| Summary: | Antarctic cyanobacteria form microbial mats in perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica. These mats demonstrate a variety of ecological and metabolic behaviors consistent with Antarctic conditions as well as specific challenges between and within the lakes. In this thesis, I explore the survival of cyanobacteria in Antarctic conditions focusing on sulfide stress, polar light availability, and their biogeographical distribution in other environments.In Lake Fryxell, Antarctica, the benthic, filamentous cyanobacterium Phormidium pseudopriestleyi creates a 1-2 mm thick layer of 50 µmol L-1 O2 in otherwise sulfidic water, demonstrating that it sustains oxygenic photosynthesis in the presence of sulfide. Sulfide inhibits oxygenic photosynthesis by blocking electron transfer between H2O and the oxygen-evolving complex in the D1 protein of Photosystem II. The ability of cyanobacteria to counter this effect has implications for understanding the productivity of benthic microbial mats in sulfidic environments throughout Earth history. A metagenome-assembled genome (MAG) of P. pseudopriestleyi indicates a genetic capacity for oxygenic photosynthesis, including multiple copies of psbA (encoding the D1 protein of Photosystem II), and anoxygenic photosynthesis with a copy of sqr (encoding the sulfide quinone reductase protein that oxidizes sulfide). The genomic content of P. pseudopriestleyi is consistent with sulfide tolerance mechanisms including increasing psbA expression or directly oxidizing sulfide with sulfide quinone reductase. However, it is unknown whether the organism can perform anoxygenic photosynthesis using sqr and PS I while PS II is sulfide-inhibited. The seasonal light availability of polar environments has implications for the functioning of circadian clocks in Antarctic cyanobacteria. However, polar cyanobacteria are underrepresented in available genomic data, limiting opportunities to study their genetic adaptations to this and other polar challenges. Chapter 2 presents four ...
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