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How Microorganisms organise their power supply via nano-wires to oxidise the greenhouse gas methane

Electrical energy from the socket - this convenient type of power supply is apparently used by some microorganisms that form nanowire connections to transfer energy. Researchers have now discovered such small power grids between dual-species microbial consortia that jointly degrade methane. Using genetic methods and electron microscopy the scientists demonstrated how the wire-like connections between the cells are relevant in energy exchange. Now the researchers report their findings in the journal Nature.

Since more than a decade, scientists from Bremen, Germany, study how methane in the seabed is degraded by microorganisms. So far, it has remained a mystery how the methane consumers tap into the energy contained in methane, and how they could share that energy. New results show that direct interspecies electron transfer between cells is likely the solution to this puzzle.

It was a spectacular finding when in 2010 researchers discovered electrical wiring between microorganisms using iron as energy source. Immediately the question came up, if electric power exchange is common in other microbially mediated reactions. One of the processes in question was the anaerobic oxidation of methane (AOM) that is responsible for the degradation of the greenhouse gas methane in the seafloor, and therefore has a great relevance for Earth climate. The microorganisms involved have been described for the first time in 2000 by researchers from Bremen and since then have been extensively studied.

In the ocean, methane is produced from the decay of dead biomass in subsurface sediments. The methane rises upwards to the seafloor, but before reaching the water column it is degraded by special consortia of archaea and bacteria. The archaea take up methane and oxidise it to carbonate. They pass on energy to their partner bacteria, so that the reaction can proceed. The bacteria respire sulphate instead of oxygen to gain energy (sulphate reducers). This may be an ancient metabolism, already relevant billions of years ago when the Earth’s atmosphere was oxygen-free. Yet, today it remains unknown, how the anaerobic oxidation of methane works biochemically.
Dr. Gunter Wegener, who authors the publication together with PhD student Viola Krukenberg, says: “We focused on thermophilic AOM consortia living at 60 degrees Celsius. For the first time we were able to isolate the partner bacteria to grow them alone. Then we systematically compared the physiology of the isolate with that of the AOM culture. We wanted to know which substances can serve as an energy carrier between the archaea and sulphate reducers.” Most compounds were ruled out quickly. At first, hydrogen was considered as energy source. However, the archaea did not produce sufficient hydrogen to explain the growth of sulphate reducers – hence the researchers had to change their strategy.

One possible alternative was to look for direct connections channelling electrons between the cells. Using electron microscopy on the thermophilic AOM cultures this idea was confirmed. Dietmar Riedel, head of electron microscopy facilities at the Max Planck Institute in Goettingen says: “It was really challenging to visualize the cable-like structures. We embedded aggregates under high pressure using different embedding media. Ultrathin sections of these aggregates were then examined in near-native state using transmission electron microscopy.”
Viola Krukenberg adds: "We found all genes necessary for biosynthesis of the cellular connections called pili. Only when methane is added as energy source these genes are activated and pili are formed between bacteria and archaea.”

Publication:
Intercellular wiring enables electron transfer between methanotrophic archaea and bacteria. Gunter Wegener, Viola Krukenberg, Dietmar Riedel, Halina E. Tegetmeyer  and Antje Boetius. Nature, 2015 doi:10.1038/nature15733

Involved research institutes:
Max Planck Institute for Marine Microbiology, Bremen, Germany
Center for Biotechnology, Bielefeld University, Bielefeld, Germany
MARUM, Center for Marine Environmental Sciences, University Bremen, Germany
Max Planck Institute for Biophysical Chemistry, Göttingen, Germany
Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany

Contact:
Dr. Gunter Wegener
Max Planck Institute for Marine Microbiology, Bremen
Telefon: 0421 2028 867
E-Mail: gwegener@mpi-bremen.de

Dr. Halina E. Tegetmeyer, Universität Bielefeld
Centrum für Biotechnologie (CeBiTec)
Telefon: 0521 106-12261
E-Mail: htegetme@cebitec.uni-bielefeld.de