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The hydrogen cycle consists of hydrogen exchanges between biotic and abiotic sources and sinks of hydrogen-containing compounds.
Hydrogen is the most abundant element in the universe. On Earth, common H-containing inorganic molecules include water , hydrogen gas , hydrogen sulfide , and ammonia. Many organic compounds also contain H atoms, such as hydrocarbons and organic matter. Given the ubiquity of hydrogen atoms in inorganic and organic chemical compounds, the hydrogen cycle is focused on molecular hydrogen, H2.
Hydrogen gas can be produced naturally through rock-water interactions or as a byproduct of microbial metabolisms. Free H2 can then be consumed by other microbes, oxidized photochemically in the atmosphere, or lost to space. Hydrogen is also thought to be an important reactant in pre-biotic chemistry and the early evolution of life on Earth, and potentially elsewhere in our solar system.