4.6 Metal ion

Fe, Ni, and Mg are required for bio-hydrogen production process. These metals are cofactors for enzymes facilitating transport processes in the microorganisms [122, 129, 130]. Fe2+ is an important element to form hydrogenase and other enzymes. Fe-S affects protein functions by acting as an electron carrier and involving in oxidation of pyruvate to acetyl-CoA, CO2, and H2 [122]. Additionally, Fe2+ induces metabolic alteration and is involved in Fe-S and non-Fe-S protein operation in hydrogenase [122, 131]. Nickel is a fundamental component of [NiFe] hydrogenase. It has the influences on the activity of [NiFe]-hydrogenase. High concentration of nickle inhibits the activity of [NiFe]-hydrogenase, promoting fermentative hydrogen production [122, 132]. Mg2+ is an element that is found abundantly in microbial cells. It stabilizes ribosomes, cell membranes, and nucleic acids and plays a crucial role as an activator of many kinases and synthetases [133]. Cu, Cr, and Zn also have influences on hydrogen fermentation process [122]. The relative toxicity of these heavy metals are Zn (most toxic) > Cu > Cr (least toxic).
