 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.