Effects of lignocellulose-derived compounds on fermentative hydrogen-producing mixed cultures

International audience; Hydrogen (H2) is one of the most promising energy carriers because it is energy dense, clean burning and can be renewably produced. Dark fermentation using mixed cultures is an attractive biotechnology to produce H2 at low cost from lignocellulosic biomass, the most abundant renewable resource. However, the dilute acid pre-treatment, generally used to convert the complex biomass into monosaccharides, also releases other degradation products which can inhibit microbial growth and fermentation. The aim of this study was to investigate the effects of added lignocellulose-derived compounds (i.e. furan derivatives such as furfural, HMF, phenolic compounds such as phenol, syringaldehyde, vanillin and kraft and organosolv lignin) on fermentative H2 production and associated bacterial communities. H2-producing batch tests were performed using anaerobic heat-treated digested sludge with various substrates: xylose alone as control, xylose in presence of inhibitory compounds and inhibitory compounds alone. Bacterial community were monitored by Capillary-Electrophoresis Single Strand Conformation Polymorphism (CE-SSCP) fingerprinting using bacterial-specific 16S rDNA primers and H2-producing bacterial-specific primers targeting functional hydA genes, encoding the catalytic sub-unit of [FeFe]-hydrogenases. Results indicated that all the lignocellulose-derived compounds tested were unable to support fermentative H2 production in the absence of additional carbon source. When added to xylose, all tested compounds significantly decreased fermentative H2 production performances. H2 yields were more affected by the addition of furan derivatives than phenol compounds. Except for lignins, the higher the molecular mass was, the shorter the lag phase time was. Noteworthily, lignins affected slightly the lag phase time but largely contributed to the H2 production instability, which can be explained by their physicochemical properties (e.g. hydrophobicity, ramification, methoxylation). Although positive correlation was observed between butyrate and H2 productions, the composition and distribution of soluble metabolites varied in accordance with the inhibitor type. Likewise, changes in bacterial community structure and diversity were also associated with inhibitor tested. Clostridial species dominated all mixed cultures and their key role in the H2 production was confirmed by the detection of clostridial hydA genes. Interestingly, Clostridium beijerinkii was found less sensitive to inhibitors, making it ideal candidate for H2 production from hydrolysates of lignocellulosic biomass. To improve the efficiency of fermentative H2 production from lignocellulosic biomass, these results suggest the application of efficient delignification and depolymerisation processes releasing low amounts of furan derivatives and phenolic compounds and/or the use of microbial communities adapted to lignocellulose-derived inhibitors.