Effect of NO₂⁃on stable isotope fractionation during bacterial sulfate reduction.

The effects of low NO2(-) concentrations on stable isotope fractionation during dissimilatory sulfate reduction by strain Desulfovibrio desulfuricans were investigated. Nitrite, formed as an intermediate during nitrification and denitrification processes in marine and freshwater habitats, inhibits the reduction of the sulfuroxy intermediate SO3(2-) to H2S even at low concentrations. To gain an understanding of the inhibition effect of the reduction of the sulfuroxy intermediate on stable isotope fractionation in sulfur and oxygen during bacterial sulfate reduction, nitrite was added in the form of short pulses. In the batch experiments that contained 0.02, 0.05, and 0.1 mM nitrite, sulfur enrichment factors epsilon of -12 +/- 1.6, -15 +/- 1.1, and -26 +/- 1.3 per thousand, respectively were observed. In the control experiment (no addition of nitrite) a sulfur enrichment factor epsilon of around -11 per thousand was calculated. In the experiments that contained no 18O enriched water (delta18O: -10 per thousand) and nitrite concentrations of 0.02, 0.05, and 0.1 mM, delta18O values in the remaining sulfate were fairly constant during the experiments (delta18O sulfate: approximately equal to 10 per thousand) and were similar to those obtained from the control experiment (no nitrite and no enriched water). However, in the batch experiments that contained 18O enriched water (+700 per thousand) and nitrite concentrations of 0.05 and 0.1 mM increasing delta18O values in the remaining sulfate from around 15 per thousand to approximately 65 and 85 per thousand, respectively, were found. Our experiments that contained isotopic enriched water and nitrite show clear evidence that the ratio of forward and backward fluxes regulated by adenosine-5'-phosphosulfate reductase (APSR) controls the extent of sulfur isotope fractionation during bacterial sulfate reduction in strain Desulfovibrio desulfuricans. Since the metabolic sulfuroxy intermediate SO3(2-) exchanges with water, evidence of 18O enriched water in the remaining sulfate in the experiments that contained nitrite also demonstrates that SO3(2-) recycling to sulfate affects sulfur and oxygen isotope fractionation during bacterial sulfate reduction to some extent. Even though reduction of adenosine-5'-phosphosulfate (APS) to sulfite of -25 per thousand was not fully expressed, SO3(2-) was recycled to SO4(2-). On the basis of the results of this study a sulfur isotope fractionation for APSR of upto approximately -30 per thousand can be assumed. However, reported NO2(-) concentrations of up to 20 microM in freshwater and marine habitats may not significantly impact the ability to use stable isotope analysis in assessing bacterial sulfate reduction