Methane-eating bacteria could have a more significant impact than we previously realized in stopping the climate-harming methane emissions from lakes, researchers from Bremen have reported. Their study clarifies who is involved in this process and how it functions.
Methane is a powerful greenhouse gas that is often generated in both marine and freshwater environments. Lakes, in particular, emit substantial amounts of this dangerous gas. Thankfully, there exist microorganisms that mitigate this issue by consuming methane for growth and energy production, preventing its escape into the atmosphere. These microorganisms, referred to as methanotrophs, are viewed as a crucial “biological filter” for methane.
Methanotrophs consist of several groups of microorganisms, and significant gaps still exist in our understanding of their lifestyles. A recent study conducted by researchers from Germany’s Max Planck Institute for Marine Microbiology in Bremen and Switzerland’s Eawag, now published in the journal Nature Communications, unveils the remarkable capabilities of some of these organisms and their previously underestimated contribution to climate stabilization.
Aerobic organisms in low-oxygen waters
For this research, scientists led by Sina Schorn and Jana Milucka from the Max Planck Institute ventured to Lake Zug in Switzerland. This deep lake, nearly 200 meters deep, remains devoid of oxygen below approximately 120 meters. Surprisingly, this anoxic water is home to aerobic methane-oxidizing bacteria (abbreviated as MOB), which typically require oxygen. Previously, it was unclear whether and how these bacteria could metabolize methane in an oxygen-free environment.
To explore the activity of these microorganisms, the team labeled methane molecules (CH4) with “heavy” carbon isotopes (13C rather than 12C) and introduced them into natural lake water samples. Subsequently, the researchers tracked the movement of the heavy carbon within individual cells using sophisticated instruments called NanoSIMS. This allowed them to observe how the bacteria converted methane into carbon dioxide—another greenhouse gas, though less harmful than methane—with part of the carbon also being integrated directly into the bacterial cells. This process helped identify which cells in the bacterial community were active and which were inactive. They also employed modern techniques such as metagenomics and metatranscriptomics to analyze the metabolic pathways the bacteria utilized.
Only one group of bacteria thrives without oxygen
“Our findings indicate that aerobic MOB can remain active in oxygen-free waters,” reports Sina Schorn, now a researcher at the University of Gothenburg. “However, this activity is limited to a specific group of MOB that are easily identified by their unique rod-shaped cells. Surprisingly, these cells maintained equal activity levels in both oxygen-rich and oxygen-deprived environments. Consequently, lower methane oxidation rates in anoxic waters are likely due to fewer of these specialized rod-shaped cells being present rather than reduced bacterial activity.”
Metabolic flexibility in combating methane emissions
The Max Planck team found yet another interesting aspect when delving into the metabolic capacity of this bacterial group. “By examining the genes present, we uncovered how these bacteria react in low-oxygen conditions,” explains Jana Milucka, who leads the Greenhouse Gases Research Group at the Max Planck Institute in Bremen. “We identified genes that are associated with a unique form of methane-based fermentation.” Although this process had been documented in laboratory cultures of MOB, it had not been observed in natural ecosystems. Additionally, the researchers found genes related to denitrification, suggesting that these bacteria might substitute nitrate for oxygen to produce energy.
The fermentation process is particularly noteworthy. “If the MOB engage in fermentation, they likely release byproducts that other bacteria can use for their growth. This means that the carbon from methane is conserved in the lake for a longer time, preventing it from entering the atmosphere. This process represents an overlooked sink for methane carbon in oxygen-free environments, which we will need to factor into future calculations,” states Milucka.
Major reduction of current and future methane emissions
The Bremen researchers elucidate who is responsible for methane breakdown in oxygen-free environments and detail the mechanisms involved in this degradation. Their findings indicate that methane-eating bacteria play a surprisingly vital role in curtailing the release of methane from these habitats into the atmosphere.
“Methane is a potent greenhouse gas accounting for about one-third of global temperature rise,” Schorn explains, highlighting the importance of the newly published results. “Microbial methane oxidation constitutes the only biological mechanism for capturing methane. Thus, the activity of these microorganisms is critical for controlling methane emissions and regulating global climate. Given the increasing prevalence of anoxic conditions in temperate lakes, the role of MOB in methane degradation is anticipated to become even more crucial. Our findings suggest that MOB will significantly contribute to greenhouse gas reduction and carbon retention in the future.”
