Anaerobic oxidation of methane (AOM) is a microbial process that consumes methane in environments devoid of oxygen. Carried out by microorganisms that use methane as an energy source, this function is a component of the planet’s carbon cycle. The process is most active in methane-rich sediment on the ocean floor. By converting methane into other substances, AOM prevents large quantities of this potent greenhouse gas from reaching the atmosphere.
The Microbial Partnership
The primary microorganisms driving AOM are single-celled organisms known as anaerobic methanotrophic archaea (ANME). These archaea, from ancient lineages of life, do not work alone. They form a mutually beneficial relationship, or syntrophy, with other microbes, most commonly sulfate-reducing bacteria (SRB).
In this partnership, the ANME archaea initiate the breakdown of methane and transfer electrons to their bacterial partners. The SRB, which often belong to the Deltaproteobacteria class, accept these electrons to fuel their own metabolism. This association is highly adapted, and some partner bacteria are rarely found living independently.
While the ANME-SRB consortium is the most studied, ANME can also partner with nitrate-reducing bacteria in environments where sulfate is scarce. This demonstrates flexibility in the microbial world, allowing AOM to occur under different geochemical conditions. The specific partners can vary, with different ANME groups associating with distinct bacterial groups.
The Biochemical Process
The mechanism of AOM is often described as “reverse methanogenesis,” running many of the same enzymatic steps backward to consume methane instead of producing it. The process is initiated by ANME archaea, which use a specialized enzyme complex to activate the stable methane molecule (CH4). This initial activation step is energetically demanding and a key area of scientific investigation.
Once the methane molecule is broken, the ANME archaea oxidize it, stripping away its electrons. These captured electrons are then passed to the syntrophic partner. In the partnership with SRB, the bacteria use these electrons in their own form of respiration. Instead of oxygen, the SRB “breathe” sulfate (SO42-), reducing it to hydrogen sulfide (HS-).
This electron transfer is believed to happen directly between the cells, possibly through conductive protein filaments. The overall chemical reaction for sulfate-coupled AOM is CH4 + SO42− → HCO3− + HS− + H2O. This equation shows methane and sulfate being consumed to produce bicarbonate, hydrogen sulfide, and water. The process is not limited to sulfate; other compounds like nitrate, nitrite, and even metal oxides can serve as the final electron acceptor for the partner microbe.
Global Hotspots for Methane Consumption
AOM occurs globally in specific oxygen-free (anoxic) locations where methane is abundant. The primary hotspots are in marine sediments, which cover vast areas of the ocean floor. Within these sediments, the process is intense at geologic features known as cold seeps and mud volcanoes, where methane from deep reservoirs provides a concentrated fuel source.
Beyond the deep sea, AOM is active in the anoxic water columns of certain basins, like the Black Sea, where a layer devoid of oxygen allows these microbes to thrive. Freshwater environments, including the sediments of lakes and reservoirs, also host this process. The rates can be influenced by factors like temperature, with warmer conditions leading to more intensive methane oxidation.
Human-altered environments can also become hotspots. The oxygen-poor soils of rice paddies and the depths of landfills create conditions suitable for both methane production and its anaerobic consumption. In these settings, microbes act as a natural filter, consuming a portion of the produced methane before it can escape.
Environmental Significance
The primary environmental role of AOM is its function as a natural biofilter. This microbial process consumes the majority of methane produced in marine sediments, with estimates suggesting over 80% is oxidized before it can leave the seafloor. This service is important for regulating Earth’s climate by preventing large quantities of a potent greenhouse gas from entering the atmosphere. Methane has a much stronger warming effect than carbon dioxide over shorter timescales.
By converting methane into bicarbonate, AOM also plays a part in the global carbon cycle. The bicarbonate can become sequestered through the formation of carbonate rocks on the seafloor. By using sulfate and producing hydrogen sulfide, the process links the global carbon and sulfur cycles, influencing ocean chemistry. The hydrogen sulfide produced can, in turn, support other unique ecosystems based on chemosynthesis.
The effectiveness of this biofilter can be influenced by environmental changes. Factors such as rising ocean temperatures could destabilize methane hydrate deposits on the seafloor, leading to an increased flux of methane. A rapid release of gaseous methane could overwhelm the capacity of these microbial communities, allowing more of the gas to bypass this natural control. Understanding the limits of this biofilter is a major focus of current research.