Formic acid as a secondary substrate for succinic acid production by metabolically engineered Mannheimia succiniciproducens

There has been much effort exerted to reduce one carbon (C1) gas emission to address climate change. As one promising way to more conveniently utilize C1 gas, several technologies have been developed to convert C1 gas into useful chemicals such as formic acid (FA). In this study, systems metabolic engineering was utilized to engineer Mannheimia succiniciproducens to efficiently utilize FA. C-13 isotope analysis of M. succiniciproducens showed that FA could be utilized through formate dehydrogenase (FDH) reaction and/or the reverse reaction of pyruvate formate lyase (PFL). However, the naturally favored forward reaction of PFL was found to lower the SA yield from FA. In addition, FA assimilation via FDH was found to be more efficient than the reverse reaction of PFL. Thus, the M. succiniciproducens LPK7 strain, which lacks in pfl, ldh, pta, and ack genes, was selected as a base strain. In silico metabolic analysis confirmed that utilization of FA would be beneficial for the enhanced production of SA and suggested FDH as an amplification target. To find a suitable FDH, four different FDHs from M. succiniciproducens, Methylobacterium extorquens, and Candida boidinii were amplified in LPK7 strain to enhance FA assimilation. High-inoculum density cultivation using C-13 labeled sodium formate was performed to evaluate FA assimilation efficiency. Fed-batch fermentations of the LPK7 (pMS3-fdh2meq) strain was carried out using glucose, sucrose, or glycerol as a primary carbon source and FA as a secondary carbon source. As a result, this strain produced 76.11g/L SA with the yield and productivity of 1.28mol/mol and 4.08g/L/h, respectively, using sucrose and FA as dual carbon sources. The strategy employed here will be similarly applicable in developing microorganisms to utilize FA and to produce valuable chemicals and materials from FA.