Implications of conservation of mass effects on mass-dependent isotope fractionations: Influence of network structure on sulfur isotope phase space of dissimilatory sulfate reduction

This paper presents worked solutions for the fractionations of all four stable sulfur isotopes (32S, 33S, 34S, and 36S) in several models of the sulfate reduction metabolism. We describe methods for obtaining solutions and how the predictions made by these solutions define different compositional fields (phase space) that can be used to gain new insights into sulfur metabolisms, specifically with respect to understanding the structure of and fractionations associated with the network of reactions that describe the transformations of sulfur within the cell. We show how this treatment can be used to evaluate data from experiments with dissimilatory sulfate reducers and to suggest that the expression of fractionations by the metabolic process is largely limited by the fraction of sulfate that is lost from the cell, and that the variation in observed fractionations reflects differences in the proportion of sulfur intermediates that are reoxidized to sulfate. This analysis provides a line of support for this assertion that depends only on the sulfur isotopic fractionations between sulfate and sulfide. This analysis also indicates that internal fractionations are consistent with a relationship given by 33α = (34α)θ where α is the fractionation factor (e.g., 33αa−b = (33 S/32S)a/(33S/32S)b and 34αa−b = (34 S/32S)a/(34S/32S)b), and where θ is restricted to a value between 0.515 and 0.514. This finding is consistent with a control on isotopic fractionation effects within the cell that is rooted in the different partition coefficients (energetics) for the different isotopologs.