Iron-based nanoparticles (Fe-NPs), often applied for pollutant remediation, inevitably interact with various organisms either directly or indirectly. The well-acknowledged toxicological mechanisms for Fe-NPs on organisms are causing oxidative damage and...

Microbial respiration using humus plays a critical role in organic matter decomposition and biogeochemical cycling of elements in diverse natural environments. A longstanding recognition is that microbial respiration is achieved through extracellular electron transfer mediated by electrical pili or c-type cytochromes. Here, we describe the capability of humus-dependent respiratory growth of a methanogen, namely Methanosarcina acetivorans, which is a model for studying acetoclastic methanogenesis. A previously unexplored gene cluster that encodes cell surface quinoproteins was identified as being responsible for extracellular electron transfer. Pyrroloquinoline quinone was found to be a cofactor for the quinoproteins, providing electrical interplay sites with quinone moieties of humus. The membrane-bound methanophenazine, which is analogous to ubiquinone, participated in extracellular electron transfer, being potentially coupled with energy-conserving humus respiration. Given that these quinoproteins are widely distributed in methanogens, our findings extend the current understanding of microbial humus respiration in the context of global methane dynamics.