Methanotrophs, bacteria and archaea that oxidize methane, provide a natural methane sink. Some strains of methanotrophs are more effective than others at oxidizing atmospheric methane while sustaining their growth. It may be possible to engineer methanotrophs to improve their ability to oxidize atmospheric methane with higher growth rates and reduced other impacts, like nitrous oxide production, compared to naturally-occurring strains. Introduction of these strains could enhance the natural methanotrophic sinks of methane in soils, termite mounds, or on plants.
These approaches may warrant additional exploration in natural systems and/or agricultural settings. This overview focuses on the subset of methanotroph enhancements that have potential for a net uptake of atmospheric methane. Applying any of these approaches for atmospheric methane removal is at the early stage of conceptualization. Little is known about the potential effectiveness and side effects.
There is insufficient peer-reviewed literature on enhanced methanotrophs for methane removal to assess feasibility. Moreover, populations of methanotrophs have evolved to be adapted well to the methane concentrations that they frequently experience, from atmospheric concentrations (2 ppm) for upland soils to much higher concentrations in landfill soils and the surface soils in wetlands and rice paddies, where methane diffuses upwards from deeper methanogenesis sources (Whalen 1996, Whalen 1990, Conrad 2007). Hence, it is unclear how engineered organisms would have mechanisms that enable greater methane oxidation rates than those of natural soil populations.
Little is known about the cost per unit of methane uptake. Cost will depend on the methane uptake rate, expenses related to the dispersal of methanotrophs and those related to the enhancement process itself, and ongoing monitoring of the effectiveness.
There is insufficient literature on enhanced methanotrophs for methane removal to assess scalability. The potential scale may depend on the suitability of the land for methane uptake enhancement, constraints in culturing and dispersing methanotrophs, and raw material availability.
As a benchmark, a net annual uptake of 10 million metric tons of methane (830 Mt CO2e using GWP20) would require enhancing the current global methane soil sink by 20% to 100%. This requirement might be somewhat less if methanogenesis suppression co-benefits are factored in.
The scale of enhanced methanotroph deployment may be limited by environmental concerns, competition with natural methanotrophs, or the availability of suitable land.
Interventions in natural or agricultural systems must be approached very carefully. 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.
Methanotrophs technology transfer would face other particular challenges connected to land use. Over 40% of ice-free land has been modified by humans, primarily for agriculture. Agricultural soils generally have lower rates of methane uptake (by as much as a factor of 7) relative to native soils, which may present an opportunity to enhance methane uptake on agricultural land without directly affecting natural ecosystems. However, it is important to understand why methane uptake rates are lower in well-drained agricultural soils, such as soil compaction that reduces diffusion of atmospheric methane into the soil, competitive inhibition of methane oxidation by ammonium, lack of sufficient micronutrients like copper, or changes in microbial communities.
Introducing methanotrophs 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.
Though certain genetically modified microorganisms have been introduced into the natural environment, and there is some research on improving methanotroph performance for biofilter applications, there has been little research specifically on engineering methanotrophs to be better at oxidizing atmospheric methane and introducing them into natural environments.
Possible approaches to engineering methanotrophs may include directed evolution, horizontal DNA transfer, and other genetic engineering approaches. Besides the risk of horizontal gene transfer, challenges may include long-term stability under changing conditions of temperature, pH, and microbial competition.
Methanotrophs may perform better when introduced as a part of a consortium of supporting microbes. Optimal consortium design should be considered in the development process.
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. The energetic limitations imposed by only having access to 2 ppm CH4, or even lower concentrations where diffusion of atmospheric CH4 is constricted, may be a barrier to any organism dependent on CH4 as a sole energy source for growth.
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. Strains of methanotrophs that are able to survive and grow at atmospheric concentrations of methane (2 ppm methane) have not yet been successfully cultivated in the laboratory. It is unknown whether engineering methanotrophs with both high affinity for methane and a high reaction rate that can sustain growth and support a viable population at atmospheric methane concentrations will be feasible.
In natural conditions, various factors can influence the impacts of methanotrophs. 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 methanotrophs 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 methanotrophs into the environment would be climate beneficial.
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.
Key questions for methanotrophs as an atmospheric methane removal approach include: