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Figure 41: Hydrogen Production from Direct and Indirect Bio-photolysis
Biological processes
Hydrogen produced through natural biological processes is known as bio-hydrogen. It can be categorised into four primary groups: water-splitting photosynthesis, photo-fermentation, dark fermentation and microbial electrolysis cells. For each group, bio-hydrogen is either evolved from single microbial species or by a mixed consortium of species, with the latter involving some H2 producing species while the rest of the species consuming the H2 for their energy requirement. Initial research in this field focused on pure cultures with a defined substrate as the carbon source. The utilisation of wastewater for substrate that is more practical makes the mixed microbial population more favourable for scaled-up production. Furthermore, the mixed consortium of species is preferred because of operational ease, stability, diversity of biochemical functions and a wider range of substrates for a source of fuel.
a. Water-splitting photosynthesis
The water-splitting photosynthesis process, also known as bio-photolysis, utilises simple steps in producing H2 using light energy and water by the oxygenic photosynthetic microorganisms, such as green algae and cyanobacteria. There are two pathways: the direct and the indirect. The direct bio-photolysis derives the electrons from the light energymediated water splitting with the assistance of photosystem II (PS II) and photosystem I (PS I) (Figure 41).
For the indirect bio-photolysis, photosynthesis converts the light energy along with CO2 fixation into carbohydrates before it is turned to H2 through other pathways, such as fermentation. The Fe-hydrogenase enzyme responsible for the evolution of H2 in green algae is O2-sensitive, which becomes the drawback of the bio-photolysis. Unlike cyanobacteria, the production of H2 occurs in the heterocyst, which protects its O2-sensitive nitrogenase from O2 exposure. Studies such as replacement of the photosynthetically evolved O2 with Argon gas, protein engineering for O2-tolerant hydrogenases, replacing hydrogenases in green algae or replacing hydrogenase with nitrogenase in cyanobacteria, mutation of the PS II proteins, changes in operational conditions and heterologous expression of hydrogenase and Pd are done to overcome these disadvantages.
