Methane-consuming archaea reveal the structure of their nanowires
Researchers grew methane-consuming archaea on electrodes and resolved the structure of protein filaments extending from their surfaces. The peer-reviewed laboratory study found closely spaced iron-containing groups inside these nanowires, a possible route for electrons leaving the cells. The work also established a culture dominated by the target organism for further microbiological experiments.
Science··Midday
Growing on an electrode
Archaea, single-celled microorganisms distinct from bacteria, include species that consume methane without oxygen. A peer-reviewed laboratory study grew one such freshwater organism on electrodes and imaged nanowires extending from its surface. Protein filaments were visible on the surfaces of cells growing in the culture. The work examines how electrons released during methane consumption leave the cell and reach an external acceptor.[1], [2]
The starting culture came from a rice paddy in Vercelli, northern Italy. Researchers supplied methane and used a carbon-felt electrode held at a controlled electrical potential as the electron acceptor. Two parallel systems were followed for 352 days. Selective growth conditions, including antibiotics that suppressed bacteria, raised the target archaeon’s share of the community to 95.8 per cent, providing a much less mixed culture for experiments.[1]
Iron groups inside protein filaments
The team proposes the name Candidatus Methanoperedens vercellensis for this methane-consuming archaeon. Microscopy showed filaments attached to its cells. A purified filament of the OmcZ protein family was resolved by cryo-electron microscopy, which images frozen samples, at 3.3 angstrom resolution. Its hemes, iron-containing groups that can carry electrons, formed a compact and nearly straight chain inside the surrounding protein.[1]
The authors interpret that close packing and protection as features that could aid electron transfer. Conductivity of individual filaments was not directly measured. The culture still contained small populations of other microorganisms, and limited biomass restricted further chemical measurements. Genetic tools for removing the relevant protein were also unavailable, preventing that direct test of its role in the archaeon’s physiology.[1]
Current follows methane supply
Removing methane sharply reduced the current, supporting methane oxidation as the electrical source. Supplying ethane did not produce the corresponding increase. After adapting to electrodes, the culture used iron oxide more readily than nitrate as an electron acceptor. These physiological observations accompany the structural images, establishing a laboratory system for studying the organism’s metabolism and the protein machinery associated with electron transfer.[1]
