Effect of Molybdate and Cell Growth on S-Isotope Fractionation During Bacterial Sulfate Reduction

Sulfur Isotope Fractionation by Sulfate-Reducing Microbes Can Reflect Past Physiology

“…The stepwise 34 ε reflects the average mass dependent isotopic discrimination over a short interval (between t x and t x−1 ), that we can relate to the true isotope phenotype of the cells. When calculated in this way and compared with the net 34 ε (Figure 3), our method of estimating the isotope phenotype reveals large 34 ε variations over the batch cycle of 30 h. In the early f values (f > 0.7), fractionation appeared largely constant at roughly −7.5‰ (Figure 3). It is later in the batch cycle that the changing 34 ε becomes important.…”

“…A single fractionation factor, which appears to describe the overall trajectory of the results when f > 0.5 ( 34 ε = −7.5 ‰ ± 0.9 ‰), provides an increasingly poor fit when f < 0.5 (Figure 3). The mean 34 ε over increasingly longer growth intervals has a calculated minimum of −6.8‰ and maximum of −9.9‰. The minimum fractionation is associated with f values closest to 1, and the maximum fractionation is associated with the smallest f values.…”

“…It is later in the batch cycle that the changing 34 ε becomes important. When f < 0.5, the measured stepwise 34 ε varied between −11.6 and −18‰ (Figure 3). Shorter steps of f than measured in this study would likely lead to even larger estimates for these stepwise fractionations.…”

“…The mean 34 ε calculated from t 0 to t x reflects an average of the net fractionation over a given range, but a calculated stepwise 34 ε value conveys a better representation of the instantaneous fractionation at a given point (that is for a given f) in the batch cycle. The stepwise 34 ε reflects the average mass dependent isotopic discrimination over a short interval (between t x and t x−1 ), that we can relate to the true isotope phenotype of the cells.…”

“…80 mL of MOLS4 was placed into 120 mL serum bottles for the assays. Bottles were crimp-sealed with chloro- 34 ε measurements on pure cultures of Desulfovibrio vulgaris Hildenborough under identical environmental conditions. 24 The uncertainty from individual measurements is much smaller than the array obtained, suggesting that a source of variability is not taken into account.…”