这个过程中，细菌催化亚砷酸盐的光依赖性氧化为砷酸盐。人们对亚砷酸盐催化的光合作用的生物学基础和环境意义知之甚少。这一过程可能在许多环境中发生，例如在陆地温泉、浅水热喷口、湖泊和溪流光带以及用富含亚砷酸盐的地下水灌溉的作物土壤中，例如在孟加拉国。由于亚砷酸盐被认为是一种毒性更强且在水文上可移动的砷形式，光合细菌氧化亚砷酸盐可以为受砷影响的地表水中砷毒性的自然衰减提供重要手段。该项目的目的是确定富砷环境中亚砷酸氧化无氧光合细菌的丰度、多样性和活性。核心假设是，依赖砷的光合细菌利用一种名为ArxA的“新型”亚砷酸氧化酶，这种酶也可以用来指示它们在富砷环境中的存在和砷转化活性。该假设将通过以下方式进行验证：(i)从加利福尼亚州Mono湖周围各种水文/地热特征的富砷环境中分离和表征新的光合亚砷酸盐氧化剂；（ii）测量内华达州大苏打湖（Big Soda Lake）的光合亚砷酸盐氧化速率，该湖富含砷，具有光合紫硫细菌的季节性爆发；（iii）在大苏打湖进行微生物生态学研究，旨在量化亚砷酸氧化酶基因（arxA）丰度与深度的关系，并将这些数据与砷生物地球化学相关联。该项目的结果将导致新的光合作用亚砷酸氧化生物的分离，定量环境样品中光依赖性亚砷酸氧化的新方法，以及针对与光驱动亚砷酸氧化相关的关键基因的基于分子的工具的开发。砷主要是一种自然产生的水污染物，影响着全世界数百万人的健康。加深我们对砷在人类获取水和食物等重要资源的环境中如何变化和移动的了解，对减轻这种有毒元素的风险具有潜在影响。微生物可以在改变砷的化学性质方面发挥关键作用，这反过来又强烈影响砷的流动性和毒性。本研究的重点是最近发现的一种微生物砷代谢形式，特别是研究在缺氧（缺氧）环境中将亚砷酸盐（毒性更强的形式）转化为砷酸盐（毒性较小的形式）的光合细菌。将进行微生物生态学和环境化学研究，以确定这些光合砷氧化细菌的活性。这些结果将为这些新发现的微生物如何影响复杂的砷生物地球化学循环提供重要的见解。此外，了解以亚砷酸盐为基础的无氧光合作用（不像标准光合作用那样产生氧气）可能有助于了解光合作用早期形式的进化。

This project will investigate a new branching point within the complex arsenic biogeochemical cycle referred to as anoxygenic photosynthetic arsenite oxidation. During this process, bacteria catalyze light-dependent oxidation of arsenite to arsenate. Little is known about the biological basis and environmental significance of arsenite-fueled photosynthesis. There are numerous environments where this process may be occurring such as in terrestrial hot springs, shallow water thermal vents, lake and stream euphotic zones, and crop soils irrigated with arsenite rich ground water, for example in Bangladesh. Because arsenite is considered a more toxic and hydrologically mobile form of arsenic, the oxidation of arsenite by photosynthetic bacteria could provide an important means for natural attenuation of arsenic toxicity within surface waters impacted by arsenic. The project aims are to determine the abundance, diversity, and activity of arsenite oxidizing anoxygenic photosynthetic bacteria in arsenic rich environments. The central hypothesis is that arsenic-dependent photosynthetic bacteria utilize a 'novel' arsenite oxidase enzyme called ArxA that can also be used to indicate their presence and arsenic-transforming activity in arsenic rich environments. The hypothesis will be tested by: (i) isolating and characterizing new photosynthetic arsenite oxidizers from arsenic rich environments in various hydro/geothermal features around the Mono Lake, CA area, (ii) measuring photosynthetic arsenite oxidation rates in Big Soda Lake, an arsenic rich lake in Nevada that has a seasonal bloom of photosynthetic purple sulfur bacteria, and (iii) conducting microbial ecology studies in Big Soda Lake aimed at quantifying arsenite oxidase gene (arxA) abundance vs. depth and correlating this data to the arsenic biogeochemistry. Results from this project will lead to the isolation of new photosynthetic arsenite oxidizing organisms, new methods for quantifying light-dependent arsenite oxidation in environmental samples, and the development of molecular-based tools that target the key genes associated with light-driven arsenite-oxidation. Arsenic is mostly a naturally-occurring water pollutant affecting the health of millions of people worldwide. Increasing our understanding of how arsenic changes and moves in the environment from which humans obtain important resources like water and food has potential implications for the mitigation of risk from this toxic element. Microbes can play a key role in changing the chemistry of arsenic, which in turn strongly affects arsenic's mobility and toxicity. This study focuses upon a recently-discovered form of microbial arsenic metabolism, specifically examining photosynthetic bacteria that change arsenite (more toxic form) to arsenate (less toxic form) in anoxic (oxygen-depleted) environments. Microbial ecology and environmental chemistry studies will be conducted to determine the activities of these photosynthetic arsenic oxidizing bacteria. The results will provide important insight regarding how these newly identified microbes may impact the complex arsenic biogeochemical cycle. In addition, understanding arsenite-based anoxygenic photosynthesis (doesn't generate oxygen like standard photosynthesis) may provide insight regarding the evolution of early forms of photosynthesis.