Journal of Energy Bioscience 2024, Vol.15, No.5, 301-313 http://bioscipublisher.com/index.php/jeb 303 No Lignin Content: Unlike terrestrial biomass, marine algae do not contain lignin, which simplifies the pretreatment process and enhances the efficiency of biohydrogen production (Wang and Yin, 2018). Versatility in Water Use: Marine algae can be cultivated in seawater, freshwater, and even wastewater, making them highly adaptable and reducing the need for freshwater resources (Adeniyi et al., 2018; Wang and Yin, 2018). 2.3 Case studies of marine algae species used in biohydrogen production Chlamydomonas reinhardtii is a well-studied microalga known for its high photosynthetic efficiency and ability to produce biohydrogen through dark fermentation. Pretreatment methods such as hydrothermal processing have been shown to enhance the hydrolysis of its biomass, leading to improved biohydrogen yields (Nagarajan et al., 2020). Ulva lactuca, a type of green macroalga, has been investigated for its potential in biohydrogen production. Studies have demonstrated that hydrothermal pretreatment can significantly increase the solubilization of its biomass, thereby enhancing the production of biohydrogen and biomethane (Lin et al., 2019). Additionally, Ulva lactuca has shown promising growth rates and biomass yields in land-based cultivation systems, making it a viable feedstock for biohydrogen production (Qarri and Israel, 2020). Saccharina latissima, a brown seaweed, has been explored for its biohydrogen production potential through dark fermentation. Hydrothermal pretreatment of S. latissima has been found to improve the solubilization of its biomass and increase the yield of fermentable sugars, which are crucial for biohydrogen production. The energy conversion efficiency of biohydrogen production from S. latissima can reach up to 72.8% under optimized conditions (Lin et al., 2019). 3 Biohydrogen Production Mechanisms 3.1 Biological pathways involved in biohydrogen production Biohydrogen production involves several biological pathways, primarily categorized into photobiological and fermentative processes. Photobiological pathways include direct and indirect biophotolysis, where microalgae use sunlight to split water molecules, releasing hydrogen. Fermentative pathways, such as dark fermentation, involve the anaerobic breakdown of organic substrates by microorganisms to produce hydrogen. These pathways are influenced by various factors, including the type of feedstock, pretreatment methods, and the specific metabolic routes employed by the microorganisms (Nagarajan et al., 2017; Ahmed et al., 2021; Sharma et al., 2021). 3.2 Photobiological hydrogen production through algae photosynthesis Photobiological hydrogen production leverages the photosynthetic capabilities of algae. In direct biophotolysis, algae directly convert solar energy into hydrogen by splitting water molecules using photosystem II. Indirect biophotolysis involves the production of hydrogen through intermediate metabolic processes, where algae first produce organic compounds that are subsequently converted to hydrogen. This method is highly efficient in terms of sunlight conversion but is sensitive to oxygen, which can inhibit the hydrogenase enzyme responsible for hydrogen production (Nagarajan et al., 2017; Ahmed et al., 2021; Sharma et al., 2021). 3.3 Dark fermentation and metabolic pathways Dark fermentation is a process where anaerobic microorganisms decompose organic substrates, such as sugars and starches, to produce hydrogen. This method does not require light and can utilize a wide range of feedstocks, including lignocellulosic biomass and algal biomass. The metabolic pathways involved in dark fermentation include glycolysis, where glucose is broken down to pyruvate, which is then converted to hydrogen and organic acids through various enzymatic reactions. The efficiency of dark fermentation can be enhanced through pretreatment methods that increase the availability of fermentable sugars (Park et al., 2013; Ergal et al., 2018; Bhatia et al., 2020; Kumar et al., 2021; Sim et al., 2021). 3.4 Enzymes and cofactors involved in hydrogen production Several key enzymes and cofactors play crucial roles in the biohydrogen production process. Hydrogenase and nitrogenase are the primary enzymes involved. Hydrogenase catalyzes the reversible oxidation of molecular
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