Syngas Fermentation for the Production of Value-added Products: A Case Study Using a Novel Isolate Clostridium sp. AWRP

Abstract

Syngas fermentation has been considered an alternative way to ferment wastes that microbes cannot efficiently digest and utilize. The major combustion product of such wastes is carbon monoxide (CO), which can be utilized by carboxidotrophic and acetogenic microorganisms. Acetogens have been receiving great attention since they are able to synthesize C2 or higher metabolites from CO by carbon-carbon ligation. In spite of their distinguished feature, syngas fermentation using the acetogens is difficult in terms of culture conditions: i) almost all of the acetogens are oxygen-sensitive, requiring the ORP level of a culture medium below a certain level; ii) paradoxically, the formate dehydrogenase-hydrogenase (Fdh-Hyd) complex, which is a key enzyme for their carbon fixation, can be easily inhibited by CO, leading to severe growth retardation in early fermentation phases. This study introduces current research progress on Clostridium sp. AWRP, which shows a great potential for production of ethanol from CO or CO2 + H2. It was found from fed-batch fermentations that the metabolic profile of this strain appeared strongly dependent on oxidoreduction potential (ORP) when syngas was the substrate. Such a decrease of ORP might be due to inhibition of the Fdh-Hyd complex by CO, while CO dehydrogenase (CODH) being still active. Our findings suggest that relative metabolic fluxes of CO dehydrogenase and FDH may be important for determining metabolic profiles from syngas. Meanwhile, when using CO2 + H2, strain AWRP still produced ca. 10 g/L of ethanol as substrates with higher ORP levels. This result suggests that ORP is not all that matters, and that the role of metabolic engineering is still important for syngas fermentation by acetogens. [This work was supported by the KIOST in-house program (PE99822)]