Nano-Fe3O4 particles accelerating electromethanogenesis on an hour-long timescale in wetland soil

Electromethanogenesis, which is different from traditional hydrogenotrophic and acetoclastic methanogenesis, is a novel means of methane production involving the direct transfer of electrons to methanogens. Nano-Fe3O4 (nano-magnetite) has been found to strengthen the link between electricity-producing bacteria and methanogens; however, whether nano-Fe3O4 changes the carbon- and/or electron-flow pathways involved in the methanogenic processes in natural soil remains unknown. Experiments on wetland soil were reported in this work. Experimental results showed that nano-Fe3O4 particles significantly stimulated methane production. Combining the application of a stable isotope tracer ((CH3COOH)-C-13) and an acetoclastic methanogenesis inhibitor (CH3F) and thermodynamic calculations, the results indicated that nano-Fe3O4 accelerated the CO2 reduction process rather than acetoclastic methanogenesis. Comprehensive utilization of a variety of methods (thermodynamic calculations, current density measurements, and model analysis) suggested that electrons, which came from syntrophic acetate oxidation (SAO), were the reducing agents for methane production. Nano-Fe3O4 did not appear to change the carbon flow but did promote electron transfer, and the whole process can be finished within an hour-long timescale. Furthermore, a dramatic increase in acetoclastic methanogenesis was observed within only 9 h incubation. This suggests that the methanogenic pathways are flexible and transitory in natural soil. Coupled with cDNA sequencing results for the active bacteria and archaea, it is suggested that SAO and electromethanogenesis were preferred when Geobacter and Methanosarcina were involved. These findings add to the knowledge of the role of nano-Fe3O4 in methanogenic processes in natural and artificial environments.