Quantifying the metabolic heat from aerobic methanotrophs

ABSTRACT Microorganisms must dissipate some energy as heat to keep energy-capturing reactions thermodynamically favorable. For methane-oxidizing bacteria (methanotrophs), circumstantial evidence, such as temperature increases within biofilters and landfills, suggests that the metabolic heat they generate might be valorized. However, the precise rate and yield of heat produced by different methanotrophic species have not been measured. Here, we measured heat flux from pure cultures of two methanotrophs, Methylomonas methanica S1 (type I) and Methylosinus trichosporium OB3b (type II). Isothermal calorimetry of sealed batch cultures at 30°C under methane concentrations of 8%, 4%, and 2% (vol/vol) in air showed that both species dissipated approximately 60% of the substrate energy as heat. Biomass-specific heat yields ranged from 1,473 to 2,000 kJ/Cmol for M. methanica S1 and from 2,638 to 3,728 kJ/C-mol for M. trichosporium OB3b. These values exceed experimentally reported heat yields for other aerobic microorganisms, suggesting that methane-oxidizing bacteria may represent the upper limit of metabolic heat production among aerobic microorganisms and are unique in their ability to dissipate significant metabolic heat at comparative biomass yields. This work provides the first direct quantification of heat generation during biological methane oxidation, establishing a necessary foundation to leverage this resource within methane mitigation biotechnologies and wider engineering applications. IMPORTANCE Methanotrophic bacteria dissipate substantial metabolic energy as heat while oxidizing methane, a trait that may have evolved to manage excess energy. If harnessed, this process positions these microorganisms as effective agents for passively mitigating methane emissions while providing an alternative source for energy recovery. Although field studies have suggested the potential for significant heat generation from biological methane oxidation, experimental quantification has been lacking. Here, we quantitatively measured the heat evolved from two species of methanotrophs across different methane concentrations. The observed heat yields are far higher than previously recorded for aerobic microorganisms, setting a new empirical boundary for aerobic metabolic heat production.