The role of iron in enhancing anaerobic toluene degradation in sulfate-reducing enrichment cultures

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Abstract
Ferrous iron enhanced the toluene degradation rate of sulfidogenic enrichment cultures inoculated with contaminated subsurface soil from an aviation fuel storage facility near the Patuxent River (Md.). Ferrous iron had an analogous effect on the degradation rate of benzoic acid, a transient metabolite of anaerobic toluene degradation in these cultures, when benzoic acid was used as a sole carbon and energy source. Two hypotheses were proposed to explain iron's effect: (a) Iron may have prevented sulfide toxicity via precipitation of sulfide as FeS, and (b) iron might have been a limiting nutrient required for degradation (i.e., amendments of iron could have compensated for iron removed from solution by precipitation as FeS). To test these hypotheses, substrate degradation rates were compared in the presence of FeSO4 (a sulfate source that both precipitates sulfide species and precludes iron limitation) versus ZnSO4 (a sulfate source that precipitates sulfide species but does not preclude iron limitation) versus MgSO4 (a sulfate source that neither precipitates sulfide nor precludes iron limitation). For both toluene and benzoic acid, FeSO4 and ZnSO4 were comparable in their enhancement of substrate degradation rates and were superior to MgSO4 in that respect. Thus, iron appears to ameliorate sulfide toxicity, not nutritional iron limitation, in these cultures. The observation that ethylenediaminetetraacetic acid, a chelating agent capable of retaining iron in solution in the presence of sulfide, did not stimulate the cultures is consistent with this conclusion. The implications of these results for bioremediation of fuel-contaminated aquifers that contain sulfate-reducing bacteria are discussed.