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Hydrogen production and its distribution in the oceans
The most widespread compound on our Earth is hydrogen, which represents around three-quarters of all the elements. In the atmosphere, its concentration is about 0.5-0.6 ppm and so here it represents the most abundant trace gas, after methane. Therefore, H2 can be used as energy source in several biological processes, also because it has a highly negative redox potential (E0’= -0.414 V). It can be coupled with different compounds:

-       O2: the oxic respiration is performed (H2+1/2O2 → H2O)

-       Oxidized compounds, such as carbon dioxide or sulfate.

In the ecosystems, hydrogen can be produced through biological and abiotic processes.

The abiotic processes are mainly due to geothermal production and serpentinization. In the first case, hydrogen is usually present as a gas and probably can be obtained by different reactions:

1.      Water may react with silicon radical at high temperature:

Si· + H2O → SiOH + H·

H· + H· → H2

2.      The proposed reaction between iron oxides and water, at temperatures higher than 800°C:

2FeO + H2O → Fe2O3 + H2

2Fe3O4 + H2O → 3Fe2O3 + H2

On the other hand, serpentinization is an exothermic geochemical mechanism that occurs when, thanks to the tectonic movements, the ultramafic rocks raise and reach the water. This process can bring to the production of large quantities of H2, but also of methane and organic substances.

The main mechanisms that lead to the formation of hydrogen, involving different microorganisms, are nitrogen fixation and fermentation. The first one happens in some bacteria, such as heterocystous and non-heterocystous cyanobacteria, that have a specialized enzyme, the nitrogenase, which catalizes the reduction of N2 to NH4+. In addition, these microorganisms have another enzyme, the hydrogenase, that oxidizes the H2 realeased as a by-product. Therefore, in this type of bacteria, the amount of hydrogen produced depends on the ratio between H2 production and consumption. In some cases, the H2 can be present in the environments because the N2-fixing bacteria can have a low quantity of hydrogenases. Instead, fermentation is performed by some strict or facultative anaerobic heterotrophic bacteria, in particular Clostridia ,that degrade organic molecules, producing hydrogen as one of the products. Therefore, this type of metabolism mainly occurs in anoxic sites, such as lake sediments, deep-sea hydrothermal vents and human gut.

Probably mainly due to the biotic processes, in the marine habitats it was observed that the concentrations of hydrogen were supersaturated. In all these environments, the highest concentrations were in the first metres, decreasing to the thermocline and reaching the lowest concentrations in the deep oceans. Globally, tropical and subtropical oceans appear to have the most abundant quantity of H2, while the least amount is present in higher latitudes. However, it was observed that the realease of hydrogen in the oceans is dependent on the solar radiation, showing a daily change with the maximum peak at noon. Nitrogen fixation, performed by cyanobacteria, leads to the production of one molecule of H2 at least. This metabolism is thought to be the major one involved in the increase of H2 in the oceans. Despite there are some evidences of this, more data need to be collected to finally correlate the two phenomena.