Electrode cultivation of methane-oxidising archaea reveals cytochrome nanowire structure

What happened

Anaerobic methanotrophic (ANME) archaea help regulate methane emissions in oxygen-free environments, but they have resisted pure culture. The authors report that growing a freshwater ANME on poised electrodes produced a near-axenic culture (95.8% relative abundance in the abstract), for which they propose the name 'Candidatus Methanoperedens vercellensis'.

The abstract gives a doubling time of 4.6 days at 30 °C; the Results text reports 5.5 ± 0.9 days at 30 °C. The culture used menaquinone as its respiratory electron carrier, did not use ethane, and, after electrode cultivation, preferred to reduce ferrihydrite (an iron oxide) over nitrate. A 3.3 Å cryo-electron microscopy structure of filaments on the archaeal surface showed polymerised OmcZ-type cytochrome nanowires, homologous to the OmcZ nanowires that Geobacter sulfurreducens uses for long-range (>10 μm) extracellular electron transfer. Unlike the bacterial nanowires, the archaeal ones carry a post-translational modification (described in the text as putative N-glycosylation), do not form interconnected networks, and have less solvent-exposed hemes. The archaea also assemble OmcZ into nanowires apparently without the prolyl isomerase involved in folding bacterial OmcZ.

Why it matters

Electron transfer over micrometre distances through protein filaments is central to electromicrobiology. This work offers a laboratory system for studying nanowire-mediated electron transfer in archaea, and shows an OmcZ-type structure in a very different domain of life from Geobacter.

Caveats

Culture purity varied across replicates and time points (roughly 87-96% in the Results), so 'near-axenic' is not pure. The doubling time differs between abstract and Results. The post-translational modification was not chemically verified. The paper's text also mentions dense filament networks in cryo-EM images, which the 'no interconnected networks' description does not fully reconcile. The study reports structure, not conductivity or in-situ function in natural communities. The abstract and the open-access full text on Europe PMC were consulted (the publisher page could not be read; figures were not checked). The authors declare no competing interests; funding includes NWO, ERC, NSF, DOE and NIH.