Hydrogen Production via Photofermentation

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.