In the environment, more than 99% of microbial life is found in biofilm structures. These 3D porous matrices are composed of cells encased in organic layers that cover mineral surfaces and are acknowledged to be the loci of astounding chemical, biological and physical gradients. Accordingly, this structured interface is thought to be extremely reactive, and is thus able to drive physico-chemical reactions. These happen in emblematic weathering environments shaping superficial element cycling, overall ranging from Amazonian alluvial plains involving continental silicates to very reactive ultramafic rocks systems of oceanic origin, all recognized as carbon sink and potentially exploited as CO2 storage sites. However, biofilm impact on rock alteration is found to be extremely complex, and the interaction dynamics between microbial biofilms and minerals remains poorly understood along with their role on elemental mobility and speciation. Understanding the process of secondary mineral formation, and their stability over time thus constitutes an important key-parameter from an environmental perspective.
The local conditions whithin the biofilm (such as pH, chemical balance, metabolic activity, presence of nucleation sites, redox potential) are parameters that control elemental mobility and biomineralization but they can strongly contrast with bulk solution. In order to investigate the importance of these parameters for secondary mineral formation and elemental behaviors, different types of biofilms accurately defined by their 3D structural and compositional properties are currently studied. To do so, several E. coli mutants biofilms (ref. Ghigo et al., 2008), expressing different types of EPS or pili densities, will be used to understand the link existing between porous and compositional structure and local microenvironments (exerting specific pH, elemental accumulation zones for instance). From there, the parameters controlling elemental transport within the biofilm will be tested in systems more representative of natural environments. In such systems, the dynamics of metal precipitation will also be investigated. First results gathered for biofilms exposed to Pb(II) indicate complex precipitation dynamics with the sequential formation of very stable lead phosphate and lead molybdate, as well as metastable lead carbonate.