Abstract
In this contribution, we studied the effect of electro‐fermentation on the butanol production of Clostridium pasteurianum strains by a targeted metabolomics approach. Two strains were examined: an electrocompetent wild type strain (R525) and a mutant strain (dhaB mutant) lacking formation of 1,3‐propanediol (PDO). The dhaB‐negative strain was able to grow on glycerol without formation of PDO, but displayed a high initial intracellular NADH/NAD ratio which was lowered subsequently by upregulation of the butanol production pathway. Both strains showed a 3–5 fold increase of the intracellular NADH/NAD ratio when exposed to cathodic current in a bioelectrochemical system (BES). This drove an activation of the butanol pathway and resulted in a higher molar butanol to PDO ratio for the R525 strain. Nonetheless, macroscopic electron balances suggest that no significant amount of electrons derived from the BES was harvested by the cells. Overall, this work points out that electro‐fermentation can be used to trigger metabolic pathways and improve product formation, even when the used microbe cannot be considered electroactive. Accordingly, further studies are required to unveil the underlying (regulatory) mechanisms.
Highlights
Cellular redox metabolism is heavily perturbed and initial intracellular Nicotinamide adenine dinucleotide (NADH)/Nicotinamide adenine dinucleotide (NAD) ratios are elevated, but subsequent activation of butanol production enables a sufficient reoxidation of NADH by exploiting the bifurcation reaction of crotonyl-CoA to butyryl-CoA
Cultivation in a cathodic bioelectrochemical system (BES) showed that C. pasteurianum is not electroactive but that the applied current still causes a momentous increase of the intracellular NADH/NAD ratio
The results suggest that the silencing of the PDO pathway and electro-fermentation represent promising approaches to improve butanol production by C. pasteurianum
Summary
Significant progress has been made in improving metabolic engineering tools for C. pasteurianum [8,9,10,11] In this context, Schmitz et al [10] and Schwartz et al [11] followed a similar approach to improve butanol production by silencing the conversion of glycerol into PDO: both targeted the first step of PDO synthesis, namely the conversion of glycerol into 3-hydroxypropanal by the enzyme glycerol dehydratase, which is encoded by the dhaBCE genes in C. pasteurianum. While Schmitz et al deleted only the gene for the subunit (dhaB), Schwartz et al knocked out all genes Both options resulted in the cessation of PDO production, but Schwartz et al reported that the modified strain could only grow redox balanced in complex media with glycerol as the sole carbon source. Fed-batch cultivations were conducted in a system that allows automated fast-filtration for the superior quantification of intracellular metabolites [12,13]
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