Cellulose Reshapes Microbial C–S–Fe Coupling to Suppress Direct and Fe(III)-Mediated Pyrite Oxidation

Pyrite oxidation generates sulfate and acidity in sulfide-bearing mine soils, but how structural carbon suppresses this process remains unresolved. We combined a 56-day pyrite-amended soil microcosm with geochemical measurements, qPCR, metagenomics, DNA stable isotope probing (DNA-SIP), genome-resolved analysis, and apparent kinetic modeling. Cellulose decreased NaOH-extractable sulfate by 72.71% and HCl-extractable Fe(III) by 55.13% at day 56. DNA-SIP showed that cellulose-derived carbon entered a phylogenetically distributed assimilating guild with genomic potential for oxidative-hydrolytic cellulose deconstruction, Fe(III) reduction, sulfate uptake, and assimilatory sulfate reduction. Enriched metagenome-assembled genomes collocated these C, Fe, and S functions within individual populations, whereas time-series metagenomes showed sustained cellulose-deconstruction and sulfur-assimilation potential without broad enrichment of Fe-reduction genes. Thus, functionally important assimilators could reshape Fe–S cycling without consistently dominating bulk community abundance. Consistent with these genomic and geochemical patterns, the apparent model indicated weaker O2-driven direct oxidation, Fe(II) oxidation/Fe(III) regeneration, and effective reactivity of extractable Fe(III). These results identify a microbial mechanism whereby cellulose utilization redirects C–S–Fe coupling toward more reductive and assimilatory transformations, suppressing pyrite oxidation without detectable mineral-surface coating.