Polski
Science, Issue 16/2025
Microorganisms in groundwater reduce methane emissions
Nov
Methane (CH4) is one of the most powerful greenhouse gases, accounting for about 30 percent of modern climate warming. The contribution of aquatic ecosystems to the budget of its natural emissions is significant, although still not fully studied. Huge amounts of this gas are contained not only in surface water, but also in groundwater, although in the case of the latter, actual emissions to the atmosphere are smaller than one might expect.
A team of researchers from Germany set out to find out why this happens. The results of their study, published in the October 13 issue of the journal Proceedings of the National Academy of Sciences (PNAS), indicate that microorganisms – an invisible army of methanotrophs that eat the methane before it can escape to the surface – play a key role in the process.
Where does methane in groundwater come from?
Methane in groundwater comes from two main sources. Most is produced by anaerobic microbial activity (methanogenesis), and a smaller portion comes from thermogenic processes, the breakdown of organic matter deposited in the earth’s crust. Scientists from leading scientific units in Germany studied two types of environments: limestone groundwater in the Hainich region, with low methane concentrations, and sandy waters near Aurich, where methane was much higher. CH4 concentrationsin the samples ranged from as low as 0.00015 mg/l to as high as 36 mg/l, a difference of as much as five orders of magnitude.
Analysis of the presence of radioactive isotopes of carbon and associated hydrocarbons (ethane and propane) has shown that the methane in Aurich is of biogenic origin – it is formed by modern microorganisms. In the limestone waters of Hainich, on the other hand, some of the gas is fossil in nature and dates back millions of years.
Invisible filter: methane oxidizing microorganisms
The only biological process capable of removing methane from the environment is oxidation by microorganisms. The researchers decided to measure the rate of this process in the field by introducing trace amounts of radioactive 14C-methaneinto collected samples. By doing so, they were able to determine how much methane was converted to carbon dioxide and how much was incorporated into the biomass of the microorganisms.
It turned out that the rate of oxidation ranged from as little as 0.001 µgC/l/d in waters in calcareous aquifers to more than 74 µgC/l/d in methane-rich waters in sandy layers. The relationship was clear: the more methane, the higher the microbial rate – the correlation coefficient was as high as 0.97.
The time required for the complete oxidation of methane varied depending on the environment: in waters in limestone layers, it averaged from 2 months to a year, and in sandy layers, from a year to more than seven years. Although the process is slow, on the time scale of groundwater flow (usually a few to several decades), it seems effective enough to essentially reduce methane emissions.
Who eats methane?
The samples analyzed were dominated by aerobic bacteria of the Type I group(Gammaproteobacteria) and archaeons of the ANME-2d clade(Candidatus Methanoperedens), capable of anaerobic methane oxidation involving iron and manganese compounds. In some samples, these microorganisms accounted for up to 25 percent of the total microbial assemblage.
The most common species were Crenothrix and Methylobacter – known for their efficient oxidation of methane in lakes and rivers. In the waters of limestone layers, where the reaction rate was lower, on the other hand, microorganisms of the genus
The authors of the study found that the vast majority of methane did not go into biomass, but was burned completely intoCO2. In only two wells was some of the methane carbon used for cell construction, 16.9 percent and 4.3 percent, respectively. Thus, for most microorganisms, methane was a source of energy, not a building material. The average rate of uptake of carbon from methane was up to ten times lower than the rate of oxidation.
Underground filter of global significance
The authors compared their results with data from 70 other studies covering lakes, rivers, estuaries, oceans and marshes. To the researchers’ surprise, the average rate of methane oxidation in groundwater (0.22 µg CH4/l/d) was in the same range as in lakes, rivers and their estuaries – and much higher than in oceanic waters.
Based on global data on the volume and composition of groundwater, the researchers estimated that microorganisms in these environments can oxidize between 167 and 778 million tons of methane per year. This means that up to two-thirds of this gas in groundwater is decomposed before reaching the surface. The scale of this process makes groundwater one of the most important, and so far underestimated, global sinks for CH4 – five times more potent than soils and up to three times more than swamps and inland waters combined.
What does this mean for the future?
The researchers stress that their data most likely underestimate the actual rate of methane oxidation because they do not take into account microorganisms settled on ground particles, which are often much more numerous than those floating in water. Nevertheless, the results make it clear: microbial filtration of methane in groundwater is a key component of the global greenhouse gas balance.
At the same time, the authors warn that the development of deep water mining may carry the risk of releasing methane trapped there. As the climate warms and groundwater temperatures rise, the activity of methanogenic microorganisms may increase, which in turn requires monitoring of this phenomenon as part of water quality assessments.
Invisible climate ally
Although microorganisms living tens of meters underground are rarely talked about, their role is proving to be invaluable. They act as a giant natural filter that reduces emissions of one of the most dangerous greenhouse gases. The study’s authors emphasize that understanding this subterranean ecosystem is not only a fascinating scientific challenge, but also an important part of a climate protection strategy – without the microscopic guardians, global warming would likely proceed even faster.
Source:
B.M. Heinze,V.F. Schwab,K. Küsel,S. Schloemer,A. Roskam,X. Xu, & S. Trumbore, Microbial oxidation significantly reduces methane export from global groundwaters, Proc. Natl. Acad. Sci. U.S.A. 122 (42) e2508773122, https://doi.org/10.1073/pnas.2508773122 (2025)
