Iron reduction under oxic conditions by Microbacterium deferre sp. nov. A1-JK^T

Microbacterium deferre sp. nov. A1-JK^T is a metabolically versatile Gram-positive bacterium isolated from the oxic-anoxic interface of freshwater sediments colonised by cable bacteria. Here, we report the metabolic ability of M. deferre A1-JK^T to simultaneously reduce oxygen and soluble Fe(III), challenging the long-standing paradigm that microbial Fe(III) reduction is restricted to anaerobic conditions. Electrochemical analyses demonstrate that M. deferre A1-JK^T is capable of extracellular electron transfer (EET) mediated by secreted flavins. It retains electroactivity under mildly alkaline and halophilic conditions, reflecting its broad environmental tolerance. Genomic analyses reveal a non-canonical system involving cytochrome FccA and flavin reductase FmnA, without the involvement of conventional flavin-based extracellular electron transfer (FLEET) components. The ability to couple oxygen and Fe(III) reduction under oxic conditions demonstrates respiratory flexibility, enabling M. deferre A1-JK^T to utilize both aerobic and anaerobic processes simultaneously in fluctuating environments. These findings offer insights into microbial adaptation to dynamic geochemical gradients and potentially explain strategies used by bacterial life during oxygenation of Earth's atmosphere.