Extracellular RNA drives Electromethanogenesis in a Methanogenic Archaeon

Abstract

Methanogenic archaea account for two-thirds of global methane emissions. Some species, including Methanosarcina barkeri, reduce CO2 by directly acquiring electrons from solid substrates. However, the mechanism of electron acquisition in M. barkeri has remained unclear because this archaeon lacks the multiheme c-type cytochromes that drive extracellular electron transfer in many other microbes. Here we show that M. barkeri releases abundant extracellular nucleic acids during early growth, primarily short RNAs (78%). These extracellular nucleic acids assemble into G-quadruplexes (G4s) and B-DNA architectures that decorate cell surfaces and link aggregates. Surface-associated G4s are folded in vivo in a conformation compatible with cofactor binding and redox chemistry. Enzymatic degradation of extracellular nucleic acids abolished electron uptake and electromethanogenesis, whereas addition of synthetic G4-RNAs doubled methane yields and lowered cell-electrode interfacial resistance. These effects were not observed when cells were grown on soluble substrates. Together, these findings identify eRNA as a previously unrecognized electron conduit in methanogens, raising the possibility that RNA-based electron transfer may predate more elaborate protein-based electron conduits, with implications for models of early earth metabolism and for the design of next-generation bioenergy systems.