Investigating some mechanisms underlying stress metal adaptations of two Burkholderia sensu lato species isolated from New Caledonian ultramafic soils

In New Caledonia, one third of the main island is covered by ultramafic soils characterized by high levels of metals and low levels of plant nutrients. In these soils, metal tolerant bacteria may play a role in plant adaptation to the edaphic constraints. Recently, two new bacteria species belonging to the genus Burkholderia sensu lato were isolated from rhizospheric roots of pioneer plant growing in New Caledonian ultramafic soils and were able to tolerate high metal concentrations. The purpose of this study was to determine the major mechanisms by which these two bacteria were able to tolerate metals and if they were able to reduce metal absorption by plants from which they were isolated. Bioaccumulation, adsorption on cell surface, ability to form biofilm and production of exopolysaccharides were assessed. A focus was carried out on Ni, generally the most toxic metal in New Caledonian ultramafic soils. All experiments were performed in comparison with a reference strain isolated from a non-ultramafic habitat, Paraburkholderia caribensis MWAP64T. The two ultramafic bacterial species showed a high tolerance to Ni with a Ni–IC50 of 5 mM for Caballeronia novacaledonica STM10272T and 30 mM for Paraburkholderia ultramafica STM10279T, values significantly higher than those of P. caribensis MWAP64T (2 mM). The major mechanism involved in the bacterial Ni tolerance was related to biofilm formation for C. novacaledonica STM10272T and to acidic exopolysaccharide production for P. ultramafica STM10279T. Both species alleviated metal contents in roots of Tetraria comosa, an endemic pioneer herbaceous species growing on New Caledonian ultramafic soils.