Electrode-associated growth reveals structure of OmcZ-type cytochrome nanowires in anaerobic methanotrophic archaea

Anaerobic methanotrophic (ANME) archaea are key regulators of methane emissions in anoxic environments, yet their physiology remains poorly understood due to the lack of axenic cultures. Here, we show that cultivation on poised electrodes yields near-axenic cultures (95.8% relative abundance) of a freshwater ANME, for which we propose the name ‘ Candidatus Methanoperedens vercellensis’. We identified key physiological properties of ‘ Ca . M. vercellensis’: doubling times of 4.6 days at 30 °C, menaquinone as respiratory electron carrier, the inability to oxidize ethane, and preferential reduction of iron oxides over nitrate after cultivation on electrodes. A 3.3 Å cryo-electron microscopy structure of filaments displayed on the archaeal surface revealed polymerized OmcZ-type cytochrome nanowires, which are homologous to OmcZ nanowires essential for long-range (>10 μm) extracellular electron transfer (EET) in Geobacter sulfurreducens . The archaeal OmcZ-type nanowires include dense, collinear stacking of protected hemesakin to bacterial OmcZ nanowires, but with post-translational modification, thus yielding morphology distinct from bacterial OmcZ nanowires. Notably, in contrast to bacterial OmcZ, archaeal OmcZ-type nanowires do not form interconnected networks, and hemes are less solvent-exposed, which can protect them from the environment. Surprisingly, these archaea assemble OmcZ into nanowires even in the apparent absence of prolyl isomerase involved in folding bacterial OmcZ. This work establishes a model system for molecular understanding of nanowire-mediated EET in archaea.