H2 production by dark fermentation (acidogenic or acetogenesis) processes shares many common features with methanogenic-anaerobic digestion [7, 11, 12]. Anaerobic conversion requires four major steps and five physiologically dis­tinct groups of microorganisms to convert hydrocarbons from complex to simple molecules through H2 and acid as intermediates finally, to carbon dioxide (CO2) and methane (CH4) (Fig. 1a). Fermentative/hydrolytic microorganisms hydrolyze com­plex organic polymers to monomers, and then ferment those monomers to a mixture of low-molecular-weight organic acids and alcohols. Obligatory H2 producing ace — togenic bacteria (AB) oxidize fermentation products to acid intermediates and H2,

which also include acetate production from H2 and CO2 by acetogens and homoace — togens and finally acetoclastic methanogens convert organic acids to CH4 and CO2 [4,11,13]. H2-producing AB grow in syntrophic association with hydrogenotrophic methanogens (H2 consuming), resulting in low H2 partial pressure thus allowing acetogenesis to become thermodynamically favorable by interspecies H2 transfer

[11] .