Photo Credit: Philip Arambula
Scientific Frontline: Extended "At a Glance" Summary: Freshwater Methane Consumption
The Core Concept: Freshwater sediments host highly adapted microorganisms that consume substantial amounts of methane under oxygen-free conditions, preventing a significant portion of this potent greenhouse gas from reaching the atmosphere.
Key Distinction/Mechanism: Unlike marine environments, microbial methane oxidation in lakes and wetlands operates efficiently at extremely low sulfate concentrations. A specific group of archaea breaks down the methane anaerobically using either trace amounts of sulfate or reactive iron minerals, a process further enhanced by natural organic matter acting as electron shuttles.
Major Frameworks/Components:
- Anaerobic Oxidation of Methane (AOM): Driven primarily by the archaeal group 'Candidatus Methanoperedenaceae'.
- Trace Sulfate Utilization: The capability of freshwater microbes to sustain highly efficient methane removal utilizing scarce sulfate resources.
- Iron Reduction Pathway: Methane breakdown coupled with high levels of reactive iron minerals.
- Electron Shuttling: Humic substances (natural organic matter) functioning as conduits to help microorganisms metabolize complex iron minerals more effectively.
Branch of Science: Biogeochemistry, Environmental Microbiology, Limnology.
Future Application: Integration of these microbial pathways into global biogeochemical models to refine calculations of methane production, natural consumption rates, and net greenhouse gas emissions from freshwater ecosystems.
Why It Matters: Lakes and wetlands are among the largest natural sources of methane. Precisely quantifying the biological pathways that naturally remove this gas is critical for accurate climate forecasting and understanding the global carbon cycle.
Methane is one of the most powerful greenhouse gases, and lakes and wetlands are among its largest natural sources. In many lakes, methane can be seen bubbling up from the bottom and escaping directly into the atmosphere.
However, considerable amounts of methane can be consumed by microorganisms living in the sediment before it reaches the surface. A new study, published in Limnology and Oceanography, provides fresh insight into the environmental factors that control this natural methane removal.
The research was carried out in the research group of Professor Bo Thamdrup in the Department of Biology, and it was led by postdoctoral researcher Alina Mostovaya and PhD student Michael Wind-Hansen, who were both based at SDU during the study. Both are now at Aarhus University: Mostovaya at the Department of Ecoscience, and Wind-Hansen at the Department of Biology.
Lake Ørn in Denmark Showed the Way
The researchers investigated sediments from Lake Ørn in Denmark and, for the first time, quantified how the availability of sulfate and iron influences methane consumption under oxygen-free conditions.
Neither sulfate nor iron is a rare element 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 the weathering of rocks and soil, or be carried by rivers and groundwater.
“Our study shows that these two elements can play an important role in regulating microbial processes that reduce methane emissions,” says corresponding author Alina Mostovaya.
Highly Adapted Microbes at Play
The methane-consuming microorganisms belong to a group of archaea known as Candidatus Methanoperedenaceae. These microbes are able to break down methane in environments without oxygen, and both sulfate and iron can play an important role in this process.
The results show that even very low concentrations of sulfate can support efficient methane removal in freshwater sediments—levels much lower than those typically found in marine environments.
“This suggests that freshwater microbes are highly adapted to making use of scarce resources,” says co-author Michael Wind-Hansen.
Natural Organic Matter Can Enhance the Process
Iron also plays an important role. The researchers found that methane breakdown linked to iron requires relatively high levels of reactive iron minerals but still represents a significant pathway for methane removal in the lake.
In addition, the study shows that natural organic matter can enhance the process. Certain components of humic substances can act as “electron shuttles,” allowing microbes to use iron minerals more effectively.
“These electron-shuttling compounds may help microorganisms take advantage of iron that would otherwise be difficult to use,” says Alina Mostovaya.
A Global Phenomenon
Overall, the findings highlight that methane removal in freshwater sediments may be an underappreciated component of the global methane cycle.
“We expect that the same patterns of methane consumption can be found in many 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.
Funding: The study was financed by the European Research Council (NOVAMOX project) and the Independent Research Fund Denmark.
Published in journal: Limnology and Oceanography
Title: Kinetics of sulfate- and iron-dependent anaerobic methane oxidation in freshwater lake sediment
Authors: Alina Mostovaya, Michael Wind-Hansen, and Bo Thamdrup
Source/Credit: University of Southern Denmark | Birgitte Svennevig
Edited by: Scientific Frontline
Reference Number: es061726_01
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