Calcium sorption and isotope fractionation in Bacillus subtilis and Pseudomonas aeruginosa

Bacteria are a key component of the critical zone, because of their role in the nutrient availability for the vegetation. There is still little knowledge on the direct role of bacteria on Ca storage/leaching in soils while it is an essential macronutrient for vegetation growth. In recent years, Ca stable isotopes have shown their potential in understanding the Ca biogeochemical cycle. Preliminary studies highlighted that in the presence of soil bacteria, the plant uptake of nutrients is increased due to the mineral bioweathering. Moreover, Ca isotope signatures of nutrient media also showed differences between growth experiments in batch in the presence and absence of bacteria. In this study, the focus is to verify if Ca adsorption and incorporation into/onto bacterial strains induce such isotopic fractionation. Batch experiments were carried out on Pseudomonas aeruginosa (a Gram-negative bacterium) and on the vegetative and sporulated forms of Bacillus subtilis (a Gram-positive bacterium). These experimentations showed that: (i) no observable isotopic fractionations were induced during calcium/bacteria contact for all experimental parameters (pH, kinetic, bacterial cell number, interaction time, dead/alive bacteria); (ii) Ca was mainly stored in the bacterial cell wall compartments. On the other hand, significant Ca isotopic differences between the spores and the sporulation medium (Δ^44/40Ca_{spores–sporulation medium} ranging from − 0.53 to − 1.15‰), suggest isotopic fractionation during the sporulation process, likely occurring during the attachment of Ca to carboxyl acid groups as calcium chelates with dipicolinic acid. The absence of Ca isotope fractionation during Ca sorption on vegetative and sporulated bacteria via passive channels indicates that the tested bacteria’s contribution to the Ca biogeochemical cycle is indirect primary enhancing bioweathering and Ca bioavailability for vegetation. If confirmed by further studies, only the sporulation mechanisms itself may directly impact the Ca biogeochemical cycle.