Methanotrophic bacteria are a group of microorganisms with a highly specialized diet. These microbes, including various bacteria and archaea, are defined by their ability to use methane as their sole source of carbon and energy. Unlike most life that relies on complex organic matter or photosynthesis, methanotrophs thrive on this simple gas. They are a specific subset of a wider group known as methylotrophs, which can consume various single-carbon compounds.
This metabolic capability is significant because methane is a powerful greenhouse gas. First identified in 1906, these bacteria are now understood to be an influential part of the planet’s carbon cycle. By consuming methane, they act as a natural control on its release into the atmosphere from both natural and human-influenced sources.
The Methane Consumption Mechanism
The process of methane consumption begins by breaking the strong carbon-hydrogen bonds of the methane molecule. Methanotrophs accomplish this using enzymes called methane monooxygenases (MMOs). This enzyme inserts an oxygen atom into a methane molecule, transforming it into the more chemically reactive compound, methanol. This initial conversion is the most energy-intensive step in their metabolism.
There are two primary forms of this enzyme, and their use is dictated by copper availability in the environment. The particulate methane monooxygenase (pMMO) is a copper-containing enzyme embedded in the bacterium’s cell membrane and is found in nearly all methanotrophs. In contrast, the soluble methane monooxygenase (sMMO) is an iron-containing enzyme in the cell’s cytoplasm, produced by some species only when copper levels are low. This regulation, called a “copper switch,” allows the bacteria to adapt to changing mineral availability.
Once methanol is produced, it is directed into two distinct pathways. A portion is funneled into assimilatory pathways, where its carbon is used as a building block to construct new cellular components like proteins and DNA, allowing the organism to grow. The remaining methanol is sent down a dissimilatory pathway for energy generation, where it is oxidized into formaldehyde, then formate, and ultimately carbon dioxide. Each step of this oxidation releases energy to power all cellular activities.
Global Impact on the Methane Cycle
Methanotrophic bacteria function as a living biofilter, regulating the flow of methane from the land and water into the atmosphere. They thrive at the interface between anaerobic (oxygen-free) and aerobic (oxygen-rich) environments. In these zones, methane produced by microbes known as methanogens must pass through the oxygenated layer where methanotrophs reside, allowing them to capture the gas before it can escape.
Wetlands are a primary example of this natural process. As organic matter decomposes in waterlogged, anoxic sediments, it generates large quantities of methane. Methanotrophs inhabiting the thin layer of oxygenated water and surface sediments can consume a substantial fraction of this methane, with estimates suggesting they oxidize up to 95% in certain freshwater ecosystems.
This dynamic is also pronounced in human-managed environments like rice paddies, a major anthropogenic source of methane. The flooded conditions create a perfect environment for methanogens, but the areas around the rice plant roots transport oxygen into the soil, creating a niche for methanotrophs. In these root zones, the bacteria can intercept a large portion of the produced methane.
A growing area of concern where methanotrophs play a part is the Arctic as permafrost thaws due to climate change. The thawing of these soils exposes vast stores of ancient organic carbon to decomposition, leading to significant methane production. While methanotrophs in the soils consume some of this newly released methane, the sheer volume of gas being generated poses a challenge to this natural filtration capacity.
Biotechnological Applications
Scientists are working to harness these bacteria for a range of technological applications. The concept is to treat methane, a potent greenhouse gas and often a waste product, as a valuable feedstock. This approach “upcycles” a harmful emission into sustainable, high-value products, creating a foundation for a methane-based bio-economy.
One of the most developed applications is the production of single-cell protein (SCP) for animal feed. The bacterial cells are naturally rich in protein. Companies cultivate methanotrophs in large bioreactors, feeding them methane from natural gas or industrial sources, and then harvest the microbial biomass. This protein-dense product serves as a sustainable alternative to traditional sources like fishmeal or soy.
Another promising avenue is the creation of biodegradable plastics. Under specific growth conditions, such as nutrient deprivation, some methanotrophs store carbon by producing polymers called polyhydroxyalkanoates (PHAs). These accumulated PHAs can constitute over half the cell’s dry weight and can be extracted and processed into a versatile, fully biodegradable bioplastic.
Methanotrophs are also being deployed for bioremediation to directly mitigate methane emissions at their source. They can be used in bio-covers or biofilters at landfills to capture and consume methane gas that would otherwise vent into the atmosphere. They can also be integrated into wastewater treatment facilities to consume dissolved methane from anaerobic digestion processes.
The metabolic capabilities of these microbes extend to the production of biofuels and other fine chemicals. The initial step of their metabolism converts methane into methanol, a liquid fuel and chemical building block. Researchers are exploring ways to optimize this conversion and halt the process before the bacteria consume the methanol. Methanotrophs also produce other commercially useful molecules, such as ectoine, a compound used in cosmetics and healthcare.