Fig. 7

Summary of the impact of the pure CO₂ injection on the bacterial community hosted at the basaltic Carbfix1 CCS site. The initial population in the groundwater sampled in monitoring well HN-04 was still flourishing in February 2012 (Feb’12) and was mainly composed of heterotrophs living under aerophilic to microaerophilic conditions. In March 2012 (Mar’12), groundwater acidification induced by arrival of the fast-flowing fraction of the pure-CO₂ injectant severely reduced bacterial richness, provoked the dissolution of the host-basalt and the release of mineral-forming cations and likely polyaromatic hydrocarbons (PAHs). Fe²⁺ and other ions (e.g., Ca, Mg, Zn) became bioavailable to the ecosystem, as a result favoring the development of Betaproteobacteria including iron-oxidizing autotrophic species related to Gallionellaceae. Because of the high insolubility of ferric iron, iron oxy(hydr)oxides as the byproducts of betaproteobacterial iron-oxidization likely precipitated at the surface of altered basaltic minerals, forming a potentially passivating layer that may have had a deleterious effect on water-rock interaction efficiency⁵⁷. In May 2012 (May12), under more anaerobic conditions, Firmicutes then bloomed along with the Thiobacillus species. The well-recognized carbonatogen potential of Firmicutes may have contributed to CO₂ mineralization in the form of carbonates at that time¹⁰. However, mobilization of Fe, Ca, Mg by microbes may have reduced the overall carbonation rates (GW stands for groundwater. Only phyla with relative abundance > 5% according to the number of retrieved pyrosequences are represented)