The subsurface of our planet, the vast zone of rock and sediment beneath the soil and seafloor, hosts an immense and still poorly understood reservoir of microbial life. It is estimated to contain around 15% of all biomass on Earth, yet most of this life lives without sunlight, relying instead on chemical energy from rocks and fluids to sustain itself. A key open question is how much carbon dioxide these subsurface microorganisms convert into living matter through processes known as carbon fixation, and how this contributes to the planet's overall carbon cycle, the same cycle that governs atmospheric CO2 levels and, ultimately, climate.
Understanding this hidden part of the carbon cycle matters for several reasons. First, it helps complete the picture of how carbon moves between the atmosphere, the surface, and deep geological reservoirs over long timescales, information relevant to climate models and to the European Union's broader climate objectives, including the European Green Deal. Second, it can reveal whether subsurface carbon capture and storage strategies, increasingly considered as tools to fight climate change, might interact with existing microbial communities and their metabolism. Third, because subsurface microorganisms often use ancient, non-canonical strategies to fix carbon rather than the more familiar plant-like photosynthesis pathway, studying them offers a window into some of the earliest forms of metabolism to have evolved on Earth, with implications for understanding the origin and limits of life more broadly, including in extreme environments elsewhere in the Solar System.
Despite its importance, no comprehensive, global picture of how carbon fixation strategies are distributed across subsurface environments existed prior to this project. Previous studies were limited to individual sites or regions, making it impossible to determine whether the strategies used by subsurface microorganisms follow consistent patterns linked to their physical and chemical surroundings, such as temperature, oxygen availability, or the composition of the fluids they inhabit.
The overall objective of this project was to address this gap by systematically surveying subsurface carbon fixation across a wide range of geological settings worldwide, and by linking the microbial strategies observed to the environmental conditions in which they occur. The ultimate goal was to produce a predictive, quantitative understanding of subsurface carbon fixation that could serve as a baseline for future research, environmental monitoring, and climate-relevant carbon budgeting.
