The University of Massachusetts Amherst
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PhD Defense, Xiangan Li, “Metabolic Models of Gas Fermentation for Renewable Fuels and Chemicals Production”


Friday, October 9, 2020 - 9:00am


via Zoom


Chair: Michael Henson

Gas fermentation has emerged as a technologically and economically attractive option for producing renewable fuels and chemicals from carbon monoxide (CO) rich waste streams. As compared to traditional catalyst technologies, microbial systems have several advantages including operation near ambient temperature and pressure, high conversion efficiencies, robustness to gas impurities and high product yields that have motivated both fundamental research and commercial development. While microbial production of high-value products from waste gases is challenging because wild-type strains capable of gas consumption tend to synthesize these products at low yields, strategy like metabolically engineering the gas fermenting acetogens have been studied to address this issue. Meanwhile, another promising alternative is to take advantage of the native capabilities for producing high-value products of wild-type strains and use coculture designs by combining gas-fermenting acetogens with bacterial strains that offer high yields of desired product.

In this study, motivated by our industrial collaborator LanzaTech, we first focused on combining hydrodynamics with a genome-scale reconstruction of Clostridium autoethanogenum metabolism and multiphase convection-dispersion equations to compare the performance of bubble column reactors with and without liquid recycle. For both reactor configurations, hydrodynamics was predicted to diminish bubble column performance when compared to bubble column models in which the gas phase was modeled as ideal plug flow plus axial dispersion. Liquid recycle was predicted to be advantageous by increasing CO conversion, biomass production, and ethanol and 2,3-butanediol production compared to the non-recycle reactor configuration. After this, we explored the possibilities of producing an important platform chemical butyrate by using wild-type strains in the continuous stirred tank bioreactors and developed two anaerobic coculture designs by combining C. autoethanogenum for CO- to-acetate conversion with environmental bacterium Clostridium kluyveri and the human gut bacterium Eubacterium rectale which offer high acetate-to-butyrate  conversion. A bubble column model developed to assess the potential for large-scale butyrate   production of the C. autoethanogenum-E. rectale design predicted that a 40/30/30 CO/H2/N2  gas mixture and a 5 meter column length would be preferred to enhance C. autoethanogenum growth and counteract CO inhibitory effects on E. rectale. This research was further developed by exploiting the diversity of 4 acetogens and 818 human gut bacteria for anaerobic synthesis of butyrate from acetate and ethanol. A total of 170 acetogen/gut bacterium/sugar combinations were dynamically simulated for continuous growth using a 70/30 CO/CO2 feed gas mixture and minimal media computationally determined for each combination. Our models generated several coculture designs with promising performance and robustness. Furthermore, our models indicate a general methodology for determining coculture designs in silico and expanding the product range for gas fermenting. We believe that our study represents an important contribution towards the development of microbial  platforms for gas fermentation.

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