Some anaerobic bacteria can respire using both ferric iron (FeIII) and elemental sulfur (S0) as electron acceptors, yet the environmental significance of the latter is often overlooked. In the presence of sulfate-reducing bacteria, however, dissolved sulfide reacts with ferric minerals such as ferrihydrite and goethite to produce two elemental sulfur atoms for each sulfide reacted. Bacteria capable of reducing both FeIII and S0 often co-occur with sulfate-reducing (and sulfide-producing) bacteria in anoxic sedimentary environments, suggesting a mechanism whereby sulfide might react with ferric minerals. We model the thermodynamic energy available from these respiratory pathways and find that while the reduction of FeIII minerals becomes less energetically favorable with increasing pH, the reduction of S0 remains strongly exergonic. We suggest that under alkaline conditions, FeIII reduction may not proceed by direct enzymatic reduction by nominally �iron-reducing� bacteria, but rather by these same bacteria reducing the S0 produced by sulfide-generating sulfate reducers.

Geochemical explanation for the prevalence of sulfur reduction among nominally iron-reducing bacteria

245th National Meeting (2013)

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