A groundbreaking collaborative study between KAIST and Stanford University has shed new light on the intricate metabolism of obligate methanotrophs, specifically the bacterium Methylosinus trichosporium OB3b, under mixed-substrate conditions involving methane and ethane. Recognized for their ability to oxidize methane—a potent greenhouse gas with a global warming potential approximately 25 times greater than carbon dioxide—methanotrophs have traditionally been understood as organisms strictly utilizing single-carbon (C1) compounds. Until now, the impact of ethane (C2H6), a significant constituent of natural gas mixtures, on these microbes has remained largely unexplored. This research reveals that ethane, though not directly metabolizable for growth by obligate methanotrophs, profoundly disrupts their core methanotrophic processes, offering novel insights with implications for sustainable biopolymer production and methane biotechnologies.
Methanotrophs occupy a critical ecological niche by consuming methane and thereby mitigating its release into the atmosphere. Among them, obligate methanotrophs rely exclusively on C1 compounds for energy and carbon assimilation. However, naturally occurring methane emissions seldom involve pure methane; instead, emissions are complex mixtures, with ethane constituting up to 15% in natural gas. This study bridges a crucial knowledge gap by systematically examining how M. trichosporium OB3b responds metabolically to the presence of ethane alongside methane under controlled laboratory conditions, replicating the mixed-gas environment these bacteria would encounter in nature.
The investigation detailed a multifaceted metabolic response induced by ethane: instead of promoting growth, ethane addition consistently suppressed cell proliferation and reduced methane consumption rates. Paradoxically, the same conditions led to an increase in intracellular accumulation of polyhydroxybutyrate (PHB), a biodegradable storage polymer with significant industrial potential as a sustainable bioplastic precursor. This triad of effects—growth inhibition, decreased methane oxidation, and enhanced PHB synthesis—intensified with rising ethane concentrations, indicating a dose-dependent relationship indicative of ethane’s potent regulatory influence.
Crucially, the study uncovered that ethane oxidation only occurs concomitantly with methane presence, signifying co-oxidation mediated by particulate methane monooxygenase (pMMO). pMMO is the key enzyme complex enabling methane activation in methanotrophs, a notoriously challenging biochemical feat due to methane’s chemical inertness. The finding that pMMO can also co-oxidize ethane provides novel insight into substrate overlap and enzymatic promiscuity within the methanotrophic system, expanding our understanding of microbial adaptation in complex gaseous environments.
Further biochemical analyses revealed that acetate, an intermediate generated during ethane oxidation, plays a pivotal regulatory role. Elevated intracellular acetate concentrations were linked to inhibited cell growth while simultaneously stimulating PHB biosynthesis, suggesting acetate functions as a metabolic signal modulating carbon flow. This dual role routes carbon towards energy storage rather than biomass accumulation, representing a strategic shift in microbial resource allocation under nutrient-variable conditions. Such carbon partitioning highlights a nuanced metabolic flexibility that challenges the traditional view of obligate methanotrophs as rigid C1 specialists.
Moreover, supplementation with external reducing equivalents such as methanol or formate enhanced ethane oxidation without significantly altering methane consumption. This decoupling implies a competitive intracellular environment where ethane oxidation demands shared reducing power, affecting resource prioritization within the cell. These dynamics reveal a complex interplay between substrate availability, enzymatic activity, and redox balance, demonstrating intricately coordinated metabolic regulation within M. trichosporium OB3b.
Notably, despite the physiological impact of ethane, the expression levels of pmoA—the gene encoding the pMMO enzyme—remained constant. This suggests that ethane’s inhibitory effects on methane uptake and cellular growth are regulated post-transcriptionally or at the enzymatic level rather than via gene expression changes. This finding underscores the complexity of metabolic control in methanotrophs and points to additional layers of regulation such as enzyme activity modulation, protein-protein interactions, or metabolic feedback inhibition mechanisms sensitive to ethane-derived metabolites.
Professor Jaewook Myung, leading the KAIST research team, emphasizes that this is the first comprehensive delineation of obligate methanotroph behavior in the presence of ethane. The study overturns previous assumptions that non-growth substrates are metabolically inert to these bacteria, instead showing ethane’s profound influence on metabolic fluxes and biopolymer synthesis pathways. This revelation opens new directions for engineering methanotroph-based biotechnologies, potentially enabling tailored bioplastic production using natural gas feedstocks comprising mixed hydrocarbons rather than pure methane alone.
The implications for environmental biotechnology are significant. Understanding how mixed hydrocarbons affect methanotrophic metabolism enhances the feasibility of leveraging these bacteria for methane mitigation strategies, bioconversion of natural gas components, and sustainable materials production. By mapping the intricate interactions among substrates, enzymes, and metabolic pathways, the study provides a foundation for optimizing microbial processes to increase yields of valuable compounds like PHB, promoting green alternatives to petrochemical-derived plastics.
This research received funding support from the National Research Foundation of Korea, the Ministry of Land, Infrastructure and Transport, and the Ministry of Oceans and Fisheries, underscoring its national importance and potential impact. The full findings are documented in Applied and Environmental Microbiology, a prestigious journal of the American Society for Microbiology, solidifying the study’s contribution to the scientific understanding of microbial ecology and industrial biotechnology.
Future research directions may build on these findings by exploring the mechanistic basis of ethane-mediated post-transcriptional regulation, the structural biochemistry of pMMO in co-oxidation scenarios, and the metabolic engineering of M. trichosporium for enhanced biopolymer production under mixed-substrate regimes. Expansion to other methanotroph species and varying substrate mixtures could reveal broader principles applicable to microbial ecology, climate mitigation, and biomanufacturing.
In summary, the revelation that ethane—historically viewed as a non-utilizable substrate—can actively perturb methane oxidation and stimulate valuable biopolymer synthesis redefines our perspective on methanotroph physiology. This study provides a vital conceptual framework for harnessing these microorganisms in mixed-gas environments, bringing us closer to realizing their full potential in sustainable biotechnology.
Subject of Research: Metabolic effects of ethane on obligate methanotroph Methylosinus trichosporium OB3b related to methane oxidation and PHB biosynthesis
Article Title: Non-growth substrate ethane perturbs core methanotrophy in obligate methanotroph Methylosinus trichosporium OB3b upon nutrient availability
Web References: http://dx.doi.org/10.1128/aem.00969-25
References: Published in Applied and Environmental Microbiology
Image Credits: KAIST
Keywords: Cell biology, Methanotrophy, Metabolism, Methane oxidation, Ethane co-oxidation, Polyhydroxybutyrate (PHB), Bioplastic production, pMMO, Acetate metabolism, Microbial biotechnology
