Ecology and biogeochemical cycling of photosynthetic arsenite-oxidizing bacteria

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

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.