Achromatium is a large Gammaproteobacterium that lives at oxic-anoxic interfaces where sulfur oxidation is a highly exergonic way of life. However, because of the shifting redox zones, the full oxidation of sulfide to sulfate isn’t always possible, but the piece-meal approach using elemental sulfur as an intermediate can cause its own problems. Sulfide to sulfur drives the pH up, while sulfur to sulfate drives the pH back down. Since free protons can be highly reactive, a buffer system which acts as a proton capacitor is a really clever way to minimize damage to macromolecules. And that’s what Achromatium has figured out: by maintaining a repository of calcium carbonate, it can suck up protons as needed, dissolving into harmless inorganic carbon and keeping essential cellular machinery safe. That’s one of the key conclusions of this new paper from Tingting Yang: paper [here], abstract below!

We investigated the intracellular dynamics of calcite and sulfur in the large sulfur-oxidizing, calcite-accumulating bacterium Achromatium, with an emphasis on oxygen exposure as a physiological control. For this purpose, morphological changes and possible accretion mechanisms of calcite granules in cells that were freshly collected from natural Achromatium-containing sediment were compared to cells from the same source after prolonged exposure to atmospheric oxygen. Intracellular sulfur is oxidized and removed in response to oxygen exposure. Calcite granules also undergo distinct oxygen-related dynamics; they alternate between tightly packaged, smooth granules with narrow but sharply defined interstitial spaces in atmospheric oxygen-exposed cells, and more loosely packaged granules with irregular, bumpy surface texture and larger interstitial spaces in cells that were not artificially exposed to oxygen. These results suggest that morphological changes of the calcite granules reflect their changing physiological role inside the cell. Sulfur oxidation and calcite dissolution appear to be linked in that proton generation during sulfur oxidation is buffered by gradual calcite erosion, visible in the smooth, rounded surface morphology observed after oxygen exposure. Our results support the hypothesis that calcite dynamics buffer the intracellular pH fluctuations linked to electron acceptor limitation during proton-consuming sulfide oxidation to sulfur, and electron acceptor abundance during proton-generating sulfur oxidation to sulfate.