Redox regulation of hydrogen production in Escherichia coli during growth on by-products of the wine industry.

Lignocellulosic wine grape waste (WGW) is a cheap medium for Escherichia coli growth and H production. The current study investigated the effect of initial redox potential (oxidation-reduction potential (ORP)) on the growth, H production, ORP kinetics, and current generation of the E. coli BW25113 parental and a mutant strain optimized for hydrogen evolution when fermenting WGW (40 g L) hydrolysate. Bacteria were cultivated anaerobically on pre-treated WGW hydrolysate with dilutions ranging from undiluted to fourfold dilution, at pH 7.5. Notably, a twofold diluted medium, with pH adjustment using KHPO, exhibited reduced acidification, prolonged H production, and enhanced biomass formation (OD, 1.5). The addition of the redox reagent DL-dithiothreitol (DTT) was found to positively influence the H production of both the E. coli BW25113 parental and mutant strains. H production started after 24 h of growth, reaching a maximum yield of 5.10 ± 0.02 mmol/L in the wild type and 5.3 ± 0.02 mmol/L in the septuple mutant strain, persisting until the end of the stationary growth phase. The introduction of 3 mM DTT induced H production from the early-exponential phase, indicating that reducing conditions enhanced H production. Furthermore, we assessed the efficacy of using intact E. coli cells (1.5 mg cell dry weight) as anode catalyst in a bio-electrochemical fuel-cell system. Whole cells of the septuple mutant grown under reduced ORP conditions yielded the highest electrical potential, reaching up to 0.7 V. The results highlight the potential of modifying medium buffering capacity and ORP as a tool to improve biomass yield and H production during growth on WGW for biotechnological biocatalyst applications. KEY POINTS: • Grape pomace hydrolysate (GPH)'s pH positively impacts on biomass and H yield • GPH with reduced ORP enhanced H yield in bacterial early-exponential growth • GPH with reduced ORP facilitates microbial current generation in the system.