fixation and fermentation. The first happens in bacteria, such as cyanobacteria, that have a specialized enzyme, nitrogenase, which catalyzes the reduction of N2 to NH4+. In addition, these microorganisms have another enzyme, hydrogenase, that oxidizes the H2 released as a by-product. If the nitrogen-fixing bacteria have low amounts of hydrogenase, excess H2 can be released into the environment. The amount of hydrogen released depends on the ratio between H2 production and consumption. The second mechanism, fermentation, is performed by some anaerobic heterotrophic bacteria, in particular Clostridia, that degrade organic molecules, producing hydrogen as one of the products. This type of metabolism mainly occurs in anoxic sites, such as lake sediments, deep-sea hydrothermal vents and the animal gut. The ocean is supersaturated with hydrogen, presumably as a result of these biotic processes. Nitrogen fixation is thought to be the major mechanism involved in the production of H2 in the oceans. Release of hydrogen in the oceans is dependent on solar radiation, with a daily peak at noon. The highest concentrations are in the first metres near the surface, decreasing to the thermocline and reaching their minimum in the deep oceans. Globally, tropical and subtropical oceans have the greatest abundance of H2. Examples Hydrothermal vents H2 is an important electron donor in hydrothermal vents. In this environment hydrogen oxidation represents a significant origin of energy, sufficient to conduct ATP synthesis and autotrophic CO2 fixation, so hydrogen-oxidizing bacteria form an important part of the ecosystem in deep sea habitats. Among the main chemosynthetic reactions that take place in hydrothermal vents, the oxidation of sulphide and hydrogen holds a central role. In particular, for autotrophic carbon fixation, hydrogen oxidation metabolism is more favored than sulfide or thiosulfate oxidation, although less energy is released (only –237 kJ/mol compared to –797 kJ/mol). To fix a mole of carbon during the hydrogen oxidation, one-third of the energy necessary for the sulphide oxidation is used. This is because hydrogen has a more negative redox potential than NAD(P)H. Depending on the relative amounts of sulphide, hydrogen and other species, energy production by oxidation of hydrogen can be as much as 10–18 times higher than production by the oxidation of sulphide. Knallgas bacteria Aerobic hydrogen-oxidizing bacteria, sometimes called knallgas bacteria, are bacteria that oxidize hydrogen with oxygen as final electron acceptor. These bacteria include Hydrogenobacter thermophilus, Cupriavidus necator, and Hydrogenovibrio marinus. There are both Gram positive and Gram negative knallgas