Bioscience Evidence 2024, Vol.14, No.2, 81-92 http://bioscipublisher.com/index.php/be 83 Figure 1 Liquid products from glucose fermentation (Adopted from Zhang et al., 2012) Image Description: The study found that, in addition to acetic acid, anaerobic activated sludge in the CSTR reactor produced significant amounts of ethanol, propionic acid, and butyric acid during glucose fermentation. The diversity of these end products suggests the presence of multiple metabolic pathways within the acid-producing microbial community (Adapted from Zhang et al., 2012) These pathways highlight the versatility and efficiency of anaerobic bacteria in biohydrogen production, making them crucial players in the development of sustainable energy solutions. 3 Metabolic Pathways for Biohydrogen Production 3.1 Fermentative pathways The production of biohydrogen by anaerobic bacteria is governed by various metabolic pathways, each with distinct biochemical mechanisms and efficiencies. Fermentative pathways are central to the production of biohydrogen in anaerobic bacteria. These pathways involve the breakdown of organic substrates, such as carbohydrates, into simpler molecules, resulting in the generation of hydrogen gas. 3.1.1 Acidogenic fermentation Acidogenic fermentation is a crucial pathway in the production of biohydrogen, where organic substrates are converted into volatile fatty acids (VFAs), carbon dioxide, and hydrogen. During this process, complex organic compounds are first hydrolyzed into simpler sugars, which are then fermented by acidogenic bacteria. The main products of acidogenic fermentation include acetic acid, butyric acid, and lactic acid, with acetic acid being particularly important for biohydrogen production due to its high hydrogen yield (Gu et al., 2020). This pathway is often coupled with other fermentative processes to enhance hydrogen production (Buckel, 2021). 3.1.2 Butyrate-type fermentation Butyrate-type fermentation is another significant pathway in biohydrogen production, characterized by the production of butyric acid alongside hydrogen and carbon dioxide. This pathway is often favored by Clostridium species and operates efficiently under anaerobic conditions. Although butyrate fermentation yields less hydrogen per mole of substrate compared to acetic acid pathways, it remains a key route for biohydrogen production due to its robustness and compatibility with various substrates (Pason et al., 2020). The pathway's ability to process complex organic materials makes it suitable for industrial-scale hydrogen production from biomass (Tang et al., 2021). 3.2 Hydrogenases and their role Hydrogenases are enzymes that play a pivotal role in the biohydrogen production process by catalyzing the reversible oxidation of molecular hydrogen. These enzymes are classified into three major types based on their metal cofactors: [NiFe]-hydrogenases, [FeFe]-hydrogenases, and [Fe]-only hydrogenases. [NiFe]-hydrogenases are commonly found in both bacteria and archaea and are known for their robustness in various environmental conditions. They play a crucial role in the metabolism of hydrogen under both anaerobic and microaerophilic conditions (Lee et al., 2019).
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