Photofermentative hydrogen production is a bioprocess in which photosynthetic purple nonsulfur
bacteria grow heterotrophically on organic acids like acetic acid, lactic acid and butyric acid and
produce hydrogen using light energy under anaerobic conditions. Two enzymes are specifically
involved in hydrogen production, namely nitrogenase and hydrogenase. While nitrogenases produce
hydrogen under nitrogen-limited conditions acting as ATP-dependent hydrogenase, hydrogenases have
the ability for both production and consumption of molecular hydrogen depending on the type of
hydrogenase and physiological conditions. Photofermentation process can be achieved in a wide variety
of conditions such as in batch or continuous mode, upon artificial or solar illumination, utilizing various
carbon and nitrogen sources including food industry wastewater and dark fermentation effluents. Panel
and tubular photobioreactors are the most applicable bioreactor types since they ensure simple design,
reasonable material and production costs and high light energy utilization. Physiological parameters
such as pH, temperature, medium composition and light intensity control the yield and hydrogen
productivity of the bacteria. Hydrogen productivity and yield can also be increased by using genetically
modified bacterial strains or immobilization of bacteria. Genetic studies focus on development of
mutant strains by disrupting the uptake hydrogenase genes, altering pigmentation and blocking
alternative by-product biosynthesis. Techno-economic evaluations show that photofermentative
hydrogen production process is very near to the commercialization stage, however demo scale
experience is necessary to solve some problems such as low rate of hydrogen production and the cost
associated with photobioreactor scale-up. Furthermore, recent studies are trying to integrate
photofermentation to dark fermentation to have an enhanced hydrogen production yield. Finally, the
whole process could end up with a fuel cell application where the produced hydrogen is stored for
future uses.
Keywords: Photofermentation, purple non-sulfur bacteria, nitrogenase, hydrogenase, photobioreactor, light
energy, R. sphaeroides, R. capsulatus, organic acids, solar illumination.
