Photofermentation

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^[1]) can be employed to convert small molecular fatty acids into hydrogen^[2] and other products.

^[3] Depicts general process of photofermentation.

Light-dependent pathways

Phototropic bacteria

Phototropic bacteria produce hydrogen gas via photofermentation, where the hydrogen is sourced from organic compounds.^[4]

${\displaystyle {\ce {C6H12O6 + 6H2O ->[{hv}] 6CO2 + 12H2}}}$^[4]

Photolytic producers

Photolytic producers are similar to phototrophs, but source hydrogen from water molecules that are broken down as the organism interacts with light.^[4] Photolytic producers consist of algae and certain photosynthetic bacteria.^[4]

${\displaystyle {\ce {12H2O ->[{hv}] 12H2 + 6O2}}}$(algae)^[4]

${\displaystyle {\ce {CO + H2O ->[{hv}] H2 + CO2}}}$(photolytic bacteria)^[4]

Sustainable energy production

Photofermentation via purple nonsulfur producing bacteria has been explored as a method for the production of biofuel.^[5] The natural fermentation product of these bacteria, hydrogen gas, can be harnessed as a natural gas energy source.^[6][7] Photofermentation via algae instead of bacteria is used for bioethanol production, among other liquid fuel alternatives.^[8]

Basic principles of a bioreactor. The photofermentation bioreactor would not include an air pathway.

Mechanism

The bacteria and their energy source are held in a bioreactor chamber that is impermeable to air and oxygen free.^[7] The proper temperature for the bacterial species is maintained in the bioreactor.^[7] The bacteria are sustained with a carbohydrate diet consisting of simple saccharide molecules.^[9] The carbohydrates are typically sourced from agricultural or forestry waste.^[9]

Variations

Depiction of algae (species not specified) in a bioreactor suitable for bioethanol production.

In addition to wild type forms of Rhodopseudomonas palustris, scientists have used genetically modified forms to produce hydrogen as well.^[5] Other explorations include expanding the bioreactor system to hold a combination of bacteria, algae or cyanobacteria.^[7][9] Ethanol production is performed by the algae Chlamydomonas reinhardtii, among other species, in cycling light and dark environments.^[8] The cycling of light and dark environments has also been explored with bacteria for hydrogen production, increasing hydrogen yield.^[10]

Advantages

The bacteria are typically fed with broken down agricultural waste or undesired crops, such as water lettuce or sugar beet molasses.^[11][5] The high abundance of such waste ensures the stable food source for the bacteria and productively uses human-produced waste.^[5] In comparison with dark fermentation, photofermentation produces more hydrogen per reaction and avoids the acidic end products of dark fermentation.^[12]

Limitations

The primary limitations of photofermentation as a sustainable energy source stem from the precise requirements of maintaining the bacteria in the bioreactor.^[7] Researchers have found it difficult to maintain a constant temperature for the bacteria within the bioreactor.^[7] Furthermore, the growth media for the bacteria must be rotated and refreshed without introducing air to the bioreactor system, complicating the already expensive bioreactor set up.^[7][9]

See also

• Dark fermentation
• Fermentative hydrogen production
• Biohydrogen
• Fermentation (biochemistry)
• Hydrogen production
• Photochemical reaction
• Photohydrogen
• Phototroph
• Photobiology
• Electrohydrogenesis
• Microbial fuel cell