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

Abstract

Sulfate-reducing bacteria contribute considerably to the degradation of organic matter in sewage contaminated soils, particularly below leaking sewers. Molybdate as a specific inhibitor of sulfate reduction is known to be present in sewage. Its influence on sulfur isotope fractionation during sulfate reduction was explored in batch experiments with pure cultures of Desulfovibrio desulfuricans and with natural populations enriched from sewage-contaminated soil. Results with D. desulfuricans show that molybdate (0.1 mmol/l) caused a decrease of 6‰ in the isotope enrichment factor compared to an uninhibited control. The decrease in sulfur isotope fractionation may be explained by a depletion of ATP resulting in a lesser amount of activated sulfate available for sulfate reduction in the organism. Experiments carried out at 15 and 37°C reveal a decrease of about 4‰ in the isotope enrichment factor at the low temperature, which is attributed to limited uptake of sulfate. The sulfate-reducing enrichment cultures have fractionated sulfur isotopes to an extent that lies within the range of that produced by the pure cultures of Desulfovibrio desulfuricans (ϵ = −13.5‰). Furthermore, the results demonstrate the influence of bacterial growth on development of the isotope enrichment factor and its possible changes during a batch-type experiment.

Keywords:

• cell growth
• isotope fractionation
• molybdate
• sewage
• soil
• sulfate reduction
• sulfur isotopes

The authors wish to thank Dr. C. Gallert (Institut für Ingenieurbiologie und Biotechnologie des Abwassers, Universität Karlsruhe) for assistance concerning the microbiological part of this work. We also thank Dr. M. A. Leosson and G. Preuss (Institut für Mineralogie und Geochemie, Universität Karlsruhe) for assistance in isotope analyses. Support for this study was provided by the “Deutsche Forschungsgemeinschaft,” within the research group “Kanalleckage” (“Risk potential assessment of leaky sewers”). S. Koydon was a recipient of a DAAD exchange grant.

Notes

¹ No. of bacteria (bac) at day 5.

² No. of bacteria (bac) at day 2.

³ Phase 1 = changes during growth of propionate forming bacteria; phase 2 = changes during growth of sulfate-reducing bacteria; results of experiment C with MoO₄ ²⁻ are comparable to those of phase 1 and therefore not displayed.

⁴ Only sulfate-reducing bacteria.