Methane is a greenhouse gas that contributes to global warming. A group of microorganisms called methanotrophs can use methane as their primary source of energy and carbon, acting as nature’s methane recyclers. By converting methane into less harmful substances or valuable products, they play a role in mitigating the gas’s effects. This ability has made them a subject of research for environmental and biotechnological applications.
Defining Methanotrophs
Methanotrophs are a diverse group of prokaryotes, including bacteria and archaea, defined by their ability to use methane. They are a type of methylotroph, an organism that can use single-carbon compounds. Methanotrophs can be either obligate, relying exclusively on methane, or facultative, capable of using other carbon sources.
These microorganisms are categorized by their oxygen requirements. Aerobic methanotrophs require oxygen to metabolize methane. In contrast, anaerobic methanotrophs thrive in oxygen-depleted environments and use other compounds like sulfate or nitrate to process methane. This distinction affects their distribution in different ecological niches.
Aerobic methanotrophs were historically classified into groups like Type I, Type II, and Type X. These classifications are based on their metabolic pathways, internal membrane structures, and phylogeny. Type I methanotrophs have bundles of stacked intracytoplasmic membranes. Type II methanotrophs have membranes arranged around the cell’s periphery, and these structural differences correspond to their biochemical processes.
How Methanotrophs Consume Methane
In aerobic methanotrophs, methane consumption begins when the enzyme methane monooxygenase (MMO) oxidizes methane to methanol. This first step allows the microbes to access the energy stored in the methane molecule.
MMO exists in two primary forms: soluble (sMMO) and particulate (pMMO). The more common pMMO is embedded within the cell’s membrane, while sMMO is found in the cytoplasm of some methanotrophs. The sMMO form can act on a wider range of hydrocarbon molecules than just methane. Environmental copper levels often regulate which form of the enzyme is produced.
After the conversion to methanol, the process continues with the oxidation of methanol to formaldehyde. This formaldehyde can then be assimilated into the cell’s biomass for growth. Alternatively, it can be further oxidized to carbon dioxide to release energy for the cell. The specific assimilation pathway used, either the ribulose monophosphate (RuMP) or serine pathway, helps classify different methanotrophs.
Ecological Roles and Natural Habitats
Methanotrophs are widespread in any environment where methane and oxygen are both present. Their habitats include forest soils, agricultural fields, lakes, streams, and ocean sediments. They are especially abundant in areas with high methane generation like wetlands, rice paddies, and landfills, where they act as a natural barrier against methane release.
In these environments, methanotrophs function as natural methane sinks or biofilters. They consume much of the methane produced by other microbes, called methanogens, before it escapes into the atmosphere. By oxidizing methane, they reduce the atmospheric impact of this greenhouse gas, which has a higher warming potential than carbon dioxide. This process is a component of the global methane cycle.
Methanotrophs are most active at the interface between anaerobic (oxygen-free) and aerobic (oxygen-rich) zones. In lake sediment or wetland soil, methanogens in the anaerobic layers produce methane. As this gas moves toward the oxygen-rich surface, methanotrophs consume it. This microbial shield is also active in marine environments near methane seeps and hydrothermal vents, preventing dissolved methane from reaching the atmosphere.
Biotechnological Applications of Methanotrophs
The metabolism of methanotrophs has several biotechnological applications. These microbes can be used to convert methane, a greenhouse gas, into valuable products. This process offers sustainable solutions for chemical and energy production.
A primary application is bioremediation. The MMO enzyme can break down a range of hydrocarbons beyond methane, allowing methanotrophs to clean up environmental contaminants. They can degrade chlorinated solvents like trichloroethylene, a common pollutant in soil and groundwater, helping to restore contaminated industrial sites.
Methanotrophs are also being developed to produce biofuels and bioplastics. They can convert methane into methanol for use as a liquid fuel or be engineered to create other energy-dense compounds. They also naturally produce biodegradable polymers called polyhydroxyalkanoates (PHAs), which can be used to make bioplastics as a renewable alternative to fossil fuel-based plastics.
The production of single-cell protein (SCP) for animal feed is another application. The biomass of methanotrophs is rich in protein and nutrients. Growing these microbes on methane from natural gas or biogas produces a sustainable protein source for aquaculture and livestock, reducing reliance on feed sources like fishmeal and soy.