Sulfur isotope fractionation during microbial sulfate reduction by toluene-degrading bacteria

Sulfate-reducing bacteria contribute considerably to the mineralization of petroleum hydrocarbons (PHC) in contaminated environments. Stable sulfur is otope fractionation during microbial. sulfate reduction was investigated in microcosm experiments with different cultures of sulfate-reducing bacteria for various initial sulfate concentrations using toluene as the sole carbo n source. Experiments were conducted with the marine strain Desulfobacula t oluolica, the fresh water strain PRTOL1, and an enrichment culture from a P HC-contaminated aquifer. Sulfate reduction rates ranged from 7 +/- 1 to 494 +/- 9 nmol cm(-3) d(-1), whereas specific sulfate reduction rates (sSRR) r anged from 8.9 X 10(-15) to 3.9 X 10(-13) +/- 9.2 X 10(-14) mol cell(-1) d( -1). Calculated enrichment factors (8) for the fractionation of stable sulf ur isotopes during microbial sulfate reduction ranged from 19.8 +/- 0.9 to 46.9 +/- 2.1 parts per thousand. In general, values of epsilon and sSRR obt ained in our experiments were similar to those reported previously for sulf ate-reducing bacteria incubated with readily available carbon sources under optimal growth conditions. Moreover, we found no obvious correlation betwe en epsilon and sSRR values when data from all our microcosm experiments wer e combined or when we combined our data with several previously published d ata sets. In contrast, epsilon values determined in our enrichment culture experiments (average 23.5 +/- 4.3 parts per thousand) agreed well with epsi lon values determined in a recent field study performed in situ in a PHC-co ntarninated aquifer. Thus, results from this laboratory study provide valua ble information on stable sulfur isotope fractionation during microbial sul fate reduction under conditions that more closely resemble those in PHC-con taminated environments, i.e., for a variety of sulfate concentrations, incl uding low sulfate concentrations, and for a an important PHC-constituent (t oluene) used as sole carbon source. Copyright (C) 2001 Elsevier Science Ltd .