Rapid hydrogen sulfide consumption by Tetrahymena pyriformis and its implications for the origin of mitochondria

Although sulfide is typically regarded as toxic to eukaryotic cells, it is avidly consumed by Tetrahymena pyriformis. That was observed only when the sulfide concentration was kept below 1 μM. Previously concentrations that were too high had been tested. A new device (Sulfidostat) was used to measure sulfide consumption in steady-state concentrations as low as 10 −12 M. The technique was validated non-biologically by slowly injecting AgNO 3 into buffer and using Ag 2S precipitation to mimic sulfide consumption, confirming that rates of sulfide consumption could be measured independently of sulfide concentrations. With T. pyriformis, sulfide consumption was 0.25 μmol (g protein) −1 s −1 in 0.5 μM sulfide. Sulfide consumption required O 2 and was inhibited by HCN or by too much sulfide. When cells were separated into fractions, sulfide consumption occurred in the particulate (mitochondrial) fraction. Unexpectedly, the soluble cytosolic fraction slowly produced sulfide even when aerated. The observations are consistent with the conjecture that mitochondria evolved from sulfidotrophic symbionts in a sulfidogenic host cell.