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Until the 1970s, any new species discovered was thought to be either eukaryotic or prokaryotic, depending on how it looked under the microscope. This changed in 1977 when Carl Woese devised a method to classify organisms based on their sequences and “accidentally” discovered a called Archaea. Based on their morphology, archaea look very much like tiny bacteria and do not have a nucleus. However, based on their genetic and biochemical make-up, they are more closely related to eukaryotes. Methanogenic archaea, or methanogens, have drawn considerable attention recently for their critical role in the global carbon cycle because of their unique ability to produce the potent greenhouse gas methane.

​Methanogens are the only known organisms capable of producing methane and do so under strictly anaerobic conditions. Their habitats range from deep-sea hydrothermal vents in the Pacific Ocean to in Siberia, all the way to the . The highest recorded increase in atmospheric methane concentration occurred in 2021 at (ppb). Anthropogenic activities, like oil and gas operations and the production of fossil fuels, are only one part of the problem related to methane emission. According to the , wetlands and other natural sources of methanogens contributed ~0.2 Gt of methane per year during 2008-2017. Agricultural activities like livestock grazing contributed another ~0.2 Gt of methane per year during this period, as methanogens are present in the rumen of many farm animals including cows and sheep.

Atmospheric methane plays a major role in global warming, as it traps heat about 30 times more efficiently than carbon dioxide, the other major greenhouse gas, over the course of 100 years. It contributes further stress to the climate when it is oxidized to CO2. The alarming rate of methane emission demands an understanding of the biogeography of methanogens and their various modes of methane production.

How Do Methanogens Produce Methane? 

Methanogenesis or methane production relies on , formate, acetate, methanol, methyl sulfides and methylamines. These substrates are primarily produced by the decomposition of organic matter by other bacteria and fungi present in the surrounding microbial communities. Methanogenesis happens in the absence of oxygen and other electron acceptors like nitrate, sulphate and iron. The production of methane, in turn, releases ATP for various cellular processes. The key enzyme in methanogenesis is the methyl-coenzyme M reductase (Mcr) complex, which catalyzes the final step of reduction of methyl-coenzyme M to methane.

Consequences of Methane Production 

Unaddressed methane production can have serious consequences for the environment by altering the global climate. It has been suggested that methanogenesis contributed to the largest extinction event in Earth’s history, the about 250 million years ago. According to the theory, a horizontal gene transfer event occurred from cellulose-degrading bacteria like Clostridia to the methanogen Methanosarcina. This resulted in Methanosarcina gaining the ability to use acetate as a substrate for methane production. Methanogenesis via acetate consumption is more efficient than hydrogenotrophic methanogenesis. This likely allowed methanogens to consume the vast trove of organic matter in marine sediments, resulting in a sharp rise in atmospheric methane that may have killed ~90% of species on the planet.

How Do Agricultural Activities Produce Methane? 

Agriculture is a major source of income in both developing and developed nations, but many agricultural activities come with a cost to the climate. Livestock, paddy fields and manure are among the top contributors to methane emissions.

Among animals, ruminants are key contributors to climate change. This is because their burps contain a huge amount of methane. In fact, cattle are among the leading cause of agricultural greenhouse gas emissions. A single cow can produce , depending on the breed. Ruminants differ from other animals in that they have specialized digestive systems comprised of stomachs that have 4 compartments instead of 1. Rumen, the largest compartment in the stomach, is the hub for digestion of feed by microbes, including bacteria and fungi. The enteric fermentation of feed produces hydrogen and carbon dioxide, which are used by methanogens present in the rumen to produce methane. The greenhouse gas is then belched or burped out of the rumen, to the atmosphere, via the esophagus.

Scientists are working on multiple fronts to reduce methane emissions by livestock. One approach is to alter their diet. A feed with higher fiber content undergoes a greater degree of fermentation and results in greater production of hydrogen and carbon dioxide. This, in turn, generates more methane. Feeding livestock corn instead of grass can reduce their methane emissions, but care has to be taken to maintain the well-being of the cattle, as corn isn’t as nutritious as grass for them.

Scientists also showed that supplementing beef cattle feed with the red algae (seaweed), , by ~70-80%, without loss in feed conversion efficiency. Seaweed contains halogenated compounds called bromoform that are structural analogs of methane. Bromoform inhibits methane production by blocking the final step of methyl transfer by the Mcr complex. However, there are several hurdles to this approach. In order for this strategy to truly be impactful, seaweed would have to be produced at large scales, which comes with environmental concerns related to ocean seaweed farming, including reduction of natural seaweeds and habitat destruction of the nearby marine flora and fauna. Bromoform and other halogenated compounds present in seaweed can also be toxic to animals and humans, and hence, dosage, would need to be tightly regulated in cattle feed.

How Does Climate Change Impact Methanogenesis? 

Methanogens have a direct impact on the climate by releasing methane, the global warming potential of which is ~30 times greater than carbon dioxide over a 100-year period. The reverse is also true. Rising global temperatures also increase the activity and abundance of methanogens in different environmental conditions. This creates a positive feedback loop, the end result of which negatively impacts all lifeforms on the planet.

An alarming demonstration of these changes is the observation that permafrost is thawing. As global temperatures rise, permafrost layers are melting in many Arctic regions, including Alaska and Siberia. Permafrost regions are a warehouse of soil organic matter. As permafrost thaws, the organic matter becomes available for that release methane. In 2014, resulted in methane emissions, the amount of which depended on the features of the landscape. During the early months of thawing in wetlands, hydrogenotrophic methanogens, which rely on hydrogen and carbon dioxide to produce methane, dominate. Later, in fully thawed marshy areas, the diversity of methanogens increases, and acetoclastic methanogens, which rely on acetate to produce methane, dominate. and results in greater methane concentrations in the atmosphere. Hence, it is important to study changes in microbial ecology that drive global changes in methane cycling.

Conclusion  

The concentration of methane in the atmosphere is ~4000-fold less than carbon dioxide, but its global warming potential is ~30 times greater. Methanogenic archaea are the only known microorganisms that produce methane, and these microbes can inhabit extreme conditions in the environment. Methanogens in agricultural activities, including livestock, rice farming and manure, contribute a significant amount of methane. Due to the potential of methane to greatly increase global warming, the and ASM’s Microbes and Climate Change Report highlight the benefits of reducing methane emissions, which have contributed ~0.5o of temperature increase since pre-industrial times. Some of the mitigation measures outlined are aimed at improving the management of water in rice fields, better feeding of livestock, as well as .

In other endeavors, methanogens are being harnessed for their ability to degrade organic matter in wastewater treatment, and efforts are being undertaken to capture methane emitted in the process for use as fuel. Implementing measures to control methane emissions from both natural and anthropogenic sources could by 0.25 °C by 2050 and by 0.5 °C by 2100.

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