It was a spectacular finding when in 2010 researchers discovered electrical wiring between microorganisms using iron as their energy source. Immediately the question came up whether 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 on the seafloor, and therefore has a great relevance for the Earth's climate. The microorganisms involved have been described in 2000 by researchers from Bremen, and have been extensively studied since then.
In the ocean, the methane source is 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; a microbial consortium consists of two or more microbial groups that live symbiotically. The archaea in the deep sea 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 - they are so-called sulphate reducers. This may be an ancient metabolism, already relevant billions of years ago when the Earth's atmosphere was oxygen-free. Yet even today it remains unclear 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 an 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 that channel electrons between 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."
With lengths of several micrometres, the wires can exceed the length of the cells by far, but their diameter is only a few nanometres. These wires provide the contact between the closely spaced cells and explain the spatial structure of the consortium, as was shown by a team of researchers led by Victoria Orphan from Caltech in the same issue of Nature.
Professor Antje Boetius, head of the Max Planck Research group, as well as of the Alfred Wegener Institute research group, and professor at Bremen University, explains how the research on the wired consortia will proceed: "Consortia of archaea and bacteria are abundant in nature. Our next step is to see whether other types also show such nanowire-like connections. It is important to understand how methane-degrading microbial consortia work, as they provide important functions in nature."
(© Max Planck Institute for Marine Microbiology, AcademiaNet)