Hydrogen in rocks: an energy source for deep microbial communities.

Deep subsurface microbial life depends on stable energy sources. Lithotrophic organisms rely on hydrogen, which is typically generated through water-mineral reactions. However, this process is limited by the availability of fresh mineral surfaces. Another potential H2 source exists in nominally anhydrous minerals. Prior research has shown that hydrogen can form from water in mineral structures. Yet, the mechanism of H2 release from these minerals remains unclear. This gap motivated further investigation into how H2 might be generated and released from such minerals. Understanding this process could clarify how deep microbial communities sustain themselves. No prior work had resolved how H2 diffuses from minerals in natural settings. This uncertainty drove the need for experimental approaches to test H2 generation from mineral hydroxyls.

The aim of this study was to determine whether hydrogen can be generated from hydroxyl groups in nominally anhydrous minerals. This investigation sought to test if such minerals can serve as a continuous H2 source for deep microbial communities. The researchers focused on two experimental approaches: single crystal fracture and crushing of igneous rocks. These methods were chosen to simulate natural processes that might release H2 from minerals. The study aimed to quantify how much H2 could diffuse from crushed minerals. By analyzing granite, andesite, and labradorite, the researchers sought to assess H2 yield across common crustal rocks. This work addresses a specific problem: whether mineral hydroxyls can reliably produce H2 in the subsurface. The motivation stems from the need to identify stable energy sources for deep microbial life.

The researchers used two experimental methods to investigate H2 generation from minerals. In single crystal fracture experiments, they observed how hydroxyl pairs convert into H2 and peroxy links. These experiments were conducted under controlled conditions to isolate the redox reaction. The second method involved crushing igneous rocks to simulate natural fracturing. This allowed H2 to diffuse from the mineral surfaces. The team selected granite, andesite, and labradorite for crushing tests. They measured the amount of H2 released per gram of rock. The crushing process mimicked the mechanical stress that occurs in natural rock fractures. The experiments were designed to replicate the conditions under which H2 might be released in subsurface environments. By comparing H2 yields across different rock types, the researchers aimed to assess the potential of each as an energy source.

The study found that hydroxyl pairs in minerals can convert into H2 molecules and peroxy links during crystal fracture. This process was observed in single crystal experiments, where H2 diffused from freshly fractured surfaces. Crushing experiments revealed that andesite released at least 70 nmol of H2 per gram of rock. At standard pressure and temperature, this equates to 5,000 cm3 of H2 per cubic meter of rock. The H2 release was consistent across different igneous rock types. The peroxy links remained in the mineral structure, while H2 diffused outward. The researchers noted that H2 saturation in intergranular water films could buffer diffusion in natural settings. These findings suggest that mineral hydroxyls may serve as a reliable H2 source for deep microbial communities.

The authors propose that hydroxyl groups in minerals can generate H2 through redox reactions during fracturing. This mechanism may provide a continuous H2 source for deep microbial life. The study supports the idea that mineral hydroxyls contribute to H2 availability in subsurface environments. The researchers suggest that this process could occur naturally in the rock column. The findings indicate that crushing experiments can simulate H2 release from minerals. The authors highlight that andesite, granite, and labradorite all released measurable H2. The study implies that H2 diffusion may be buffered by water saturation in natural settings. These conclusions are based on the observed H2 yields and the proposed redox mechanism.