Bacterial strains resistant to inorganic and organic forms of mercury isolated from polluted sediments of the Orbetello Lagoon, Italy, and their possible use in bioremediation processes

Bacteria are able to adapt to heavy metals in contaminated environments, by developing specific mechanisms of resistance. A mercury (Hg)-resistant bacterial community was isolated from polluted sediments of the Orbetello Lagoon, Italy. The members of the Hg-resistant bacterial community showed high levels of resistance both to the inorganic and to the organic forms of Hg. 16S rRNA gene sequencing showed the presence of different genera, and a bacterial strain resistant to Hg belonging to the genus Luteimonas was evidenced for the first time. The merA gene coding for mercury-reductase conferring resistance to inorganic Hg, and the merB gene coding for mercury-lyase enzymes, for resistance to the organic forms of Hg, were detected in the tested bacterial strains. The community showed the presence of bacteria belonging to the genera Pseudomonas and Psychrobacter that highlighted the capability to reduce Hg2+ to the volatile form of Hg0. Experiments carried out with immobilized cells of the Hg-resistant strains removed 96% of Hg in sediments leachates in a bioremediation laboratory scale pilot plant. A methylating activity of the sulphate-reducing bacteria of the same polluted sediments was moreover evidenced. These results evidenced the presence of microbial communities highly adapted to the presence of Hg into the sediments of the lagoon. The use of the isolated autochthonous bacterial strains for bioremediation of the native sediments contaminated by Hg is suggested.