Iron Oxides Forms and Salinity Jointly Regulate Iron Reduction and Methane Metabolism by Shifting Microbial Communities

Dissimilatory iron reduction （DIR） is a key anaerobic process linked to methane cycling in lake sediments， but how Fe（III） mineral forms influence DIR and methane metabolism across contrasting saline lake settings remains unclear. Here， we established anaerobic microcosms amended with ferrihydrite， goethite， or hematite using sediments from two Tibetan Plateau lakes with contrasting salinity: Daze Co （hyposaline） and Mang Co （hypersaline）. Fe²⁺ accumulated mainly in the solid phase， while dissolved Fe²⁺ increased early and declined later. DIR responses to Fe（III） minerals differed between lakes， and ferrihydrite transformed into more crystalline minerals in Mang Co sediments. Methanogenesis occurred in both lakes but showed a longer lag phase in Daze Co， and CH₄ accumulation depended on lake type and mineral amendment. Iron oxide additions also suppressed CO₂ accumulation and early-stage H₂ production， indicating altered metabolic competition. Microbial communities were strongly reshaped during incubation， with more diverse putative DIRB taxa enriched in Daze Co， whereas Clostridium sensu stricto dominated in Mang Co. Metagenomic analyses showed that canonical multi-heme cytochrome pathways were less abundant than quinone/flavin-associated electron-transfer modules in iron-amended sediments， suggesting that the latter may be important contributors to extracellular electron transfer under these conditions. These findings suggest that DIR-methane coupling differed between the two lake systems and was associated with mineral transformation， microbial shifts， and electron-transfer pathways under contrasting salinity backgrounds.