Date

2017

Department or Program

Biological Sciences

Primary Wellesley Thesis Advisor

Vanja Klepac-Ceraj

Abstract

The metabolic pathway of oxygenic photosynthesis is fueled by sunlight and the oxidation of water; oxygen is produced as a waste product. Before the rise of oxygenic phototrophs 2.3 billion years ago, microbes utilized anoxygenic photosynthesis to form organic carbon compounds using electron donors such as iron [Fe(II)] and reduced sulfur compounds. Anoxygenic phototrophs can be found in modern environments like meromictic lakes. We studied enrichment cultures of microorganisms driven by anoxygenic photosynthesis to determine whether and how they use another possible electron donor, manganese [Mn(II)]. Cultures reduced with sulfide [H2S] and enriched with Mn(II) as a potential electron donor oxidized Mn(II) and formed various manganese oxide minerals in the presence of light. To further characterize these anaerobic microbial communities and the role of manganese, we grew the cultures on different concentrations of Mn(II) and H2S and isolated individual microbes. We characterized the composition of the microbial communities by extracting copies of a specific gene, 16S rRNA, which is highly conserved and a standard species-identifying marker. Communities grew only in the presence of light and obligately anaerobic green sulfur bacteria from the phylum Chlorobi were the only phototrophic organisms. From these communities, we identified several taxa using 16S rRNA gene and whole genome sequencing; most notably, a taxon belonging to Chlorobi identified as Chlorobium limicola and a taxon belonging to Desulfomicrobium identified as Desulfomicrobium baculatum. We found a robust microbial community capable of oxidizing Mn(II) in the absence of molecular oxygen. These communities are driven by anoxygenic photosynthesis and dependent on the concentration of sulfur sources, like sulfide. Optimal conditions for anaerobic oxidation of Mn(II) suggest this process occurs in source environment.

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