KAIST: "Greenhouse Gas Reduction and Bioplastic Production Using Ethane"

A team of Korean researchers has presented the potential for utilizing ethane (C2H6) in both greenhouse gas reduction and bioplastic production.

On August 7, KAIST announced that a research team led by Professor Jaeuk Myung from the Department of Civil and Environmental Engineering, in collaboration with Stanford University in the United States, has identified the effects of ethane?a major minor component of natural gas?on the core metabolism of the "obligate methanotroph" (Methylosinus trichosporium OB3b).

(From left) Professor Jaeuk Myung, Doctoral Candidate Sunho Park, Dr. Chungheon Shin, Professor Craig S. Criddle. Provided by KAIST

Methanotrophs are bacteria that grow using methane as an energy source under aerobic conditions. Among them, "obligate methanotrophs" are characterized by their ability to use only C1 compounds such as methane or methanol as growth substrates. For this reason, prior studies had not investigated how obligate methanotrophs respond to ethane, which is not a growth substrate.

In contrast, through this joint research, the team discovered that ethane, a C2 substrate, significantly affects key metabolic pathways of obligate methanotrophs?including methane oxidation, cell growth, and the synthesis of the biodegradable polymer polyhydroxybutyrate (PHB)?even though ethane itself is not used as a growth substrate.

When methanotrophs were cultured under various methane and oxygen concentrations with added ethane, three consistent metabolic responses were observed: inhibition of cell growth, reduction in methane consumption, and increase in PHB synthesis. Notably, these changes became more pronounced as the concentration of ethane increased.

Based on these findings, ethane does not react independently in methanotrophs, and the bacteria do not grow when only ethane is provided. Conversely, when methane is present together with ethane, a phenomenon called "co-oxidation" was observed, in which ethane is oxidized simultaneously via the key methane-oxidizing enzyme, particulate methane monooxygenase (pMMO).

Methane is approximately 25 times more potent as a greenhouse gas than carbon dioxide (CO2), making it one of the most urgent targets for reduction in climate change mitigation. In other words, it can be inferred that using ethane to co-oxidize methane may contribute to greenhouse gas reduction.

From another perspective, during the oxidation process, ethane produces an intermediate metabolite called acetate, which was found to inhibit cell growth in methanotrophs while simultaneously promoting PHB production. PHB is a polymer material gaining attention as a raw material for biodegradable bioplastics, and these findings suggest the possibility of producing bioplastics using ethane.

This action varies depending on the nutritional status of the bacteria. When nutrients are sufficient, ethane negatively affects cell growth. However, under nutrient imbalance, ethane instead induces PHB accumulation, resulting in a positive effect.

Additionally, while the addition of ethane reduces methane consumption, there was no significant change in the expression level of the pmoA gene, which encodes a subunit of the methane-degrading enzyme pMMO.

This demonstrates that ethane does not affect the transcription level of the gene, but instead influences the actual functional activity (activity level) of the enzyme or post-transcriptional regulatory stages.

Based on these results, the joint research team concluded that ethane acts as a regulator that indirectly controls the metabolic flow of methanotrophs and, when present with methane, unintentionally affects both cell growth and PHB production.

Professor Jaeuk Myung stated, "This study is significant as it is the first to systematically elucidate how obligate methanotrophs metabolically respond under mixed substrate conditions with ethane, rather than in a single-substrate environment. By revealing the effects of non-growth substrates like ethane on methane metabolism and biodegradable polymer production, the joint research team has not only suggested new possibilities for biological methane mitigation but also for bioplastic production."

This research was supported by the National Research Foundation of Korea, the Ministry of Land, Infrastructure and Transport, and the Ministry of Oceans and Fisheries. Sunho Park, a doctoral candidate in the Department of Civil and Environmental Engineering at KAIST, participated as the first author. The research results (paper) were published on July 10 in the journal "Applied and Environmental Microbiology" of the American Society for Microbiology.