Methane is a powerful greenhouse gas and is responsible for about a third of the current global temperature rise. This gas, which is released from the seabed and many bodies of water, would probably be even more harmful to the climate if it were not stopped by certain bacteria. These microbes absorb the methane and use it to generate energy in their metabolism. Biologists have now discovered how the tiny beneficial organisms manage to do this even in oxygen-free water.
After carbon dioxide, methane is the most harmful greenhouse gas in our atmosphere. It is responsible for about a third of the global rise in temperature. We humans release it, for example, through animal husbandry or through landfills, but there are also large natural sources such as oceans and lakes. Fortunately, these are equipped with their own methane filter: bacteria that need methane for their growth and to generate energy and thus prevent parts of the gas from entering the atmosphere.
Bacteria fishing in Switzerland
Despite their important function as a natural methane sink, there are still many unanswered questions about the exact way in which such methanotrophic bacteria live. To learn more about the metabolic processes of these microorganisms, researchers led by Sina Schorn from the Max Planck Institute for Marine Microbiology in Bremen have now studied aerobic methane-oxidizing bacteria (MOB) in Lake Zug in Switzerland. The lake is almost 200 meters deep, but permanently stratified, so that its water is oxygen-free from a depth of about 120 meters. Methane-oxidizing bacteria should not normally be found here, as they depend on oxygen for their metabolism, but they live here anyway. How do they manage that?
To uncover the secrets of these bacteria, Schorn and her colleagues took seawater samples and added methane molecules that they had previously labeled with heavy carbon atoms. Using special instruments, they were then able to use molecular biology to track how and whether the various bacteria in the water samples metabolized the methane. It was found that only a certain group of MOB – recognizable by their elongated cell shape – was permanently active even in oxygen-free water. “To our surprise, these cells were equally active under oxic and anoxic conditions – that is, with and without oxygen,” explains Schorn. “So if we measure lower rates of methane oxidation in anoxic waters, this is probably because there are fewer of these special rod-shaped cells there and not because the bacteria are less active.”
Metabolic tricks against oxygen deficiency
The researchers were able to use the genes of the microbes to determine how this group of aerobic methane-oxidizing bacteria thrives, even when oxygen is scarce. For example, they found some genes for denitrification, which probably enables the bacteria to use nitrate instead of oxygen to generate energy. Schorn and her colleagues also discovered genes for a special methane-based fermentation that converts methane into fatty acids such as acetate and lactate and into hydrogen when there is a lack of oxygen, thus ensuring the bacteria’s survival. During this form of fermentation, the microbes probably also release substances that can in turn be used by other bacteria in the lake and incorporated into their cells.
“The carbon contained in the lake, which originally comes from the climate-damaging methane, is retained in the lake for even longer and does not enter the atmosphere. This is a previously unconsidered sink for methane carbon in anoxic habitats, which we will have to include in our calculations in the future,” says senior author Jana Milucka from the Max Planck Institute for Marine Microbiology.
Source: Max Planck Institute for Marine Microbiology; Article: Nature Communications, doi: 10.1038/s41467-024-49602-5