# Photo-Fermentative Bacteria Used for Hydrogen Production
## 2 Photo-Fertanative Bacteria for Hydrogen Production  
The production of hydrogen by photo-fermentation was recognised in the year 1949 by [PERSON] and [PERSON] as it produces hydrogen with high purity without generating oxygen [14]. In this process, the organic matter is decomposed by photosynthetic bacteria in the presence of light to produce hydrogen through a reaction catalyzed by nitrogenase.  
\[\mathrm{C_{6}H_{12}O_{6}+6~{}H_{2}O\to 6~{}CO_{2}+12~{}H_{2}~{}(photosynthetic bacteria)} \tag{1}\]  
Bacteria commonly used for photo-fermentation include purple non-sulfur bacteria (PNSB) because they have high metabolic flexibility and can grow as photoautotrophs and chemoheterotrophs. They do not require water, unlike plants, algae, and cyanobacteria, nor do they require much free energy for hydrogen production, so no oxygen is produced in the system. Inorganic ions, carbon compounds or hydrogen are the electron sources for carrying out metabolic activities, and the generation of energy is produced by photosynthesis. PNSB show a versatile ability to utilize a wide range of organic carbon compounds for metabolic activity including pyruvate, acetate, amino acids, alcohols, organic acids, and carbohydrates. In particular, certain species within this group can use C1 compounds, such as formate and methanol, as viable carbon sources. In addition, PNSB can also utilize aromatic organic compounds, such as benzoate, cinnamate, and chlorobenzeneate, to meet their carbon requirements [15]. PNSB also demonstrate the ability to assimilate a range of organic acids, including acetic, butyric, propionic, malic, and lactic acids, which becomes relevant when using organic waste as a substrate for hydrogen production. During the acidogenic phase of anaerobic digestion, PNSB can use these organic acids as carbon sources for their conversion to hydrogen and carbon dioxide. Thus, the main significance and advantage of photo-fermentative bacteria is their ability to achieve high hydrogen yields. The theoretical yield achieved from 1 mol of glucose is 12 mol H\({}_{2}\), as given in equation 1 above, but the yield can vary slightly depending on the type of microbe. As  
Figure 1: Schematic illustration representing hydrogen production by various pathways (adapted from Ref. [12]. Article published in Int. J. Hydrogen Energy, 45, [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON], Recent advanced biotechnological strategies to enhance photo-fermentative biohydrogen production by purple non-sulfur bacteria: An overview, 13211–13230, Copyright Hydrogen Energy Publications LLC. Published by Elsevier Ltd. (2020)).  
shown in Figure 2, the following PNSB species show the ability to produce hydrogen using photo-fermentation: _Rhodobacter_ species [16], such as _Rhodobacter__sphaeroides_[17] and _Rhodobacter__capsulatus_[18]; _Rhodospseudomonas_ species [19], such as _Rhodospseudomonas__palustris_[20], _Rhodospseudomonas__capsulata_; _Rhodospirillum rubrum_[21]; _Rhodovolium_ species, including _Rhodovolium sulfidophilium_[22]. These microbes follow the pathways supported by ATP-dependent nitrogenase, as the metabolic pathway is similar in all these bacteria; among them, _Rhodospirillum rubrum_ was reported as the first PNSB to produce hydrogen.