Total solid content drives hydrogen production through microbial selection during thermophilic fermentation

[Display omitted] •Higher hydrogen production under wet than dry conditions.•Hydrogen-producing pathways first impacted by dry conditions.•H2-producing pathways shifts to lactic acid metabolism at 19%TS.•Solid-state fermentation favours the growth of Bacilli and Bacteroidetes species. In this study, the effect of total solid content (TS) on thermophilic hydrogen production from wheat straw was investigated. Six TS contents ranging from wet to dry conditions (10–34%TS) were tested in batch tests. A decrease of H2 yields was observed and three statistical groups were distinguished according to the TS content: wet conditions (10% and 14%TS) with 15.3±1.6NmlH2gTS−1, intermediate conditions (19%TS) with 6.4±1.0NmlH2gTS−1 and dry conditions (25–34%TS) with 3.4±0.8NmlH2gTS−1. Such a decrease in biohydrogen yields was related to a metabolic shift with an accumulation of lactic acid under dry conditions. Concomitantly, a microbial population shift was observed with a dominance of species related to the class Clostridia under wet conditions, and a co-dominance of members of Bacilli, Clostridia classes and Bacteroidetes phylum under dry conditions.