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Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Santa Cruz Electronic Theses and Dissertations (2017)

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.

    Cover page: Ecology and molecular genetics of anoxygenic photosynthetic arsenite oxidation by arxA
    • Article
    • Peer Reviewed
    UC Santa Cruz Previously Published Works (2016)
    The full genome sequence of Ectothiorhodospira sp. strain BSL-9 is reported here. This purple sulfur bacterium encodes an arxA-type arsenite oxidase within the arxB2AB1CD gene island and is capable of carrying out "photoarsenotrophy" anoxygenic photosynthetic arsenite oxidation. Its genome is composed of 3.5 Mb and has approximately 63% G+C content.
      Cover page: Genome Sequence of the Photoarsenotrophic Bacterium Ectothiorhodospira sp. Strain BSL-9, Isolated from a Hypersaline Alkaline Arsenic-Rich Extreme Environment
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2017)
      Three novel strains of photosynthetic bacteria from the family Ectothiorhodospiraceae were isolated from soda lakes of the Great Basin Desert, USA by employing arsenite (As(III)) as the sole electron donor in the enrichment/isolation process. Strain PHS-1 was previously isolated from a hot spring in Mono Lake, while strain MLW-1 was obtained from Mono Lake sediment, and strain BSL-9 was isolated from Big Soda Lake. Strains PHS-1, MLW-1, and BSL-9 were all capable of As(III)-dependent growth via anoxygenic photosynthesis and contained homologs of arxA, but displayed different phenotypes. Comparisons were made with three related species: Ectothiorhodospira shaposhnikovii DSM 2111, Ectothiorhodospira shaposhnikovii DSM 243T, and Halorhodospira halophila DSM 244. All three type cultures oxidized arsenite to arsenate but did not grow with As(III) as the sole electron donor. DNA-DNA hybridization indicated that strain PHS-1 belongs to the same species as Ect. shaposhnikovii DSM 2111 (81.1% sequence similarity), distinct from Ect. shaposhnikovii DSM 243T (58.1% sequence similarity). These results suggest that the capacity for light-driven As(III) oxidation is a common phenomenon among purple photosynthetic bacteria in soda lakes. However, the use of As(III) as a sole electron donor to sustain growth via anoxygenic photosynthesis is confined to novel isolates that were screened for by this selective cultivation criterion.
        Cover page: Arsenite as an Electron Donor for Anoxygenic Photosynthesis: Description of Three Strains of Ectothiorhodospira from Mono Lake, California and Big Soda Lake, Nevada
        • Article
        • Peer Reviewed
        UC Santa Cruz Previously Published Works (2017)
        'Photoarsenotrophy', the use of arsenite as an electron donor for anoxygenic photosynthesis, is thought to be an ancient form of phototrophy along with the photosynthetic oxidation of Fe(II), H2 S, H2 and NO2-. Photoarsenotrophy was recently identified from Paoha Island's (Mono Lake, CA) arsenic-rich hot springs. The genomes of several photoarsenotrophs revealed a gene cluster, arxB2AB1CD, where arxA is predicted to encode for the sole arsenite oxidase. The role of arxA in photosynthetic arsenite oxidation was confirmed by disrupting the gene in a representative photoarsenotrophic bacterium, resulting in the loss of light-dependent arsenite oxidation. In situ evidence of active photoarsenotrophic microbes was supported by arxA mRNA detection for the first time, in red-pigmented microbial mats within the hot springs of Paoha Island. This work expands on the genetics for photosynthesis coupled to new electron donors and elaborates on known mechanisms for arsenic metabolism, thereby highlighting the complexities of arsenic biogeochemical cycling.
          Cover page: The genetic basis of anoxygenic photosynthetic arsenite oxidation
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