Invited Talk: Lactic acid production using C1 or C6 carbon source

Biorefinery is defined as producing useful chemicals and fuels mainly via microbial fermentation. Currently, glucose and other biomass-derived sugars are used as major carbon sources for microbial fermentation, but other abundant and inexpensive carbon sources such as CO₂, CO and methane are considered as promising alternatives to sugars in biorefinery. We engineer Saccharomyces cerevisiae and Methylomonas sp. DH-1 to produced D-lactic acid using glucose and methane, respectively. Both organisms cannot naturally produce lactic acid. Therefore L or D form-specific lactic acid can be produced by introducing stereo-specific heterologous lactate dehydrogenase. By eliminating ethanol and glycerol production pathways, and adaptive laboratory evolution to increase lactate tolerance, we generate S. cerevisiae strain, which can produced D-lactic acid under acidic conditions with high titer, yield, and productivity. Methylomonas sp. DH-1 is a methanotroph which can use methane as a sole carbon and energy source. By adaptive laboratory evolution, lactic acid-tolerant strain is generated. Expression of D form-specific lactate dehydrogenase gene in the evolved strain leads to about 8-fold increase in D-lactic acid production from methane compared to that in wild type strain.