Crustal rocks harbor diverse, often unique, microbial populations that influenced the Earth’s chemistry throughout geological time by mediating elemental fluxes from the lithosphere to the oceans and atmosphere. Although low biomass and diversity are the norm in the majority of subsurface environments, the oceanic lithosphere appears to be an attractive habitat where a diverse and active biosphere is thriving. This habitat is generated by vigorous hydrothermal circulation in the porous and fractured architecture of the subseafloor crust. This process has major implications for seafloor engineering operations but also for our understanding of the habitability of our planet.
Our group is involved in the Research Center of CO2 geological storage (IPGP/TOTAL/SCHLUMBERGER/ADEME) and in this framework, we focus on the study of the deep biosphere and its reactivity during an injection of anthropogenic CO2 in the subsurface. Indeed, the recently-recognized existence of deep ecosystems logically raises the questions of their response to CO2 injections and their action on the reactions of sequestration at all timescales. To address these issues, we conducted a set of analog studies using both experimental and natural samples, along with pilot site monitoring. Experiments under controlled conditions allow to quantitatively evaluate the metabolic response of microorganisms to CO2 injection and to characterize the kinetics and mechanisms of microbially-enhanced solid carbonate formation leading to permanent mineral storage (see also our Biomineralization/Bioweathering theme). Monitoring on the pilot site allows studying whole environmental consortia and enables the understanding of biogeochemical pathways. With this aim we have monitored since 2008 a CO2 pilot site in Iceland, in relation to the CARBFIX project.
These works are carried out in collaboration with national and international partners that allow integrated and pluridisciplinary approaches. They were funded by the French National Agency ANR through the projects GeoCarbone-Carbonatation (2005-2008) and CO2FIX (2008-2012), both co-coordinated by the IPGP, along with our industrial partners and the participation to the ANR project CARMEX.
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Evaluating to which extend the rock-hosted deep biosphere constitute a major and impacting component of the Earth factory remains a pending and challenging question. Over the last 10 years researchers have speculated that the oceanic lithosphere could constitute the largest microbial habitat on Earth, but until now only scarce evidence has been provided. In order to characterize its nature, extent, and functioning, we continue its exploration, notably through the study of deep microbial ecosystems hosted in the oceanic lithosphere and supported by the hydration of basalts and peridotites-forming minerals (i.e. serpentinization). These reactions have the peculiar capability to produce molecular hydrogen that constitutes a valuable source of energy for deep ecosystems. Implementing our analytical tools developed for the characterization of microbe-mineral interactions (see our Microbial imaging theme) allow us to assess the impact of this rock-hosted life on the global geochemical fluxes from the mantle to the oceans/atmosphere notably for the deep carbon cycle. These studies also allow to address the problem of the emergence of a biological activity under extreme and hostile conditions and to establish the physical and chemical conditions for its development. These environments and associated processes (notably serpentinization) are indeed close analogues for Archaean hydrothermal sites, where life is thought to have emerged and developed.
This work is financially supported by the Deep Carbon Observatory (Sloan Foundation/Carnegie Institution of Washington) through the projects of the Deep Energy community along with the CNRS/INSU (INTERRVIE project)
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