1
H
Hydrogen
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Essential: all life
Hydrogen
Major functions in cells: (1)
- Hydrogen is part of water, which is essential for all life
- H2 availability limits growth of methanogens
Environmental and health impacts:
- Hydrogen gas is an environmentally friendly fuel
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Reduce:
- Thermophilic H2 syntrophy between methanogens and H2 producing heterotrophs (2)
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(1) Hydrogen: Essential for All Life
Hydrogen is essential for all organic molecules. In general, atomic hydrogen is readily available because it is one of the elements that makes up water, and all life is based in aqueous chemistry. Water is both the universal solvent for biochemical reactions and an active participant.
Water (H2O) rapidly and reversibly dissociates into hydrogen ion (H+) and hydroxide (-OH). At neutral pH (7), the concentration of H+ is 100 nM. Although this concentration corresponds to only ~100 ions per cell (Eco), every water molecule can provide a H+ ion to enzymes on an as-needed basis (e.g. acid-base catalysis). Water is consumed by many enzymes as a substrate (hydrolases) and generated by others as a product (dehydration synthesis). Remarkably, up to 70% of intracellular H2O in rapidly growing Eco may be generated by metabolism, rather than derived from the environment (Martin et al., 2005).
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Molecular hydrogen (H2) is also relevant for some bacteria, known as methanogens, that metabolize H2 and produce methane as a byproduct.
(2) H2 Syntrophy
Although elemental H is rarely limiting because it is found in water, molecular H2 can be a limiting nutrient for methanogens that metabolize H2. There is no known H2 sparing response in methanogens, but they have developed mechanisms to survive in low H2 conditions. In hyperthermophilic and thermophilic growth temperatures, methanogens are limited by the availability of H2. Their growth can be supported by syntrophy with hyperthermophilic heterotrophs, primarily Thermococcales, that are capable of H2 production (TopçuoÄŸlu et al., 2016).