 H2 is a product of some types of anaerobic metabolism and is produced by several microorganisms , usually via reactions catalyzed by iron- or nickel @-@ containing enzymes called hydrogenases . These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electrons . Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water . The natural cycle of hydrogen production and consumption by organisms is called the hydrogen cycle . 
