Assembly Dynamics and Functional Divergence of Anaerobic Communities Driven by Iron Oxides

Iron oxides play an important role in regulating global biogeochemical cycles, yet how their physicochemical properties influence community structure, function, and assembly remains poorly understood. Here, we investigated the effects of four representative iron oxides-ferrihydrite (Fh), goethite (Gt), haematite (Ht), and magnetite (Mt)-serving as terminal electron acceptors on microbial communities enriched from activated sludge. Metagenomic profiling revealed mineral-dependent divergence in community composition and functional potential. The communities developed on amorphous Fh demonstrated low microbial diversity and were heavily dominated by Pseudomonas_A. In contrast, the communities associated with crystalline oxides (Gt and Ht) maintained intermediate diversity. Notably, the Mt. communities exhibited a more polycentric structure, possessing the highest richness and evenness, alongside a significant enrichment of Geobacter. Methane was detected in crystalline-oxide systems but remained below detection in Fh systems. Mineralogical analyses revealed substantial Fe(III) reduction and secondary mineral formation across treatments. Null-model analysis of pairwise turnover supports mineral-associated community turnover while highlighting that the relative contributions of selection versus undominated processes vary among minerals. Overall, these findings demonstrate that iron oxide identity is associated with differences in anaerobic community structure and biogeochemical outcomes.