Hg bioavailability and impact on bacterial communities in a long-term polluted soil

Abstract

Different soil samples characterised by a long-term Hg-pollution were studied for Hg total content, fractionation, phytotoxicity and influence on the bacterial community. Hg pollution ranged from 1 to 50 mg kg⁻¹ and most of it was speciated in scarcely soluble forms. In agreement with this, the biochemical quality indexes were investigated (biomass, enzyme activities) and the bacterial community (viable heterotrophic (VH) bacteria, functional diversity) apparently was not influenced by the degree of Hg pollution. In particular, the investigated soils exhibited a low percentage of Hg-resistant (Hg^R) bacteria ranging from less than 0.001% to 0.25% of the VH and the addition of available Hg in the form of HgCl₂ induced an enrichment of resistant Hg^R populations. The general biodiversity of the bacterial community was evaluated by denaturing gradient gel electrophoresis of DNA of Hg spiked soil microcosms and of control soils. Hg^R bacteria capable to grow in a minimal medium containing HgCl₂ were also isolated and identified. MerA and merB gene PCR fragments were obtained from different Hg^R strains and the range of similarities at the DNA level and at the deduced amino acid level showed that they carried mercuric reductase and lyase. Differently from bacteria, some influence of soil Hg content on seeds' germination and root elongation was observed for Lepidium sativum L. and Solanum lycopersicum L. In conclusion, most of the Hg in these long-term polluted soils was scarcely mobile and available and did not significantly influence the soil bacterial community. The risk of potential Hg remobilisation over time, that could be naturally favoured by the activity of plant roots or other inorganic processes occurring in soil, can be extenuated since bacterial community was resistant and resilient to subsequent Hg stress.