Hydrogen in Rocks: An Energy Source for Deep Microbial Communities

Abstract

To survive in deep subsurface environments, lithotrophic microbial communities require a sustainable energy source such as hydrogen. Though H₂ can be produced when water reacts with fresh mineral surfaces and oxidizes ferrous iron, this reaction is unreliable since it depends upon the exposure of fresh rock surfaces via the episodic opening of cracks and fissures. A more reliable and potentially more voluminous H₂ source exists in nominally anhydrous minerals of igneous and metamorphic rocks. Our experimental results indicate that H₂ molecules can be derived from small amounts of H₂O dissolved in minerals in the form of hydroxyl, OH^- or O₃Si-OH, whenever such minerals crystallized in an H₂O-laden environment. Two types of experiments were conducted. Single crystal fracture experiments indicated that hydroxyl pairs undergo an in situ redox conversion to H₂ molecules plus peroxy links, O₃Si/^OOSiO₃. While the peroxy links become part of the mineral structure, the H₂ molecules diffused out of the freshly fractured mineral surfaces. If such a mechanism occurred in natural settings, the entire rock column would become a volume source of H₂. Crushing experiments to facilitate the outdiffusion of H₂ were conducted with common crustal igneous rocks such as granite, andesite, and labradorite. At least 70 nmol of H₂/g diffused out of coarsely crushed andesite, equivalent at standard pressure and temperature to 5,000 cm³ of H₂/m³ of rock. In the water-saturated, biologically relevant upper portion of the rock column, the diffusion of H₂ out of the minerals will be buffered by H₂ saturation of the intergranular water film.