Efficient Production of D-Lactate from Methane Using Lactate-Tolerant Methylomonas Sp. DH-1 Strain Generated By Adaptive Laboratory Evolution

Metabolic engineering of methanotrophs, which can use methane as a sole carbon and energy source, has emerged as a promising strategy to utilize methane, an abundant and inexpensive carbon source. In this study, we engineered Methylomonas sp. DH-1 to produced D-lactate from methane. Methylomonas sp. DH-1 belongs to type I methanotrophs, which assimilate methane by ribulose monophosphate (RuMP) pathway. Wild type strain can survive only up to 1.0 g/L lactate. Therefore, we improved lactate tolerance of Methylomonas sp. DH-1 by growing the cells with increasing concentrations of lactate up to 8.0 g/L, generating a lactate-tolerant strain Ev 8.0. D-Lactate production was achieved by integrating D form-specific lactate dehydrogenase gene (ldhA) from Leuconostoc mesenteroides subsp. mesenteroides ATCC 8293 into the genome of Ev 8.0. In a flask fed-batch culture with pH and medium optimization, the engineered strain produced 1.36 g/L D-lactate from methane after 144 h.