Fermentative iron reduction by a psychrotolerant Clostridium-dominant consortium enriched from Antarctic...

Microbial iron cycling regulates nutrient availability and redox balance in global ecosystems, yet its pathways remain underexplored in ice-free Antarctic terrestrial ecosystems. This study reports the enrichment of a psychrotolerant microbial consortium from penguin-impacted soils on Beaufort Island, Antarctica, capable of reducing Fe(III) to Fe(II) at 4 °C via an anaerobic (likely fermentative) iron-reducing pathway. The consortium was dominated by Clostridium sensu stricto 13 and completely reduced 230 mg L^-1 Fe(III) citrate within three months and drove the biogenic formation of magnetite (Fe₃O₄). Metagenomic binning yielded four high-quality Clostridium genomes harboring multiple hydrogenases and cold-shock proteins (csp), revealing genomic strategies for energy conservation and psychrotolerance. Hydrogen production was strongly suppressed in the presence of Fe(III) citrate, indicating an intimate coupling of fermentation-derived electron flow to Fe(III) reduction. Our findings reveal a previously unrecognized low-temperature iron reduction mechanism and highlight the ecological significance of anaerobic (likely fermentative) iron reducers in ornithogenic soils-microhabitats enriched in organic matter and metals by penguin guano. This work expands the known diversity of Fe(III)-reducing microorganisms, demonstrates their role in magnetite biomineralization under extreme conditions, and provides insights into microbial modulation of iron speciation in Antarctic ornithogenic soils.