Concept
Photofermentation is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of photosynthetic bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion. Photofermentation differs from dark fermentation because it only proceeds in the presence of light. For example, photo-fermentation with Rhodobacter sphaeroides SH2C (or many other purple non-sulfur bacteria) can be employed to convert small molecular fatty acids into hydrogen and other products. Phototropic bacteria produce hydrogen gas via photofermentation, where the hydrogen is sourced from organic compounds. C6H12O6 + 6H2O ->[{hv}] 6CO2 + 12H2 Photolytic producers are similar to phototrophs, but source hydrogen from water molecules that are broken down as the organism interacts with light. Photolytic producers consist of algae and certain photosynthetic bacteria. 12H2O ->[{hv}] 12H2 + 6O2(algae) CO + H2O ->[{hv}] H2 + CO2(photolytic bacteria) Photofermentation via purple nonsulfur producing bacteria has been explored as a method for the production of biofuel. The natural fermentation product of these bacteria, hydrogen gas, can be harnessed as a natural gas energy source. Photofermentation via algae instead of bacteria is used for bioethanol production, among other liquid fuel alternatives. The bacteria and their energy source are held in a bioreactor chamber that is impermeable to air and oxygen free. The proper temperature for the bacterial species is maintained in the bioreactor. The bacteria are sustained with a carbohydrate diet consisting of simple saccharide molecules. The carbohydrates are typically sourced from agricultural or forestry waste. In addition to wild type forms of Rhodopseudomonas palustris, scientists have used genetically modified forms to produce hydrogen as well. Other explorations include expanding the bioreactor system to hold a combination of bacteria, algae or cyanobacteria. Ethanol production is performed by the algae Chlamydomonas reinhardtii, among other species, in cycling light and dark environments.
