section_path: Biological reactions
section_name: Biological reactions
paragraph_text: Biological reactions
  
  H 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 H 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. Bacteria such as Mycobacterium smegmatis can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking, using a hydrogenase with small channels that exclude oxygen and so permits the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.
  
  Hydrogen is the most abundant element in the human body by numbers of atoms but the third most abundant by mass. H occurs in human breath due to the metabolic activity of hydrogenase-containing microorganisms in the large intestine and is a natural component of flatus. The concentration in the breath of fasting people at rest is typically less than 5 parts per million (ppm) but can be 50 ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostic hydrogen breath tests.
  
  Water splitting, in which water is decomposed into its component protons, electrons, and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms, including the alga Chlamydomonas reinhardtii and cyanobacteria, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H gas by specialized hydrogenases in the chloroplast. Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H gas even in the presence of oxygen. Efforts have also been undertaken with genetically modified alga in a bioreactor.