Freshwater sediments may play a bigger role in slowing methane emissions than previously thought
image:
Lake Ørn in Denmark, where the study was done
view moreCredit: Professor Bo Thamdrup/University of Southern Denmakr
Methane is one of the most powerful greenhouse gases in Earth’s atmosphere, and wetlands together with inland waters are among its largest natural sources. But in the freshwater sediments, specialized microorganisms consume part of this methane before it can escape into the air.
A new study sheds light on the environmental controls governing this natural methane consumption. The study was published in Limnology and Oceanography and is available here: https://aslopubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/lno.70373
The study was performed in the group of Bo Thamdrup, who is a Professor of geomicrobiology at the Department of Biology, University of Southern Denmark (SDU). The experimental efforts were led by postdoc Alina Mostovaya and PhD student Michael Wind-Hansen, who are now both at Aarhus University but were at SDU when the work was conducted.
The researchers investigated sediments from Lake Ørn in Denmark and managed to quantify, for the first time, how the availability of sulfate and iron controls anaerobic oxidation of methane in freshwater sediments.
Their findings provide new insight into the microbial processes that regulate methane emissions from lakes, wetlands, and other aquatic environments.
Freshwater environments like lakes are often considered important natural contributors to climate change, because methane is released from the water surface. Often, one can see the methane leave the water as bubbles that burst once they reach the surface, releasing methane directly into the air.
But according to the researchers, a considerable amount of methane could very well be consumed in the sediment.
“If this mechanism was not at play, more methane would leave the lakes”, says corresponding author Alina Mostovaya.
The researchers describe the mechanism as an underappreciated methane sink that should be considered when making models for balancing production and consumption of methane in freshwater environments.
The driver of the investigated methane consumption in the studied Lake Ørn is microbial activity. Certain microbes use sulfate and iron in the sediment to consume methane under oxygen-free conditions.
Neither sulfate nor iron are rare elements in freshwater sediments. Sulfate may, for example, enter with rain and runoff from soils, nearby fertilized fields, wastewater or seawater intrusion. Iron is one of the most abundant elements on Earth and may come from weathering of rocks and soil or is carried by rivers and groundwater.
The methane-consuming microbes at play belong to the archaeal group ‘Candidatus Methanoperedenaceae’, and they appear to be quite efficient even in low-resource freshwater environments:
“Our work in Lake Ørn shows that even relatively low concentrations of sulfate can support efficient methane removal in freshwater sediments,” says co-author Michael Wind-Hansen.
The researchers found that sulfate-dependent methane oxidation in Lake Ørn operates efficiently at sulfate concentrations in the low micromolar range — far lower than typical values reported from marine systems. This suggests freshwater microbial communities have evolved high-affinity strategies for scavenging scarce resources.
The team also showed that iron-dependent methane oxidation requires relatively high concentrations of reactive iron minerals, but nevertheless represents an important pathway for methane removal in the lake.
Their experiments further revealed that dissolved organic compounds resembling natural humic substances can shuttle electrons between microbes and iron minerals, significantly stimulating methane oxidation under certain conditions.
“These electron-shuttling compounds may help microorganisms take advantage of iron minerals that would otherwise be difficult to use,” Alina Mostovaya says, “That means natural organic matter may play a dual role in many freshwater environments, both as a source of methane and as a regulator of methane consumption and emissions.”
The findings have broader implications beyond a single Danish lake.
“We expect that the same pattern can be found in many other lakes and freshwater environments in other parts of the world, so this is a factor that should be considered when making global models of methane production, consumption, and emissions in these environments”, says Professor Bo Thamdrup.
Journal Limnology and Oceanography, April 23, 2026: Kinetics of sulfate- and iron-dependent anaerobic methane oxidation in freshwater lake sediment. doi: 10.1002/lno.70373. Authors: Alina Mostovaya, Michael Wind-Hansen, Bo Thamdrup.
The study was financed by the European Research Council (project NOVAMOX) and the Independent Research Fund Denmark.
