Fractionation of sulfur isotopes by the yeast Saccharomyces cerevisiae

Growing and resting cell suspensions of Baker's yeast (Saccharomyces cerevisiae) were permitted to metabolize sulfite and sulfate under a variety of environmental conditions. Enrichment of S³² in the H₂S released during reduction of sulfate under growing conditions was consistently lower (δS³⁴ < ?25%.) than enrichment during reduction of sulfite under similar conditions (δS³⁴ ~ ?30 to ?50%.). Yeast cells harvested from a sulfite medium released small quantities of H₂S when suspended in glucose-sulfate solution; cells from a sulfate growth medium did not. The enrichment of H₂S in S³² by sulfite-grown resting cells suspended in either SO^2?₄ or SO^2?₃ solutions was about 10%.in less than the maximum obtained during growth on sulfite. Isotopic fractionation was always significantly less in assimilated, than in dissimilated sulfur.