Dynamics of bacterial communities and substrate conversion during olive-mill waste dark fermentation: Prediction of the metabolic routes for hydrogen production.

The aim of this work was to study the biological catalysts and possible substrate conversion routes in mesophilic dark fermentation reactors aimed at producing H from olive mill wastewater. Bacillus and Clostridium were the most abundant phylotypes during the rapid stage of H production. Chemical analyses combined with predictive functional profiling of the bacterial communities indicated that the lactate fermentation was the main H-producing route. In fact, during the fermentation process, lactate and acetate were consumed, while H and butyrate were being produced. The fermentation process was rich in genes that encode enzymes for lactate generation from pyruvate. Lactate conversion to butyrate through the generation of pyruvate produced H through the recycling of electron carriers via the pyruvate ferredoxin oxydoreductase pathway. Overall, these findings showed the synergy among lactate-, acetate- and H-producing bacteria, which complex interactions determine the H production routes in the bioreactors.