LAUSR

Abstract Microbiological Fe(III) reduction under oxic and acidic conditions in acid mine drainage (AMD)-impacted systems is surprising, given the greater energetic favorability of O2 reduction than Fe(III) reduction. A possible explanation for this observation is that the energetic favorability of Fe(III) reduction approaches that of O2 reduction at pH < ∼2. However, some microorganisms require O2 for optimal Fe(III) reduction, and the process has still been observed at pH ≥ 3.5 under oxic conditions. We set up acidophilic Fe(III) reducing enrichment cultures with and without O2. The time of initiation and extent of Fe(III) reduction in oxic and anoxic enrichments were identical, and Fe(III) reduction by the enrichment cultures occurred to similar extents with subsequent transfers of the cultures. Desulfosporosinus- and Clostridia-attributable phylotypes dominated the anoxic enrichment cultures and Alicyclobacillus- and Acidibacillus-attributable phylotypes dominated the oxic enrichment cultures. Our results indicate that while O2 does not impact Fe(III)-reducing activities, it does impact the types of organisms that can be enriched in its presence or absence. The ability of AMD-associated organisms to reduce Fe(III) under both oxic and anoxic conditions may allow them to adapt to fluctuating O2 levels that are encountered in AMD-impacted settings.