Abstract
Methane is the second most abundant greenhouse gas (GHG), with nearly 60% of emissions derived from anthropogenic sources. Microbial conversion of methane to fuels and value-added chemicals offers a means to reduce GHG emissions, while also valorizing this otherwise squandered high-volume, high-energy gas. However, to date, advances in methane biocatalysis have been constrained by the low-productivity and limited genetic tractability of natural methane-consuming microbes. Here, leveraging recent identification of a novel, tractable methanotrophic bacterium, Methylomicrobium buryatense, we demonstrate microbial biocatalysis of methane to lactate, an industrial platform chemical. Heterologous overexpression of a Lactobacillus helveticus L-lactate dehydrogenase in M. buryatense resulted in an initial titer of 0.06 g lactate/L from methane. Cultivation in a 5 L continuously stirred tank bioreactor enabled production of 0.8 g lactate/L, representing a 13-fold improvement compared to the initial titer. The yields (0.05 g lactate/g methane) and productivity (0.008 g lactate/L/h) indicate the need and opportunity for future strain improvement. Additionally, real-time analysis of methane utilization implicated gas-to-liquid transfer and/or microbial methane consumption as process limitations. This work opens the door to develop an array of methanotrophic bacterial strain-engineering strategies currently employed for biocatalytic sugar upgrading to “green” chemicals and fuels.
Highlights
IntroductionWe report microbial biocatalysis of methane to an industrial platform chemical, lactate, a precursor to the biodegradable polylactide (PLA) polymer used in bioplastics
In order to facilitate regulated, heterologous gene expression in the M. buryatense, we constructed an inducible, broad-host range vector, pCAH01, (Fig. 2a) by fusing the tetracycline promoter/operator from pASK75 with the IncP-based pAWP78 vector that can be replicated by Methylomicrobium spp.[22,23]
CH4-rich biogas derived from energy crops, crop residues, biofuel residues, manure, and other organic waste streams, through anaerobic digestion in agricultural/food waste digesters, waste water treatment plants (WWTP), and landfills, offers a versatile, high-volume, renewable source of CH428,29
Summary
We report microbial biocatalysis of methane to an industrial platform chemical, lactate, a precursor to the biodegradable polylactide (PLA) polymer used in bioplastics. We demonstrate effective genetic engineering strategies in a methanotrophic bacterium, enabling production of lactate from both CH4 and CH3OH as sole carbon sources. The presented route circumvents competition with food substrates, such as corn, utilized in conventional sugar-based lactate production, and offers a potentially transformational path to concurrent mitigation of GHG emissions and biological CH4 upgrading
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