New greenhouse gas-eating bacteria found in highly acidic sulphur cave

A team of speleologists, ecologists and microbiologists has identified a unique organism in samples from a Romanian cave nicknamed 'Stinky Mountain'. The novel bacteria can feed on methane, an important greenhouse gas that contributes to global warming. The discovery means greenhouse gas projections may have to be recalculated.

When speleologists took samples from the greyish film covering the walls of the Sulphur Cave in eastern Transylvania, they didn't expect to find anything alive. But a team from VU Amsterdam, Utrecht University and NIOO-KNAW discovered the samples were teeming with bacteria belonging to the same genus as the pathogens that cause tuberculosis and leprosy.

These hitherto unknown bacteria managed to survive the extremely acidic circumstances in the volcanic cave by feeding on methane. Other methane-eating bacteria do exist, but this is the first species in the phylum Actinobacteria. Closely-related species from the same group are known to be widespread in other high-methane environments.

That's why the researchers believe the discovery may hold an important key to breaking down methane worldwide. Their findings have just been published in the scientific journal Nature Microbiology.

Valuable to our climate

One of the lead researchers in the consortium is NIOO's Paul Bodelier. He has successfully grown the novel bacteria in the laboratory, mimicking the Romanian cave’s gas composition and confirming that the bacteria can indeed survive exclusively on methane.

Paul Bodelier: "So far, we knew about only two groups of methane-eating bacteria that are aerobic, which means they live in open, oxygen-rich air just like we do: Protobacteria and Verrucobacteria.  We now have a completely unrelated group from the genus Mycobacterium. It's a major addition to a category of microbes that is of great value to our climate."

Bodelier says it had long been suspected that some mycobacteria could have this ability, but there was no proof until now. The acidity of the cave, he says, is comparable to that of our own gastrointestinal tract. But mycobacteria are also widespread in the soil, "so we may expect to find species there that have the same ability as this one."

The lab experiments at NIOO proved to be quite tricky, says Bodelier. The bacteria grew very slowly, and only at pH levels of lower than 4. "They really do love acidic conditions!" He also found that out of the three Mycobacterium-species found in the cave, only this one fed on methane. "We believe it provides energy and carbon dioxide for the entire community of micro-organisms in the cave's biofilm."

Important piece of the puzzle

The discovery has consequences for our understanding of the dynamics of methane emissions, believes Bodelier. "There are still major gaps in our knowledge, and this could be an important piece of the puzzle. We tend to focus on what we know, but now there's this whole new species that has so far been outside our analyses when it comes to breaking down methane."

The fact that these bacteria are 'extremophiles' doesn't matter according to the NIOO-researcher. "They occur in places where there's a lot of methane. The other methane-eating organisms we know of can't cope with such extreme circumstances." Bodelier says evidence is now emerging from other research groups that some other mycobacteria can also feed on methane, under less extreme circumstances. "And besides, isn't the fact that it's from a genus that also includes notorious pathogens just cool?"

The most obvious practical use for the researchers' discovery, believes Bodelier, is in improving methane emissions models. "These bacteria grow very slowly, so I don't think they're suitable for biotechnical applications. But if we want to use models to accurately track and predict changes in methane emissions worldwide, and if we believe that traits of specific organisms play a key role, then of course it helps if we at least know what they are."