Anaerobic breakdown of organic material can yield hydrogen in a number of cases. Anaerobic digestion of sewage sludge by a consortium of microorganisms can produce small amounts of hydrogen in addition to the major product biogas (section ‘Anaerobic digestion’, Chapter 5, this volume). Anaerobic digestion or fermentation of organic compounds by Clostridium sp. and some microalgae was also found to produce hydrogen under specific conditions. One of the best-known examples is the production of acetone and butanol by Clostridium acetobutylicum growing anaerobically on glucose (molasses). This process was used from 1915 until the 1950s to produce acetone and butanol for the munitions and chemical industries. The biological process has now been replaced by the production of acetone and butanol from petrochemicals. However, other products are formed along with acetone and butanol and include ethanol, butyrate, acetic acid, carbon dioxide and hydrogen. Depending on the culture conditions, and the strain used, the amount of the various products formed can vary including the amount of hydrogen produced. The pathway involved in the production of acetone and butanol is shown in Fig. 5.11. It has been estimated that 2 mol of hydrogen are formed per mole of glucose consumed (Ni et al., 2006).

Green algae such as C. reinhardtii respond to anaerobic conditions or nutrient reduction (sulfur) by producing hydrogen. The induction of anaerobic conditions switches the organism’s metabolism to fermentative which produces a number of harmful end products such as ethanol and organic acids. Under these conditions hydrogenase activity is inhibited and hydrogen acts as an electron sink, avoiding some of the problems of aerobic conditions. The low sulfur condition causes the downregulation of the photosystem II where the lack of sulfur-containing amino acids blocks the repair cycle for photosystem II.