Carbon nanotubes accelerate acetoclastic methanogenesis: From pure culture to anaerobic soil

Abstract Direct interspecies electron transfer (DIET) between electricigens and methanogens has been shown to favour methane production. Furthermore, DIET is accelerated by conductive materials. However, whether conductive materials can promote other methanogenic pathways is unclear due to a lack of detailed experimental data and the poor mechanistic studies. Here, we hypothesized that conductive carbon nanotubes (CNTs) stimulate acetoclastic methanogenesis independent of electricigens in pure cultures of Methanosarcina spp. and anaerobic wetland soil. A statistically significant increase in the methane production rate was observed in the logarithmic phase, e.g. from 0.169 mM to 0.241 mM after addition of CNTs on the 3rd day. CNTs did not increase the abundance of electromicroorganisms or the electron transfer rate in anaerobic soil via microbial diversity and electrochemical analysis. 13C–CH3COOH labelling, stable carbon isotope fractionation and the CH3F inhibitor of acetoclastic methanogenesis were used to distinguish methanogenic pathways. CNTs mainly accelerated acetoclastic methanogenesis rather than CO2 reduction in both pure culture and anaerobic soils. Furthermore, the presence of CNTs can slightly alleviate the inhibition of CH3F on acetoclastic methanogenesis through the pure culture of Methanosarcina barkeri and Methanosarcina mazei with the production of more than 0.3 mM methane. CNTs closely attached to the cell surface manifested by Transmission electron microscopy. Proteome analysis presented a stimulation of protein synthesis with about twice the improvement involved in –COOH oxidation and electron transfer. Overall, a potential strategy of conductor-enhanced acetate dismutation for methane production is proposed, and this study increase the understanding of the methane production process in anaerobic ecosystems.