Microbial and functional characterization of an allochthonous consortium applied to hydrogen production from Citrus Peel Waste in batch reactor in optimized conditions

Energy recovery from lignocellulosic waste has been studied as an alternative to the problem of inappropriate waste disposal. The present study aimed at characterizing the microbial community and the functional activity of reactors applied to H2 production through lignocellulosic waste fermentation in optimized conditions. The latter were identified by means of Rotational Central Composite Design (RCCD), applied to optimize allochthonous inoculum concentration (2.32–5.68 gTVS/L of granular anaerobic sludge), pH (4.32–7.68) and Citrus Peel Waste (CPW) concentration (1.55–28.45 g/L). After validation, the conditions identified for optimal H2 production were 4 gSTV/L of allochthonous inoculum, 29.8 g/L of CPW (substrate) and initial pH of 8.98. In these conditions, 48.47 mmol/L of H2 was obtained, which is 3.64 times higher than the concentration in unoptimized conditions (13.31 mmol H2/L using 15 g/L of CPW, 2 gTVS/L of allochthonous inoculum, pH 7.0). Acetogenesis was the predominant pathway, and maximal concentrations of 3,731 mg/L of butyric acid and 3,516 mg/L of acetic acid were observed. Regarding the metataxonomic profile, Clostridium genus was dramatically favored in the optimized condition (79.78%) when compared to the allochthonous inoculum (0.43%). It was possible to identify several genes related to H2 (i.e dehydrogenases) and volatile fatty acids (VFA) production and with cellulose degradation, especially some CAZymes from the classes Auxiliary Activities, Glycoside Hydrolases and Glycosyl Transferase. By means of differential gene expression it was observed that cellulose degradation and acetic acid production pathways were overabundant in samples from the optimized reactors, highlighting endo-β-1,4-glucanase/cellulose, endo-β-1,4-xylanase, β-glucosidase, β-mannosidase, cellulose β-1,4-cellobiosidase, cellobiohydrolase, and others, as main the functions. [Display omitted] •Hydrogen production (48.47 mmol/L) was 3.64 times higher in optimized conditions.•Acetogenesis was the main pathway involved in Citrus Peel Waste (CPW).•Clostridium was the main genus involved in CPW digestion.•Cellulose degradation pathway was overabundant for optimized conditions.•H2 production was related with the gene pyruvate-ferredoxin oxidoreductase.