Scientists from the Institut de physique du globe de Paris and the laboratoire Matériaux et Phénomènes Quantiques (Université de Paris, CNRS), in collaboration with researchers from the Museum national d’Histoire naturelle and the Institut Pasteur, have for the first time succeeded in characterizing, under hydrated conditions and at nanometer scale, the influence of bacterial cells and the organic polymers they secrete on mineral formation. The nucleation and growth of the mineral depend closely on the nature and distribution of functional groups within these structures, which ultimately determine the morphology, size and location of the mineral formed.
Biomineralization is a process of mineral synthesis performed by living organisms, in particular by bacteria that interact with metals in their environment and can thus immobilize them in the form of solid minerals. This widespread phenomenon plays an important role in the metals cycle in the environment, but its modus operandi has never before been observed " live ".
In the environment, microorganisms are mainly organized as biofilms, which are hydrated assemblies of microbial cells and organic polymers that these cells secrete for mineral surface colonization. These biofilms could play a major, yet poorly known, role in the transport of metals in the environment and their immobilization as minerals. A major obstacle to understanding the mechanisms governing metal behaviors within biofilms is technological, since these mechanisms must be studied under hydrated conditions and at the microbial cell scale (i.e. submicrometric) to be representative of environmental processes.
Taking advantage of recent advances in liquid cell-transmission electron microscopy, scientists from the Institut de physique du globe de Paris and the laboratoire Matériaux et Phénomènes Quantiques (University of Paris, CNRS), in collaboration with researchers from Sorbonne Université and the Institut Pasteur have for the first time succeeded in characterizing, under hydrated conditions and at the nanometer scale, the influence of the nature and structure of bacterial cells and the organic polymers they secrete on mineral formation. By using two mutant strains of Escherichia coli producing different types of polymers, and by inducing manganese precipitation thanks to the electron beam irradiation, they show that nucleation and growth of manganese precipitates are closely dependent on the nature and distribution of functional groups within these structures. Thus, the organic polymers constitute supports that govern morphology, size and location of the mineral formed.
This opens up new avenues for the study of these major environmental processes regulating the cycle of metals in the critical zone. This external layer of our planet, in constant evolution, is the site of complex interactions between rocks, soils, water, air and living organisms that regulate the natural habitat and determine the availability of resources necessary for life, including our food production and water quality.
Liquid-cell transmission electron microscopy images of mutant strains of E. coli producing different types of organic polymers. On the left, the strain forming proteinaceous pili induces the formation of a manganese crust homogeneously distributed on the cell envelope. On the right, for the strain producing cellulose on its surface but also in its extracellular environment, the precipitated manganese forms more localized nanospheres. This variability in the distribution, size and morphology of the manganese minerals produced is linked to the nature and density of charged functional sites which vary according to the exopolymer produced by bacteria (credits: IPGP/MPQ).
> T. Couasnon, D. Alloyeau, B. Ménez, F. Guyot, J.-M. Ghigo, A. Gélabert, In situ monitoring of exopolymer-dependent Mn mineralization on bacterial surfaces. Sci. Adv. 6, eaaz3125 (2020).
Thaïs Couasnon, équipe de géomicrobiologie de l'IPGP