| Summary: | Sulfide inhibits oxygenic photosynthesis by blocking electron transfer between H<sub>2</sub>O 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. In Lake Fryxell, Antarctica, the benthic, filamentous cyanobacterium <i>Phormidium pseudopriestleyi</i> creates a 1–2 mm thick layer of 50 µmol L<sup>−1</sup> O<sub>2</sub> in otherwise sulfidic water, demonstrating that it sustains oxygenic photosynthesis in the presence of sulfide. A metagenome-assembled genome of <i>P. pseudopriestleyi</i> indicates a genetic capacity for oxygenic photosynthesis, including multiple copies of <i>psbA</i> (encoding the D1 protein of Photosystem II), and anoxygenic photosynthesis with a copy of <i>sqr</i> (encoding the sulfide quinone reductase protein that oxidizes sulfide). The genomic content of <i>P. pseudopriestleyi</i> is consistent with sulfide tolerance mechanisms including increasing <i>psbA</i> expression or directly oxidizing sulfide with sulfide quinone reductase. However, the ability of the organism to reduce Photosystem I via sulfide quinone reductase while Photosystem II is sulfide-inhibited, thereby performing anoxygenic photosynthesis in the presence of sulfide, has yet to be demonstrated.
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