# Photo-Fermentative Bacteria Used for Hydrogen Production
## 1 Introduction  
Hydrogen is a clean and alternative energy source to fossil fuels [1]. It is the most abundant element in the universe and can be produced from a variety of renewable resources, including water, biomass, and solar energy. Hydrogen is an attractive fuel for transport, and its environmental impact is determined in terms of greenhouse gases, acidification, eutrophication, toxicity potential, and eco-cost [2]. The energy content of hydrogen is 122 kJ/g, which is 2.72 times higher than that of petrol [3]. However, the current production methods are based on fossil fuels, which cause greenhouse gas emissions. Methods, such as the thermal cracking of water or water electrolysis, have been used to produce hydrogen, but they are energy-intensive and emit gases, such as carbon dioxide, that cause global warming [4]. Hydrogen gas is advantageous because it can be used to produce electricity in fuel cells with heat and water as end products [5], which helps to reduce air pollution and mitigate climate change. The development of fuels with net-zero emissions is a current research priority [6]. Hydrogen has a higher energy content per unit weight than fossil fuels and can, therefore, be used in fuel cells for electric vehicles, power generation, and in industrial processes. Hydrogen can be stored and transported, making it a flexible gas carrier. However, there are challenges associated with the production and useof hydrogen, including the high cost of production and storage, its corrosiveness to metals, the need for infrastructure development, and safety concerns related to handling and storage. In any case, the production and use of hydrogen is an important area of research and development as we strive to improve both economic and environmental sustainability in the near future [7]. A promising method of producing hydrogen from biomass using microorganisms has been favored because it is environmentally friendly [4]. Such a biological method contributes to the production of renewable and carbon-neutral hydrogen, referred to as \"biohydrogen\" [8]. Biological hydrogen production includes photo-fermentation and dark fermentation. Photo-fermentation, which is the main focus of this review, involves bacteria that can convert organic matter into hydrogen using solar energy. It has been widely used to produce clean, renewable energy sources [8]. Substrates, such as simple sugars, volatile fatty acids, and industrial and agricultural wastes under anaerobic conditions in the presence of light, can be used by purple non-sulfur bacteria in photo-fermentation [9]. Based on this, several photobioreactors (PBRs) have been developed to improve the production of hydrogen in continuous operation, including different configurations, such as flat panel, tubular, or torus reactors [10]. The operation of PBRs at ambient conditions reduces the need for additional energy input, thereby reducing the process costs compared to thermochemical and electrolytic methods [11]. Some of the microbes that have been studied to carry out photo-fermentation include anaerobic bacteria of the genera _Rhodobacter_, _Rhodobium_, _Rhodospseudomonas_, and _Rhodospirillum_, which help to convert organic acids into hydrogen and carbon dioxide. The amount of hydrogen produced in this process is comparable to biophotolysis and it is influenced by the type of microbe, the medium, and the design of the PBR [12]. Other hydrogen production methods include microbial electrohydrogenesis cells, which produce hydrogen simultaneously with wastewater treatment, biophotolysis, which dissociates the water molecule into hydrogen and oxygen and, finally, dark fermentation [7]. Dark fermentation involves strict anaerobes or facultative bacterial strains using organic matter, biomass, and wastewater as substrates.