Temperature controls of the sulfur isotope fractionation during sulfate reduction by Thermodesulfobacterium and Desulfovibrio strains

Microbial sulfate reduction is a major mechanism driving anaerobic mineralization of organic matter in global ocean. While sulfate-reducing prokaryotes are well known to fractionate sulfur isotopes during dissimilatory sulfate reduction, unraveling the isotopic composition of sulfur-bearing minerals preserved in sedimentary records could provide invaluable constraints on the evolution of seawater chemistry and metabolic pathways. Variations in sulfur isotope fractionations are partly due to inherent differences among species and also affected by environmental conditions (e.g. sulfate abundance and temperature). Sulfur isotope fractionations caused by microbial sulfate reduction have been interpreted to be caused by a sequence of enzyme-catalyzed kinetic isotope fractionation steps. The fractionation factor mainly depends on (1) the sulfate flux into and out of the cell, and (2) the flux of sulfur compound transformation between the internal pools.
This study examined the multiple sulfur isotope fractionation patterns catalyzed by a thermophilic Thermodesulfobacterium-related strain and a mesophilic Desulfovibrio gigas over a wide temperature range. The Thermodesulfobacterium-related strain grew between 34 and 79 oC with an optimal temperature at 72 oC and the highest cell-specific sulfate reduction rate at 77 oC. The isotope fractionation (ε34Ssulfate-sulfide) ranges between 8.2 and 31.6‰ with a maximum at 68 oC. The D. gigas grew between 10 and 45 oC with an optimal temperature at 30 oC and the highest cell-specific sulfate reduction rate at 41 oC. The isotope fractionation ranges between 10.3 and 29.7 ‰ with higher fractionations at both lower and higher temperatures. The isotope fractionation causing by these two strains is similar to previous reports, but the maximum fractionation is greater than that by the same species. Apparently, the differences in growth conditions may cause the different isotope fractionation. In addition, the change of fractionation with temperature is different for the two strains and cannot be predicted by a standard model considering physiological characteristics of cells. The result of multiple sulfur isotope measurements in this study cannot be described by a sulfate reduction network, which calculated the Δ33S and δ34S values by assuming the equilibrium fractionation among internal steps. Indeed, the sulfate reduction network has to be reevaluated.
Although there are many experiments and several models to study the sulfur isotope fractionation by microbial sulfate reduction, but the result is not conclusive. Temperature is one of the most important environmental factors, but it may not make systemic influence on the physiology of strains and also the isotope fractionation. Further studies regarding physiological responses to environmental factors with the multiple sulfur isotope analysis may probably offer a linkage between sulfate isotope fractionation and growth conditions by sulfate reducing microorganisms.