Role of indigenous bacteria in dark fermentation of organic substrates

[Display omitted] •Seven substrates were used to produce H2 with and without microbial inoculum.•H2 and metabolite amounts were equal for indigenous and treated exogenous bacteria.•Thermal pretreatment did not improve H2 production when no inoculum was added.•Key role of Enterobacteriales and Clostridiales proportions in H2 yield. Hydrogen production by dark fermentation of complex organic substrates, such as biowaste, can naturally take place with indigenous bacteria or by adding an external microbial inoculum issued from various natural environments. This study aims to determine whether indigenous bacteria associated with thermal pretreatment could impact dark fermentation performances. Biochemical hydrogen potential tests were carried out on seven organic substrates. Results showed a strong influence of the indigenous bacteria which are as effective as thermally pretreated exogenous bacteria to produce H2 and metabolites. High abundance in Clostridiales and/or Enterobacteriales was associated with high H2 yield. This study shows that no inoculum nor pretreatment are required to achieve satisfactory dark fermentation performances from organic waste.