Of the many ways hydrogen can be produced, this chapter focuses on biological hydrogen production by thermophilic bacteria and archaea in dark fermentations. The thermophiles are held as promising candidates for a cost-effective fermentation process, because of their relatively high yields and broad substrate palette. Yet many challenges remain to be faced, including improving productivity, tolerance to high osmolality and growth inhibitors, and reactor configuration. This review consolidates current insights in the quest for high yields and productivities within thermophilic hydrogen production. Important is to understand how environmental parameters affect the redox- and energy metabolism of the microorganism(s) involved. This knowledge is required for designing an optimal bioreactor configuration and operation.
Keywords: Central carbon metabolism, hydrogenase, hydrogen productivity, hydrogen yield, hydrogen partial pressure, hydrolysate, medium composition, osmolality, physiology, reactor design, thermodynamics, thermophile.