Is there someone else who is the “culprit” of methane production?

Produced by: Science Popularization China

Author: Yang Changjialian (Institute of Microbiology, Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

In 1977, scientist Carl Woese accidentally discovered a new domain of life, Archaea, by analyzing biological sequences, which was like adding a mysterious new member to the known biological family tree.

Although these microorganisms are morphologically similar to bacteria and do not have a nucleus, their genetic and biochemical characteristics show that they are closer to eukaryotic organisms (such as plants and animals). This discovery not only overturns our understanding of the evolution of life, but also opens up new explorations of the microbial world for scientists.

Three-domain system of life

(Image source: Reference 4)

Methanogenic Archaea: Unsung Heroes of Earth's Carbon Cycle

Among all archaea, methanogens are the most eye-catching. They are one of the earliest prokaryotic microorganisms on Earth and can degrade organic matter into methane in an anaerobic environment, commonly known as "biogas fermentation." This unique metabolic method makes them the main "contributors" to global methane emissions.

The traditional view is that methanogenic archaea mainly belong to the Euryarchaeota in the Archaea domain, but recent studies have found that other archaeal branches also have the potential to produce methane.

Scientists have discovered many methanogenic archaea outside the phylum Euryarchaeota in nature. These new archaea can not only produce methane, but may also participate in multiple metabolic pathways such as fermentation and sulfur metabolism, making their role in the global carbon cycle more complex and diverse.

Methane production: from simple molecule to greenhouse gas

Methane (CH4) is a simple but important organic molecule consisting of one carbon atom and four hydrogen atoms. Although its concentration in the atmosphere is much lower than that of carbon dioxide, its global warming potential (GWP) (a measure of the impact of greenhouse gases on global warming) is 30 times that of carbon dioxide, and can even reach 84-87 times on a 20-year scale.

The production of methane mainly depends on methanogenic archaea, which can convert organic matter (such as carbon dioxide, hydrogen, formate, acetate, etc.) into methane in an anaerobic environment. This process is like a miniature chemical factory, involving a series of complex biochemical reactions, among which the key step is catalyzed by the methyl-CoA reductase (Mcr) complex.

Greenhouse effect diagram

(Image source: Reference 1)

New discoveries on methanogenesis in non-Euphrarchaea

Although it is widely believed that only Euryarchaea can produce methane, recent metagenomic studies have challenged this view. Genes encoding the Mcr complex have been found in multiple non-Euryarchaea phyla, indicating that these archaea also have the potential to produce methane.

Although genome analysis has shown the potential methane production ability of these archaea, the lack of pure culture has prevented in-depth study of their metabolic mechanisms and ecological functions. These archaea have been in a "dark matter" state, and their actual methane production ability cannot be verified experimentally.

A research team from the Chengdu Biogas Science Research Institute of the Ministry of Agriculture and Rural Affairs of China, in collaboration with Wageningen University in the Netherlands, spent seven years to successfully isolate and cultivate a new type of methanogenic archaea, Methanosuratincola petrocarbonis LWZ-6, from oil fields through the cocktail separation method (providing diverse growth conditions to help screen and isolate specific microbial species from complex environmental samples, similar to adding various ingredients to a "cocktail" to find the most suitable combination of ingredients to successfully cultivate the target microorganism).

This is the first time that humans have obtained pure culture from non-Euphrarchaea, providing valuable experimental materials for studying the carbon metabolism mechanisms and ecological functions of these archaea.

LWZ-6 is a thermophilic archaeon belonging to the Methanosuratincolia class of the Thermoproteinae. The LWZ-6 strain only uses methanol and monomethylamine as electron acceptors (accepting electrons transferred from other molecules and generating energy through redox reactions) and hydrogen as electron donors to produce methane, but does not have the ability to ferment sugars, peptides or amino acids. This discovery is like a key that unlocks our new understanding of the methane production pathway and reveals the importance of non-euryarchaea in global methane emissions and carbon cycles.

Metabolic pathways of LWZ-6 strain

(Image source: Reference 2)

Environmental impacts of methane and response strategies

As a potent greenhouse gas, methane's impact on global warming cannot be underestimated. Since the Industrial Revolution, the concentration of methane in the atmosphere has increased 2.5 times. It is predicted that if no effective measures are taken, methane emissions will cause global temperatures to rise by 0.5°C by 2100.

To address the climate challenge of methane emissions, we have proposed a series of emission reduction strategies. For example, in the agricultural sector, measures such as improving water resource management in rice fields, optimizing livestock breeding methods, and adjusting dietary structure can all help reduce methane emissions; in the industrial sector, using methanogenic archaea to degrade organic matter in wastewater and capture methane released during fuel acquisition is also an effective means of emission reduction.

Methane cycle diagram

(Image source: Reference 3)

Conclusion

Methanogenic archaea are among the oldest and most important microorganisms on Earth. They not only played a key role in the formation of the Earth's early environment, but also play an important role in modern carbon cycles and climate change. By studying these archaea in depth, scientists can better understand the complex mechanisms of the Earth's carbon cycle and develop more effective methane reduction strategies, providing a scientific basis for addressing global climate change.

The discovery of Methanosuratincola petrocarbonis LWZ-6 marks a major breakthrough in archaea research, revealing the importance of non-euryarchaea in methane production. In the future, with the advancement of technology and in-depth research, we are expected to further uncover the mysteries of more archaea and explore their unique roles in the Earth's ecosystem.

References:

1.Fady Jameel. Not just hot air

2. Wu, K., Zhou, L., Tahon, G. et al. Isolation of a methyl-reducing methanogen outside the Euryarchaeota. Nature (2024).

3.Kanika Khanna, How Methanogenic Archaea Contribute to Climate Change, May 6, 2022

4.Woese CR, Kandler O, Wheelis ML. Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proc Natl Acad Sci US A. 1990;87(12):4576-4579.

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