Methane-oxidizing bacteria, include methanotrophs and ammonia oxidizing bacteria which co-oxidize methane. Prevalent in diverse soil types, termite mounds, and on tree bark, twigs, and leaves, methane-oxidizing bacteria contribute to natural methane sinks. Some strains of methane-oxidizing bacteria are more effective than others at oxidizing atmospheric methane while sustaining their growth. Methane-oxidizing bacteria can also have side effects that negatively impact the climate, such as nitrous oxide emissions. It may be possible to engineer methane-oxidizing bacteria to oxidize atmospheric methane with higher growth rates and reduced side effects compared to naturally occurring strains.
Introducing methane-oxidizing bacteria which are selected or engineered to be better at oxidizing atmospheric methane to soil, plants, or artificial constructions may enable sustainable methane consumption at concentrations low enough to enhance atmospheric methane uptake. So far this approach has only been proposed. As yet little is known about its potential effectiveness and side effects.
Since research on methane-oxidizing bacteria as an atmospheric methane removal approach is limited, its overall feasibility cannot be fully assessed at this time. One key question is whether it is possible to cultivate and/or engineer bacteria which have higher growth rates at low methane concentrations (atmospheric methane is about 2 ppm), and which can survive and thrive under natural conditions.
Methanotrophs are widespread in nature, including some that must depend on atmospheric methane. Extant populations of these bacteria often have enzymes with a very high affinity for methane (i.e., a low half-saturation constant, Km, for methane). But the price of high affinity (low Km) is usually low maximum capacity (Vmax) of the enzymatic reactions. Therefore, even with high affinity for methane, the rate of reaction is typically low, which limits overall methane uptake.
Another limiting factor is diffusion of atmospheric methane through the substrate, i.e. through soils and across air-water interfaces, so it reaches reactive sites and contacts methane-oxidizing enzymes. The amount of energy available to methanotrophs depends on this rate of diffusion. If it’s too low, it may not be possible to maintain robust populations of methanotrophs.
Researchers are currently working on engineering bacteria to increase their growth rates at low concentrations of methane. Currently these bacteria cannot grow below about 60 ppm of methane. Few strains of methane-oxidizing bacteria which are atmospheric-compatible (i.e. they can grow at 2 ppm of methane) can be cultivated in laboratories. It will require innovation to engineer methane-oxidizing bacteria with both high affinity for methane and a high reaction rate. that can sustain growth and support a viable population at atmospheric methane concentrations.
In natural conditions, various factors can influence methane-oxidizing bacteria’s impacts. For example in nitrogen-rich conditions, methanotrophs may produce nitrous oxide, a powerful greenhouse gas. Abundant ammonium in N-rich conditions can also competitively inhibit methane oxidation. It may be possible to engineer methane-oxidizing bacteria to reduce their nitrous oxide emissions. Nitrous oxide emissions, soil carbon dynamics, and methane fluxes would need to be assessed before deployment to ensure that introducing methane-oxidizing bacteria into the environment would be climate-positive.
Methanotrophy’s potential may be limited by resources including copper, a nutrient methane-oxidizing enzymes require. It is currently not known if existing populations of methanotrophs could be stimulated by adding more micronutrients, or if these micronutrients would also be required for engineered methanotrophs to survive.
The cost and climate impacts of transplant or cultivation of methane-oxidizing bacteria are currently unclear, and would need to be assessed to determine whether deployment would be feasible. Ongoing monitoring for greenhouse gas dynamics and side effects would also be needed, which may increase costs.
Methane-oxidizing bacteria technology transfer would face other particular challenges connected to land use. Introducing methane-oxidizing bacteria in agricultural settings or natural landscapes around the world would have to contend with various cultural norms, traditions, policy mindsets, capacity limitations, and scarce finances.
Since methane-oxidizing bacteria’s effectiveness is not yet known, and research is limited, scalability cannot yet be determined. It may depend on the suitability of the land and constraints in culturing and transferring bacteria.
A net methane uptake of 10 Mt/yr would require enhancing the current methane soil sink by between 20% to 100%.
Not all land is suitable for net methane uptake. High soil moisture conditions favor methanogenesis over methanotrophy, resulting in net methane production. Nitrogen-rich soils, including fertilized soils, often exhibit lower rates of methanotrophy as well as high rates of nitrous oxide emissions.
Over 40% of ice-free land has been modified by humans, primarily for agriculture. Agricultural soils appear to have lower rates of methane uptake (perhaps by as much as a factor of seven) relative to native soils. That presents an opportunity to enhance methane uptake on agricultural land without affecting natural ecosystems, provided that nitrogen, which is essential for agricultural productivity, does not inhibit methane oxidation.
Introducing foreign bacteria into a natural or agricultural environment, whether natural or genetically modified, may have impacts on the microbial structure, soil structure, health, and nutrient profile, affecting plant, fungi, and animal health. Genes added by transgenic methods may transfer to other bacteria through horizontal gene transfer, though this risk can be mitigated somewhat by “suicide genes”. Genetic modification may also have impacts on nitrous oxide co-generation.
Assessing such impacts through lab testing, modeling, and small-scale field tests would be necessary before deployment, as would ongoing monitoring of effects after deployment.
Genetically modified microorganisms have been deployed for pollutant degradation, environmental monitoring, and other applications. Regulatory regimes vary by country. In the U.S. they are regulated by the EPA.