Abstract
Gas fermentation processes have attracted considerable attention in recent years as they hold high potential for capturing and converting C1 waste gases into a range of biofuels and commodity chemicals. The production of solvents in gas fermentation is typically achieved by exploiting the solventogenic metabolism of acetogenic cultures, which is generally triggered upon exposure to stressful conditions, e.g. low pH. Although the oxidoreduction potential (ORP) is a well-known trigger of the cellular stress response, it has been scarcely investigated as a process control parameter in gas fermentation. Thus, this study focused on evaluating the potential of ORP control strategies for boosting the productivity of ethanol by exploiting the metabolic response to oxidative stress of acetogenic cultures. The dynamics of the redox cofactor pool and ratio as a function of the extracellular ORP and other operational parameters were also studied by monitoring the intracellular levels of the redox cofactor NADH/NAD+. The results showed that increasing the ORP to oxidizing conditions using dilute H2O2 triggered a 3.7-fold increase in the specific ethanol productivity, from 0.63 ± 0.04 mmol∙gCDW−1 h−1 at an ORP of -210 mV to 2.32 ± 0.19 mmol∙gCDW−1 h−1 at 160 mV. Additionally, the concentration and product selectivity towards ethanol also increased considerably due to the partial inhibition of the chain elongation under oxidative stress. Boost in ethanol productivity and inhibition of the chain elongation were both found to be driven by the presence of H2O2 rather than by the ORP per se. Studying the profile of the redox cofactors revealed a highly dynamic nature in the pool and ratio of NADH/NAD+ as a function of the specific uptake rate and the ratio of acetate-to-ethanol, respectively. The latter was explained by analyzing the thermodynamics of the aldehyde:ferredoxin oxidoreductase (AOR) pathway, which showed that the intrinsic thermodynamic limitation of this pathway imposes a high Fdred/Fdox ratio (>88 % of reduced ferredoxin) while forcing a highly dynamic NADH/NAD+ ratio in order to maintain the thermodynamic drive in the forward direction. The dynamics of the NADH/NAD+ ratio were also found to be significantly affected by the oxidative stress triggered by dilute H2O2, which confirmed the involvement of the AOR pathway in the detoxification of reactive oxygen species.
Highlights
The threat of global warming and the insufficient CO2 emission reduction scenarios projected based on the Paris agreement and the national pledges make the implementation of carbon capture and utili zation technologies more imperative than ever
The results showed that the modulation of the oxidor eduction potential (ORP) causes severe alterations on the metabolic activity of the mi crobial community, with the latter being translated in sig nificant changes in the solventogenic and chain elongation activity of the culture
This study evaluated the use of ORP control strategies through the addition of dilute H2O2 for boosting the specific productivity of ethanol in an open-culture-based gas fermentation system
Summary
The threat of global warming and the insufficient CO2 emission reduction scenarios projected based on the Paris agreement and the national pledges make the implementation of carbon capture and utili zation technologies more imperative than ever. The need of decoupling transportation fuels from fossil resources along with the fact that carbon-based fuels and chemicals will not be phased out in the future, evidences that the latter must be sourced from renewable feed stocks. Gas fermentation has attracted significant attention as a viable approach to tackle both issues simultaneously, as this technology holds high potential for displacing fossil fuels by capturing and converting C1 waste gases, such as CO and CO2, into a range of biofuels and commodity chemicals [1,2].
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