title: Bioconversion of methane to lactate by an obligate methanotrophic bacterium.
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.
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
authors.forename: Calvin A
authors.initials: CA
authors.lastname: Henard
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
authors.forename: Holly
authors.initials: H
authors.lastname: Smith
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
authors.forename: Nancy
authors.initials: N
authors.lastname: Dowe
authors.affiliation: Department of Biology, San Diego State University, San Diego, CA 92182 USA.
authors.forename: Marina G
authors.initials: MG
authors.lastname: Kalyuzhnaya
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
authors.forename: Philip T
authors.initials: PT
authors.lastname: Pienkos
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Guarnieri
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D055162:Biocatalysis; D019149:Bioreactors; D005285:Fermentation; D015870:Gene Expression; D007700:Kinetics; D007770:L-Lactate Dehydrogenase / Q000235:genetics / Q000378:metabolism*; D019344:Lactic Acid / Q000096:biosynthesis*; D052198:Lactobacillus helveticus / Q000201:enzymology / Q000235:genetics*; D060847:Metabolic Engineering; D008697:Methane / Q000378:metabolism*; D008772:Methylococcaceae / Q000235:genetics / Q000378:metabolism*; D010957:Plasmids / Q000737:chemistry / Q000378:metabolism; D014169:Transformation, Bacterial; D019076:Transgenes
chemical_list: D001426:Bacterial Proteins; D019344:Lactic Acid; D007770:L-Lactate Dehydrogenase; D008697:Methane