In the fight against climate change, a lesser-known battle is taking place deep within our lakes. Methane, a potent greenhouse gas, is being held at bay not by policy or technology but by microscopic warriors—methanotrophs. These remarkable microorganisms have the unique ability to consume methane, preventing its escape into the atmosphere—earning them the title of f "biological methane filters"—and offering a natural solution to one of the most pressing environmental challenges of our time. As scientists explore the capabilities of these tiny climate protectors, a new understanding of their crucial role is emerging, one that could reshape our approach to mitigating greenhouse gas emissions.

Methanotrophs comprise various groups of microorganisms, each with distinct characteristics. Despite their importance, much about their behavior remains a mystery. A recent study conducted by researchers from the Max Planck Institute for Marine Microbiology in Bremen, Germany, and the Swiss Eawag and published in Nature Communications, sheds light on the extraordinary capabilities of these organisms and their crucial yet underappreciated role in climate regulation.

Methane Consumption in Oxygen-Starved Waters

The research team, led by Sina Schorn and Jana Milucka from the Max Planck Institute, ventured to Lake Zug in Switzerland for their study. This lake, with a depth nearing 200 meters, is devoid of oxygen from around 120 meters down. Surprisingly, the researchers found aerobic methane-oxidizing bacteria (MOB) thriving in these oxygen-free waters. As their name suggests, these bacteria typically rely on oxygen to break down methane. However, how they managed to function in an oxygen-starved environment was previously unknown.

To unravel this mystery, the researchers employed methane molecules labeled with “heavy” carbon atoms (13C) and introduced them into natural lake water samples containing the microorganisms. Using advanced instruments like NanoSIMS, they tracked the path of the heavy carbon within individual cells, observing how the bacteria converted methane into carbon dioxide, a less harmful greenhouse gas. They also discovered that some carbon was absorbed directly into the bacterial cells, identifying the active members of the microbial community. Through modern techniques like metagenomics and metatranscriptomics, they further explored the metabolic pathways employed by these bacteria.

Discovering the Unusual Capabilities of Methanotrophs

“Our results show that aerobic MOB remains active even in oxygen-free water,” explains Sina Schorn, now a researcher at the University of Gothenburg. Interestingly, only a specific group of MOB, identifiable by their distinct rod-shaped cells, maintained activity in both oxygen-rich and oxygen-poor conditions. Lower methane oxidation rates in anoxic waters, the team discovered, were due not to reduced bacterial activity but to the scarcity of these specialized rod-shaped cells.

The researchers also uncovered a surprising level of metabolic versatility in these bacteria. “We identified genes that activate when oxygen is scarce, enabling a unique type of methane-based fermentation,” says Jana Milucka, head of the Greenhouse Gases Research Group at the Max Planck Institute. Although this fermentation process had been observed in laboratory MOB cultures, its presence in natural environments had not been confirmed until now. Additionally, the bacteria were found to possess genes for denitrification, suggesting they could use nitrate in place of oxygen to generate energy.

The implications of this fermentation process are particularly intriguing. If MOBs engage in fermentation, they may release compounds that other bacteria can utilize for growth, effectively retaining methane carbon within the lake for longer periods and preventing its release into the atmosphere. This represents an often-overlooked methane carbon sink in oxygen-free environments, which future climate models must consider.

A Critical Role in Mitigating Methane Emissions

This study illuminates the crucial role that methane-oxidizing bacteria play in controlling methane emissions from oxygen-free habitats. The findings highlight the surprising importance of these microorganisms in mitigating methane release into the atmosphere.

“Methane is a potent greenhouse gas responsible for about a third of the current global temperature rise,” says Schorn, underscoring the significance of their research. “Microbial methane oxidation is the only biological mechanism for removing methane. The activity of these microorganisms is, therefore, vital for regulating methane emissions and, consequently, the global climate. With the expected increase in anoxic conditions in temperate lakes, the importance of MOB in methane degradation is likely to grow. Our results suggest that MOB will play a significant role in future efforts to reduce greenhouse gases and enhance carbon storage.”