Lake Ørn in Denmark, where the study was done
Credit
Professor Bo Thamdrup/University of Southern Denmark
Journal
Limnology and Oceanography
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Kinetics of sulfate- and iron-dependent anaerobic methane oxidation in freshwater lake sediment
Fungi help lock carbon into Arctic fjord sediments
LMU-Researchers show that fungi play an important role in the marine carbon cycle.
Arctic fjords are among the most efficient natural systems for absorbing and long-term storage of carbon. However, as the Arctic is warming around four times faster than the global average, fjord ecosystems are changing rapidly. Against this backdrop, understanding the biological processes that regulate carbon storage is becoming increasingly important. Yet the microbial mechanisms that control whether carbon is stored in sediments or returned to the environment are still not fully understood.
A new study led by Professor William Orsi of the Department of Earth and Environmental Sciences now shows that fungi may play a surprisingly important role in keeping carbon locked into the seafloor. Working in Kongsfjorden, Svalbard, an international team of researchers found that marine fungi living in sediments efficiently assimilate dissolved organic matter and retain it as microbial biomass, rather than allowing it to be rapidly remineralised.
A poorly understood part of the marine carbon cycle
Fungi are known to play important roles in how carbon is processed, retained, and stored in terrestrial soils, yet their contribution to carbon cycling in marine sediments has remained largely unknown. Understanding this role is particularly important in Arctic fjords, where microbial activity at the sediment surface influences whether dissolved organic matter (DOM) is converted into microbial biomass, remineralized to CO₂, or ultimately buried and sequestered as organic carbon in the sediments.
To investigate this, the researchers sampled sediments, seawater, soils, and glacial environments across Kongsfjorden, a high Arctic fjord on the west coast of Svalbard. They then used isotope-tracing techniques to follow how fungi and bacteria consumed DOM and contributed to carbon cycling across these interconnected habitats.
The results showed that fungi in fjord sediments assimilated dissolved organic matter (DOM) with relatively high efficiency. As a result, more carbon was retained in microbial biomass rather than being remineralized to CO₂. Higher fungal assimilation was associated with increased fungal-to-bacterial biomass ratios, suggesting that fungal metabolism promotes carbon retention at the seafloor and may enhance long-term carbon sequestration in fjord sediments.
A distinct fungal community in the fjord
The study also showed that the fungal communities in Arctic fjord sediments are distinct from those in nearby soils and overlying seawater. Using quantitative stable isotope probing, the researchers linked amino acid assimilation – free amino acids are an essential component of DOM - to more than 80 fungal taxa in fjord sediments.
These findings suggest that free-living marine fungi are not just present in Arctic fjords, but are active participants in carbon cycling, helping stabilise labile organic matter in sediments that are important carbon sinks.
„Our study shows that fungi in the Arctic ocean can contribute significantly to carbon storage in sediments via their highly efficient metabolism. This is important because it is a previously unknown mechanism of microbial carbon storage in fjords, key geological settings that store more than ten percent of all the carbon buried below the seafloor”, resumes Orsi.
Juan Carlos Trejos-Espeleta, PhD student at LMU and first author of the study, adds: "The Arctic is changing before our eyes at unprecedented rates and the efforts to understand its ecosystems functioning remain insufficient. Only recently we are looking at marine fungi as important participants in the marine carbon cycle, having a potential role in carbon sequestration, just as it is known for terrestrial environments. Future research should not ignore fungi anymore as key agents of carbon cycling."
James Bradley, CNRS researcher at the Mediterranean Institute of Oceanography and co-author of the study, says: “Sampling and developing experiments in the High Arctic is still a challenging task, just like understanding fragile, dynamic ecosystems such as a glaciated fjord. This is why studies in these parts of the planet are rare but paradoxically have a high level of urgency."
Journal
PLOS Biology
Article Title
Fungi enhance microbial carbon retention in high Arctic fjord sediment
Article Publication Date
16-Jun-2026
