Deciphering subsurface carbon fixation

ongoing

Despite containing the vast majority of carbon on the planet, the Earth’s subsurface represents a mostly uncharacterized frontier for biogeochemistry and microbial ecology. While carbon fixation at the Earth’s surface has been drawing a lot of attention for decades, subsurface carbon fixation remains poorly understood. Yet, metabolically, it engages a rich diversity of non canonical pathways, contrasting to surface carbon fixation dominated by the canonical Calvin cycle. Also, its contribution to the global carbon cycle, though long overlooked, is now garnering increased attention. In recent years, several studies aiming to characterise the diversity of carbon-fixing organisms within the subsurface have emerged. However, these studies are isolated and do not accurately describe the dependence of these metabolisms on the geoenvironmental context. I propose here to provide a blueprint of subsurface carbon fixation on Earth. In a top-down approach, I will systematically sample various subsurface environments for metagenomic and metaproteomic analysis of carbon fixing metabolisms, in search of the main environmental factors determining the carbon fixation strategy used. In parallel to this, I will undertake a bottom-up experimental approach to explore and confirm this metabolic versatility in response to changing environments, through controlled culturing experiments. Finally, I will integrate the acquired results with stable isotope analysis to build a metabolic model, aiming to comprehensively predict carbon fixation fluxes in function of the geoenvironmental context. The proposed holistic approach will increase our understanding of the role of subsurface microbial metabolisms in global carbon cycling.