Valorising fermentation effluent rich in short-chain fatty acids and sugars for biohydrogen via photofermentation by Rhodobacter sphaeroides KKU-PS1

Abstract

Growing fermentative chemical production will increase effluents from industrial fermentations containing short-chain fatty acids and residual sugars, which are exploitable for biohydrogen through photofermentation. Previous studies have concentrated on single substrates and photofermentation study using fermentation effluent from bio-succinate production containing residual sugars and short-chain organic acids has yet to be reported to the best of authors' knowledge. Rhodobacter sphaeroides KKU-PS1 grown on succinate was used for hydrogen production from medium containing mixture of substrates mimicking final effluent from bio-based succinate production. Prior to that, hydrogen producibility test with succinate-only medium was carried out. Photofermentation from succinate by this strain yielded 1217 ml H2/l of maximum cumulative hydrogen with maximum hydrogen rate of 6.7 ml H2/l/h, comparable to malate which was previously reported as best single substrate for the strain. Hydrogen production profiles using mixed substrates was well-fitted by modified Gompertz model with maximum cumulative hydrogen and maximum hydrogen production rate of 1005 ml H2/l and 4.1 ml H2/l/h, respectively. Only glucose, xylose and succinate followed modified Gompertz model for substrate consumption. Instantaneous succinate consumption compared to extended lag time of 100h for consumption of both sugars indicated higher affinity towards short-chain fatty acid utilization during initial growth phase. Xylose showed highest overall substrate consumption signifying its importance for hydrogen generation, which continued after stationary growth phase started reaching a total of 91.9% consumption. Significant remaining substrate levels other than xylose suggested that the process was not inhibited by limited substrates. The study highlighted potential of fermentation effluents containing mixed substrates for biohydrogen, with further optimization needed.