UC Santa Cruz

Thesis statement:

Anoxygenic photosynthetic arsenite oxidation encoded by arxA is a bacterial arsenic metabolism that contributes to the biogeochemical cycle of arsenic in extreme environments.

Abstract:

This dissertation provides molecular genetics and environmental insight into the poorly-understood phenomenon of a photosynthetic microbial metabolism fueled by arsenic. The hypothesis is that arxA is critical for photosynthetic arsenite oxidation and actively found in the environment, which has an impact on the arsenic cycle in euphotic arsenite rich extreme environments.

The discovery of light dependent arsenic metabolisms that allows bacteria to conserve energy by oxidizing arsenite to arsenate in the absence of oxygen was described in 2008. This light dependent (photo) arsenic fueled (arsenotrophy) metabolism is referred to as “photoarsenotrophy” or more specifically anoxygenic photosynthetic arsenite oxidation. These earlier findings have yielded interesting molecular genetics and environmental ecology questions that are address in this dissertation. The first question is that of a molecular genetic nature, namely: Is the arsenite oxidase, ArxA, essential to the photoarsenotrophy mechanism? This question was validated after a genetic system was developed in Ectothiorhodospira sp. strain BSL-9, since there were no genetic models to study photoarsenotrophy.

The second question that was investigated was: Do anoxygenic photosynthetic arsenite oxidizing bacteria transform arsenite to arsenate in arsenic rich environments? We surveyed Paoha Island Mono Lake in California and Big Soda Lake, NV, two extreme environments rich in arsenic known to have ideal physicochemical conditions, photosynthetic purple sulfur bacterial and the arsenite oxidase arxA gene. The results provide evidence of anoxygenic photosynthetic arsenite oxidation activity by demonstrating in situ arxA gene expression within the hot spring pool biofilms of Paoha Island, and also provide evidence for a microbial arsenic cycle that was light dependent within anoxic water column samples that were collected from Big Soda Lake, Nevada. The environmental-functional gene ecology study demonstrated that Big Soda Lake contains microbial populations that can perform anoxygenic photosynthesis coupled to arsenite oxidation and arsenate reduction. These ecological studies further elucidate the biogeochemical cycle of photoarsenotrophy in extreme environments, lay the foundation for future fundamental molecular genetics and raising many questions that have yet to be addressed.