Life without sunlight? Earthquake fractures fuel deep underground microbes

Chinese researchers have recently challenged the long-held belief that "all life depends on sunlight." In a study published in Science Advances, the researchers identified how microbes in deep subsurface areas can derive energy from chemical reactions driven by crustal faulting, offering critical insights into life deep below Earth's surface.

The research was led by Prof. Hongping he, a member of the Chinese Academy of Sciences (CAS), and Prof. Jianxi Zhu, both from the Guangzhou Institute of Geochemistry of CAS.

Long regarded as inhospitable to life due to the absence of sunlight and organic matter, the deep subsurface has in recent years been found to host a large-scale, highly active biosphere teeming with diverse microorganisms. These microbes derive energy from abiotic redox reactions during water–rock interactions. Hydrogen (H2) serves as their main energy source and oxidants are also essential for metabolic activities, but their origins were not previously well understood.

To tackle this mystery, the research team simulated crustal faulting activities and discovered that free radicals produced during rock fracturing can decompose water, generating both hydrogen and oxidants such as hydrogen peroxide (H2O2). These substances create a distinct redox gradient within fracture systems, which can further react with iron (Fe) in groundwater and rocks—oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) or reducing ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), depending on local redox conditions.

In microbe-rich fractures, hydrogen production driven by earthquake-related faulting was found to be up to 100,000 times greater than that from other known pathways, such as serpentinization and radiolysis. The team demonstrated that this process effectively drives iron's redox cycle, which in turn influences the geochemical processes of elements like carbon, nitrogen, and sulfur—sustaining microbial metabolism in the deep biosphere.

This study sheds new light on the energy sources and ecological diversity of the deep-subsurface biosphere. Profs. He and Zhu also noted that fracture systems on other Earth-like planets could potentially provide habitable conditions for extraterrestrial life, offering a new avenue for the search for life beyond Earth.

The study was financially supported by the National Science Fund for Distinguished Young Scholars and the Strategic Priority Research Program of CAS, among other sources.