Abstract
The fermentation of glucose using microbial mixed cultures is of great interest given its potential to convert wastes into valuable products at low cost, however, the difficulties associated with the control of the process still pose important challenges for its industrial implementation. A deeper understanding of the fermentation process involving metabolic and biochemical principles is very necessary to overcome these difficulties. In this work a novel metabolic energy based model is presented that accurately predicts for the first time the experimentally observed changes in product spectrum with pH. The model predicts the observed shift towards formate production at high pH, accompanied with ethanol and acetate production. Acetate (accompanied with a more reduced product) and butyrate are predicted main products at low pH. The production of propionate between pH 6 and 8 is also predicted. These results are mechanistically explained for the first time considering the impact that variable proton motive potential and active transport energy costs have in terms of energy harvest over different products yielding. The model results, in line with numerous reported experiments, validate the mechanistic and bioenergetics hypotheses that fermentative mixed cultures products yielding appears to be controlled by the principle of maximum energy harvest and the necessity of balancing the redox equivalents in absence of external electron acceptors.
Highlights
Carbohydrates anaerobic fermentation towards volatile fatty acids (VFAs) has an increased interest due to its potential to provide building blocks from wastes towards a plethora of diverse valuable products
Did fall short in accurately predict the experimental product formation yields as function of operational conditions beyond very small ranges [24, 27], arising from the incompletely defined roles of electron carriers, the use of incomplete metabolic networks and the specific modelling approaches used for the transport processes across the cell membrane. We have identified these three aspects as the key factors limiting the predictive capacity of the existing energy-based Mixed culture fermentation (MCF) models which have to be addressed
FAD reduction can be coupled to energy harvest by proton translocation [17]; FAD(H2) electron carrier plays the role of an intermediate facilitating the coupling of a highly energetic metabolic reaction with the generation of proton motive force described with the oxidation of NADH following the mechanism presented in Eqs (2–4) [58,59,60,61] (Section B in S1 File and Section J in S1 File)
Summary
Carbohydrates anaerobic fermentation towards volatile fatty acids (VFAs) has an increased interest due to its potential to provide building blocks from wastes towards a plethora of diverse valuable products. Did fall short in accurately predict the experimental product formation yields as function of operational conditions beyond very small ranges [24, 27], arising from the incompletely defined roles of electron carriers, the use of incomplete metabolic networks and the specific modelling approaches used for the transport processes across the cell membrane.
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