Contribution of magnetotactic bacteria forming intracellular carbonate mineral phases to the sequestration of inorganic carbon and alkaline earth elements – CarboMagnet

Identifying the diversity of carbon sequestering processes and their dynamics is more than ever fundamental to identify carbon sinks and predict how climate change will affect ecosystems functioning. In this context, the environmental formation of CaCO3 by bacteria has recently received increasing attention because of its impact on both C and Ca cycles, its ability to cement/stabilize sediments and soils or to bioremediate sites polluted by alkaline earth elements (AEE) (e.g., 90Sr, 226Ra). Usually, such calcification processes have been considered as extracellular and most of the times occur in solutions that are already supersaturated with CaCO3 mineral phases. However, some studies, including by our consortium, have shown that several species of Cyanobacteria and a giant uncultured sulfoxidizing gammaproteobacterium of the genus Achromatium form intracellular amorphous CaCO3 (iACC). This biomineralization can occur in undersaturated solutions, where CaCO3 precipitation would not occur otherwise. However, while intracellular carbonatogenesis by bacteria appears as a geochemically interesting process, its phylogenetic and environmental distribution, the involved geochemical mechanisms and its quantitative importance in geochemical cycles remains poorly explored. The CarboMagnet stems from our very recent discovery of diverse magnetotactic bacteria (MTB) forming intracellular carbonates at the oxic-anoxic boudary (OAB) of aquatic environments, where strong chemical gradients (e.g., fO2, [H2S]…) prevail. We will test several hypotheses: (i) these bacteria are an significant reservoir of inorganic carbon and alkaline-earth elements (AEE), and participate to their biogeochemical cycling; (ii) the dynamics of this reservoir depends on varying concentrations of of AEE and/or other chemical parameters; (iii) this sequestration process is associated with a repertoire of specific genes and metabolisms linked, in particular, with carbon and sulfur; (iv) these bacteria preferentially incorporate heavy AEEs and fractionate Ca and/or Mg isotopes specifically; (v) the carbonate phases are partly conserved in the sediments upon cell death by crystallization and/or protection within cell membranes. The molecular and geochemical processes of bacterial biomineralization will be studied by a combination of approaches in isotope and aqueous geochemistry, mineralogy, genomics and microbial ecology, from the single cell to the community level and from the natural environment to the laboratory. Together, CarboMagnet deliverables will provide new microbial models for efficient inorganic carbon / AEE sequestration and decipher the contribution of this reservoir to the cycling of C and AEE at the OAB. The CarboMagnet project sets on strong foundations to answer the different questions that are raised, providing us a unique positioning on this topic. First, we will study two, relatively close-by and easily accessible, field sites (i.e. Lake Pavin, Carry-le-Rouet) hosting abundant magnetotactc bacteria forming intracellular carbonates. These sites are environmentally contrasted (freshwater vs marine) and they are already well characterized geochemically and microbiologically. Second, we have demonstrated the possibility to use magnetic sorting procedures providing a unique advantage to efficiently study/enrich/concentrate these iACC-forming bacteria (compared with e.g., other iACC-forming bacteria). Third, we are bringing together a multidisciplinary consortium who has been working together for several years and includes all the complementary and crucial expertise needed for this project: the microbiological and genomic study of MTB; the mineralogical and microscopy characterization of iACC and derived crystalline products; isotope tracing and characterization of physicochemical conditions prevailing where these MTB thrive.