Convert methane into lactic acid
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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.
Methane (CH4), the primary component of natural gas and anaerobic digestion-derived biogas, offers a promising, high-volume petroleum replacement for fuel and chemical bioprocesses. Recent advances in gas-recovery technologies have facilitated access to previously inaccessible natural gas reserves, while biogas generated from anaerobic digestion of waste streams offers a versatile, renewable CH4 source. However, the gaseous state of CH4 makes for a lack of compatibility with current transportation and industrial manufacturing infrastructure, limiting its utilization as a transportation fuel and intermediate in biochemical processes. Importantly, CH4 is also the second most abundant greenhouse gas (GHG), with nearly 60% of emissions derived from anthropogenic sources1. Microbial conversion of CH4 to value-added chemicals using natural CH4-consuming bacteria offers valorization potential2,3,4, while reducing GHG emissions.
Obligate methanotrophic bacteria (methanotrophs) are a unique group of microorganisms capable of utilizing CH4 or methanol (CH3OH) as their sole carbon and energy source. These bacteria use the enzyme methane monooxygenase (MMO) to convert CH4 to CH3OH, which is further oxidized to formaldehyde (CH2O), formate (CHOOH) and CO2. Depending on the metabolic arrangement, CH4-derived carbon is assimilated at the level of CH2O (via the Ribulose-monophosphate cycle), methylene tetrahydrofolate and CO2 (Serine cycle), or CO2 (Calvin cycle)5,6. In the past, methanotrophs have been exploited for the conversion of CH4 to an array of products7, including bioprotein8,9, polyhydroxybutyrate10, carotenoids11,12,13, vitamins14, and CH3OH15,16. However, advances in CH4 biocatalysis and methanotroph strain engineering have largely been limited by the low-productivity of methanotroph cultures and lack of genetic tools for use in these organisms3,7,17.
Recently, an active Embden–Meyerhof–Parnas (EMP) pathway was identified in novel gammaproteobacterial methanotrophs that are resistant to the toxic components of natural gas and biogas18,19,20,21, and a set of genetic tools, including expression vectors, have been developed for the halotolerant, alkaliphilic methanotrophic bacteria Methylomicrobium buryatense21,22. Given the conserved nature of their downstream metabolic machinery, conventional industrial strain-engineering routes from sugars to biochemical intermediates and products can potentially be paralleled in these methanotrophs. Here, 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.