Construction Of Lactic Acid Producing Saccharomyces Cerevisiae And Application To Microbial Consortium For Electricity Production

The overuse of fossil fuels has raised concerns about global pollution,prompting the development of clean energy sources.Microbial fuel cell(MFC)is an environment-friendly energy producing device that converted chemical energy into electrical energy.Shewanella oneidensis MR-1,one of widely used exoelectrogens in MFCs,could only utilize limited spectrum of carbon sources,such as lactic acid.Compared to microorganisms cultured individually,microbial constrium could perform more complex tasks through labor division.Lactic acid is a naturally occurring organic acid,which has important applications in food and chemical industries,and is also an important carbon source for many microorganisms.Traditional lactic acid production mainly relied on Lactobacillus fermentation.However,the complex culture media and poor acid tolerance limited the large-scale lactic acid production.S.cerevisiae,as the first eukaryotic organism having its whole genome sequenced,is an important eukaryotic model organism and is suitable for industrial production due to the good acid tolerance,robustness,and requiring simple and economical media.However,S.cerevisiae has a Crabtree effect,and ethanol is the main byproduct during the glucose fermentation process.To obtain yeast strain with high lactic acid production,the ethanol synthesis pathway was firstly blocked to construct an ethanol-free yeast strain by knocking out the genes encoding pyruvate decarboxylase(PDC1,PDC5,PDC6)and the genes encoding ethanol dehydrogenase(ADH1 and ADH4).Subsequently,the LDH gene was integrated so that the yeast strain could metabolize glucose to produce lactic acid without ethanol.Finally,a xylose metabolism pathway was further integrated to enable yeast to produce lactic acid using mixed sugars of glucose and xylose.To expand the carbon source scopes available to S.oneidensis,the lactic acid-mediated S.cerevisiae-S.oneidensis microbial consortium was constructed in this study.S.cerevisiae(fermenter)converted glucose and xylose to lactic acid,while S.oneidensis(exoelectrogens)utilized lactic acid to produce electricity.Most importantly,S.cerevisiae stays at the bottom of the culture of MFC anode chamber,and does not form biofilms on anode surfaces under the anaerobic MFC conditions,which enabled more effective anode surfaces for exoelectrogens to generate electricity.The microbial consortium can produce electricity from glucose and xylose by adjusting the pH and components of the co-culture environment.To achieve the best MFC performance,the ratio of the strains and the concentration of carbon source were further optimized.Meanwhile,the flavins synthetic pathway was integrated into S.oneidensis,and thus a maximum output power density of 238.7±14.5 mW/m~2 in the microbial consortium was reached.The microbial consortium achieved a mutually beneficial symbiosis for converting the chemical energy in the carbon source into electrical energy,promoting the efficient conversion of the carbon metabolic flow to electrical energy through the pulling effect of the downstream electron flow.This research providesd new ideas for microbial fuel cell research.