Dark hydrogen fermentations

The production of hydrogen is a ubiquitous, natural phenomenon under anoxic or anaerobic conditions. A wide variety of bacteria, in swamps, sewage, hot springs, the rumen of cattle etc. is able to convert organic matter to hydrogen, CO2 and metabolites like acetic acid, lactate, ethanol and alanine. In general, these bacteria live in the close vicinity of other bacteria which consume these metabolites, including hydrogen, producing their own endproducts like methane and CO2. In this way, a stable ecosystem is formed where potential feedback inhibition of the hydrogen producers by hydrogen, is annulled by the action of the hydrogen consumers. In view of the design of a bioprocess for the production of hydrogen from biomass, extreme thermophilic anaerobic bacteria have been selected because of their high yield with respect to hydrogen production. The yield is reported to be approximately 83-100¿f the maximal theoretical value of 4 mol hydrogen/mol glucose, in contrast to the strict anaerobic Clostridia which produce hydrogen with an approximate yield of 2 mol/mol and the facultative anaerobes which show a H2 yield of less than 2. Besides optimal H2 molar yields, high hydrogen production rates are needed. Product formation appeared to be dependent on cell densities. Thermophiles usually grow to low densities and, therefore production rates are expected to be low. High production rates are reported for Clostridia and Enterobacter of maximal 23 and 58 mmol/L.h, respectively. Hydrogen fermentations by co- and mixed cultures showed production rates of approximately 30-50 mmol/L.h