Abstract

The bioavailability of Fe oxides critically governs microbial redox processes in natural and engineered systems. Acidimicrobium sp. A6 (A6), an autotrophic Feammox bacterium that oxidizes ammonium anaerobically coupled to Fe(III) reduction, has attracted interest for its potential relevance in the bioremediation of per- and polyfluoroalkyl substances (PFAS). However, its role in modulating Fe mineral transformations and interactions with other iron reducing bacteria under environmentally relevant conditions remains unclear. Here, we investigated Fe oxide transformations and microbial activity in A6, Geobacter sulfurreducens, and co-cultures of the two organisms amended with ferrihydrite or goethite across contrasting pH and nutrient conditions. Under acidic, oligotrophic conditions, Feammox activity was sustained while G. sulfurreducens was inhibited, demonstrating that selective A6 stimulation can be achieved through environmental control. Under circumneutral, nutrient-rich conditions, A6 and G. sulfurreducens coexisted, especially in goethite-amended systems. Fe K-edge X-ray absorption spectroscopy showed that A6 facilitated the transformation of crystalline goethite into less crystalline goethite, along with the emergence of Fe(II)-bearing phases, consistent with enhanced Fe(III) accessibility. Correspondingly, goethite-amended incubations supported sustained Feammox and stimulated acetate oxidation in co-cultures, highlighting the role of A6 in promoting Fe bioavailability for heterotrophic Fe(III) reducers. Together, these findings establish A6 as a robust Feammox performer and biological modulator of crystalline Fe oxides, supporting its potential for in situ applications in Fe(III)-rich environments.