S9-6 Elemental sulfur reduction by eukaryotic cytoplasm consistent with an ancient sulfur symbiosis

Abstract

An accepted theory for the origin of eukaryotic cells postulates a methanogenic ancestor, although no evidence of methanogenesis exists in modern eukaryotic cells. Instead, sulfur reactions are ubiquitous. The seminal event in eukaryotic evolution might have been a sulfur-based symbiosis between a S°-reducing host and a sulfide-oxidizing bacterium. A vestige of those activities might remain in modern cells. To test this, Tetrahymena thermophila was examined as a representative eukaryotic cell. Aerobic cells consumed H 2 S at a rate of 140 nmol (g protein) −1 s −1 . Anoxic cells produced H 2 S at 1.1 nmol g −1 s −1 , but H 2 S production was suppressed by AOAA, an inhibitor of cysteine catabolism. When aerobic cells were fed H 2 S slowly so that all the H 2 S was oxidized, and then made anoxic, H 2 S was produced at up to 3.6 nmol g −1 s −1 , and not inhibited by AOAA. Evidently, H 2 S is oxidized to a compound that is available for H 2 S production. When aerobic incubation was continued after H 2 S addition had stopped, subsequent H 2 S production decreased with t 1/2 = 11 min. When S° was added to 7 mg cells at 85 pmol s −1 , H 2 S production was 4.4 nmol g −1 s −1 and not inhibited by AOAA. When cells were lysed and fractionated, there was NADH-stimulated S° reductase activity in the membrane fraction, distinguishing it from NADPH disulfide oxidoreductases and linking S° reduction to glycolysis. Thus, there is evidence of a sulfur cycle in which S° is reduced in the cytoplasm and H 2 S is oxidized in mitochondria.