Elemental sulfur reduction to H₂S by Tetrahymena thermophila

Eukaryotic nucleocytoplasm is believed to be descended from ancient Archaea that respired on elemental sulfur. If so, a vestige of sulfur reduction might persist in modern eukaryotic cells. That was tested in Tetrahymena thermophila, chosen as a model organism. When oxygenated, the cells consumed H₂S rapidly, but when made anoxic they produced H₂S mostly by amino acid catabolism. That could be inhibited by adding aminooxyacetic acid, and then H₂S production from elemental sulfur became more evident. Anoxic cell lysates produced H₂S when provided with sulfur and NADH, but not with either substrate alone. When lysates were fractionated by centrifugation, NADH-dependent H₂S production was 83% in the soluble fraction. When intact cells that had just previously oxidized H₂S were shifted to anoxia, the cells produced H₂S evidently by re-using the oxidized sulfur. After aerobic H₂S oxidation was stopped, the oxidation product remained available for H₂S production for about 10 min. The observed H₂S production is consistent with an evolutionary relationship of nucleocytoplasm to sulfur-reducing Archaea. Mitochondria often are the cellular site of H₂S oxidation, suggesting that eukaryotic cells might have evolved from an ancient symbiosis that was based upon sulfur exchange.