New Caledonian serpentine (ultramafic) outcrops are metal-rich soils, containing high levels of toxic heavy metals particularly the nickel (20 g.kg-1). These are deficient in essential elements like carbon, nitrogen, phosphorous, and have high Mg-Ca ratio, resulting in altered structure of soil microbial communities. The effect of ultramafic soils on ectomycorrhiza and mycorrhizosphere bacterial diversities as well as on their genetic determinants of adaptation to Ni associated to widespread Tristaniopsis spp. has been investigated for fist time. About 200 ectomycorrhizas were sampled from four different ultramafic sites (3 in Koniambo and 1 in Desmazures forest) vs two non-ultramafic ones from volcano-sedimentary soils (Arama). Molecular characterization of fungi (through partial sequencing of the ITS rRNA gene) and related bacteria (through16S rRNA sequencing) revealed the presence of different dominant fungi (Pisolithus albus, Russula spp., Boletellus spp.) and bacteria (Burkholderia spp., Bacillus spp., Pseudomonas spp.). However, bacteria isolated from ultramafic soils could grow in the presence of Ni up to 20 mmol L-1, exhibited P-solubilizing, contained cnrT and nreB genes, known to confer heavy metal tolerance, contrary to bacteria isolated from non-ultramafic soils, likely indicating their adaptation to ultramafic soils and would help in the understanding of plant functioning on these mine sites.(Texte intégral)