Iron-respiring bacteria mediate regulatory mechanisms and pathways of electron donors and acceptors in carbon and nitrogen cycles

ABSTRACT Iron-respiring bacteria play important roles in global material cycles owing to their electron transfer capabilities, yet their electron-transfer connections to carbon and nitrogen cycles in complex environments remain unclear. Through the investigation of sediments, this study found multidimensional associations between the abundance of iron-respiring bacteria and microbial community structure, as well as carbon and nitrogen cycles. At the community level, the abundance of iron-respiring bacteria showed negative correlations with certain populations, implying a potential influence on population expansion through competition for electron donors. Meanwhile, iron-respiring bacteria might have a symbiotic role that promoted the Shannon index. In terms of cycles, iron-respiring bacteria and carbon metabolism mutually promoted each other, with the mdh gene contributing 4.2% to these bacteria. In the nitrogen cycle, the iron transport sit gene was widely present in areas with high ammonia nitrogen concentrations. This spatial coupling suggests that it may provide necessary conditions for iron reduction-coupled anaerobic ammonium oxidation (Feammox). Network analysis revealed that iron-respiring bacteria serve as core nodes coupling carbon and nitrogen cycling functional modules. Through potential pathways such as electron-shuttle mediation and their own multifunctional metabolism, they may promote the synergy of carbon and nitrogen reactions. The abundance of iron-respiring bacteria was significantly positively correlated with the electron exchange capacity (EEC) of sediments ( R ² = 0.797), suggesting their possible involvement in forming long-distance electron transfer networks. This study broadens the understanding of potential coupling mechanisms between iron-respiring bacteria and carbon-nitrogen cycling under varying environmental factors and offers new insights into the management of sediments. IMPORTANCE Iron‑respiring bacteria in sediments have the potential to drive carbon and nitrogen cycles through electron transfer. They link these cycles by using organic carbon or ammonium as electron donors and iron or nitrate as electron acceptors. Moreover, these bacteria may also produce mobile electron shuttles and have the potential to build long‑distance electron transfer networks within sediments. Electron exchange benefits the metabolism of surrounding microorganisms, helping them decompose recalcitrant contaminants. By understanding the direction of electron flow, we can learn whether the bacteria convert nitrogen into harmless gases or retain it as ammonium, and whether carbon is released as carbon dioxide or stored as carbohydrates. Understanding this electron‑driven coupling provides a scientific basis for the management of lake sediments.