Dark fermentation consists in converting simple sugars or disaccharides
to hydrogen, carbon dioxide and organic acids. Suitable
heterotrophic bacteria include strict anaerobes (such as Clostridia
and thermophiles), facultative anaerobes (like Enterobacter and E.
coli) and aerobes (for example Alcaligenes and Bacillus). Anaerobic
bacteria produce hydrogen from hexoses in acetic acid, butyric acid
and acetone-butanol-ethanol fermentations. The maximal theoretical
value of 4 mol H2/mol glucose can be reached in acetic
acid fermentations. When other organic acids and alcohols are
produced by facultative anaerobic bacteria, the yield of hydrogen is
decreased to about 2 mol H2/mol hexose. The hydrogen yields and
production rates of thermophilic bacteria (e.g., Caldicellulosiruptor
saccharolyticus and Thermotoga neapolitana) growing at temperatures
above 60 C, are often higher than those of mesophilic bacteria
growing at ambient temperature because thermophilic
bacteria produce acetic acid as the main fermentation byproduct.