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.
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]
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.
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]
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, algaeorcyanobacteria.[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]
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]
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]
^Tao Y, Chen Y, Wu Y, He Y, Zhou Z (February 2007). "High hydrogen yield from a two-step process of dark-and photo-fermentation of sucrose". International Journal of Hydrogen Energy. 32 (2): 200–6. doi:10.1016/j.ijhydene.2006.06.034.
^ abcdefGhimire A, Frunzo L, Pirozzi F, Trably E, Escudie R, Lens PN, Esposito G (April 2015). "A review on dark fermentative biohydrogen production from organic biomass: Process parameters and use of by-products". Applied Energy. 144: 73–95. doi:10.1016/j.apenergy.2015.01.045.
^ abcdCorneli E, Adessi A, Olguín EJ, Ragaglini G, García-López DA, De Philippis R (December 2017). "Biotransformation of water lettuce (Pistia stratiotes) to biohydrogen by Rhodopseudomonas palustris". Journal of Applied Microbiology. 123 (6): 1438–1446. doi:10.1111/jam.13599. hdl:2434/837874. PMID28972701. S2CID4312887.
^Laurinavichene T, Tekucheva D, Laurinavichius K, Tsygankov A (March 2018). "Utilization of distillery wastewater for hydrogen production in one-stage and two-stage processes involving photofermentation". Enzyme and Microbial Technology. 110: 1–7. doi:10.1016/j.enzmictec.2017.11.009. PMID29310850.
^ abCosta RL, Oliveira TV, Ferreira J, Cardoso VL, Batista FR (April 2015). "Prospective technology on bioethanol production from photofermentation". Bioresource Technology. 181: 330–7. doi:10.1016/j.biortech.2015.01.090. PMID25678298.
^ abcdZhang Q, Wang Y, Zhang Z, Lee DJ, Zhou X, Jing Y, Ge X, Jiang D, Hu J, He C (April 2017). "Photo-fermentative hydrogen production from crop residue: A mini review". Bioresource Technology. 229: 222–230. doi:10.1016/j.biortech.2017.01.008. PMID28108074.
^Chen CY, Yang MH, Yeh KL, Liu CH, Chang JS (September 2008). "Biohydrogen production using sequential two-stage dark and photo fermentation processes". International Journal of Hydrogen Energy. 33 (18): 4755–4762. doi:10.1016/j.ijhydene.2008.06.055.
^Keskin T, Hallenbeck PC (May 2012). "Hydrogen production from sugar industry wastes using single-stage photofermentation". Bioresource Technology. 112: 131–6. doi:10.1016/j.biortech.2012.02.077. PMID22420990.
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