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.