Molecular hydrogen: An abundant energy source for bacterial activity in nuclear waste repositories

A thorough understanding of the energy sources used by microbial systems in the deep terrestrial subsurface is essential since the extreme conditions for life in deep biospheres may serve as a model for possible life in a nuclear waste repository. In this respect, H2 is known as one of the most energetic substrates for deep terrestrial subsurface environments. This hydrogen is produced from abiotic and biotic processes but its concentration in natural systems is usually maintained at very low levels due to hydrogen-consuming bacteria.A significant amount of H2 gas will be produced within deep nuclear waste repositories, essentially from the corrosion of metallic components. This will consequently improve the conditions for microbial activity in this specific environment. This paper discusses different study cases with experimental results to illustrate the fact that microorganisms are able to use hydrogen for redox processes (reduction of O2, NO3-, Fe III) in several waste disposal conditions. Consequences of microbial activity include: alteration of groundwater chemistry and shift in geochemical equilibria, gas production or consumption, biocorrosion, and potential modifications of confinement properties.In order to quantify the impact of hydrogen bacteria, the next step will be to determine the kinetic rate of the reactions in realistic conditions.

► We demonstrate that microorganisms use hydrogen for redox processes in disposal conditions.
► We evidenced strategies developed by bacteria to access energetic substrates for Fe(III).
► Consequences of microbial activity include shift in geochemical equilibria and gas consumption.
► Biocorrosion is observed but modifications of confinement properties must be assessed.
► Future studies will focus on parameters governing the rate of these mechanisms.