Industrial wasteland soils with aged PAH and heavy metal contaminations are environments where pollutant toxicity has been maintained for decades. Although the communities may be well adapted to the presence of stressors, knowledge about microbial diversity in such soils is scarce. Soil microbial community dynamics can be driven by the presence of plants, but the impact of plant development on selection or diversification of microorganisms in these soils has not been established yet. To test these hypotheses, aged-contaminated soil samples from a field trial were collected. Plots planted with alfalfa were compared to bare soil plots, and bacterial and fungal diversity and abundance were assessed after 2 and 6 years. Using pyrosequencing of 16S rRNA gene and ITS amplicons, we showed that the bacterial community was dominated by Proteobacteria, Actinobacteria, and Bacteroidetes and was characterized by low Acidobacteria abundance, while the fungal community was mainly represented by members of the Ascomycota. The short-term toxic impact of pollutants usually reduces the microbial diversity, yet in our samples bacterial and fungal species richness and diversity was high suggesting that the community structure and diversity adapted to the contaminated soil over decades. The presence of plants induced higher bacterial and fungal diversity than in bare soil. It also increased the relative abundance of bacterial members of the Actinomycetales, Rhizobiales, and Xanthomonadales orders and of most fungal orders. Multivariate analysis showed correlations between microbial community structure and heavy metal and PAH concentrations over time, but also with edaphic parameters (C/N, pH, phosphorus, and nitrogen concentrations).

Due to human activities, large volumes of soils are contaminated with organic pollutants such as polycyclic aromatic hydrocarbons, and very often by metallic pollutants as well. Multipolluted soils are therefore a key concern for remediation. This work presents a long-term evaluation of the fate and environmental impact of the organic and metallic contaminants of an industrially polluted soil under natural and plant-assisted conditions. A field trial was followed for four years according to six treatments in four replicates: unplanted, planted with alfalfa with or without mycorrhizal inoculation, planted with Noccaea caerulescens, naturally colonized by indigenous plants, and thermally treated soil planted with alfalfa. Leaching water volumes and composition, PAH concentrations in soil and solutions, soil fauna and microbial diversity, soil and solution toxicity using standardized bioassays, plant biomass, mycorrhizal colonization, were monitored. Results showed that plant cover alone did not affect total contaminant concentrations in soil. However, it was most efficient in improving the contamination impact on the environment and in increasing the biological diversity. Leaching water quality remained an issue because of its high toxicity shown by micro-algae testing. In this matter, prior treatment of the soil by thermal desorption proved to be the only effective treatment.

The arsenic resistance gene cluster of Microbacterium sp. A33 contains a novel pair of genes (arsTX) encoding a thioredoxin system that are cotranscribed with an unusual arsRC2 fusion gene, ACR3, and arsC1 in an operon divergent from arsC3. The whole ars gene cluster is required to complement an Escherichia coli ars mutant. ArsRC2 negatively regulates the expression of the pentacistronic operon. ArsC1 and ArsC3 are related to thioredoxin-dependent arsenate reductases; however, ArsC3 lacks the two distal catalytic cysteine residues of this class of enzymes.Arsenic is widely dispersed in the environment and occurs primarily in two oxidation states, arsenate [As(V)] and arsenite [As(III)], and both are toxic to the majority of living organisms. The frequent abundance of arsenic in all environmental compartments has guided the evolution of detoxification systems in almost all microorganisms. Of these, the arsenic resistance system (ars) appears to be widely distributed among prokaryotes. It involves an arsenate reductase (ArsC), an arsenite efflux pump (ArsB or ACR3), and a transcriptional repressor (ArsR) (32), encoded by a set of genes that display large variations in their number and genomic organization. The early identified ars system of Escherichia coli plasmid R773 (41) has two additional components, ArsA, which acts as the catalytic subunit of the ArsAB arsenite extrusion pump (33), and ArsD, a metallochaperone protein that transfers As(III) to ArsA (18). In addition to these well-studied ars components, a variety of ars clusters contain additional genes whose functions in arsenic resistance have not been clearly established in many cases (31).Members of the Microbacterium lineage of actinobacteria that can tolerate various metals, including nickel, chromium, and uranium (1,16,25), have been isolated from metal-rich environments. New examples of arsenic-resistant isolates of Microbacterium are continuously being reported (1,2,8,10,12,21). In each case, however, the tolerance mechanism was not investigated, probably due to the lack of efficient genetic systems in this genus. Among actinobacteria, only Streptomyces sp. FR-008 (40) and Corynebacterium glutamicum ATCC 13032 (28) have been subjected to molecular characterization of determinants of defense against arsenic. In the former, the linear plasmid pHZ227 carries an arsenic resistance gene cluster with two novel genes, the arsO and arsT genes, which encode a putative flavin-binding monooxygenase and a putative thioredoxin reductase, respectively (40). The latter strain was recently shown to possess two members of a new class of arsenate reductases (Cg_ArsC1 and Cg_ArsC2) (30) and a transcriptional repressor (Cg_ArsR1) with a metalloid binding site unrelated to other previously characterized members of the ArsR/SmtB metalloregulatory proteins (29).The present study focuses on Microbacterium sp. strain A33, a soil isolate previously shown to tolerate high concentrations of arsenite and arsenate (2). Here, we report on the isolation and functional characteriz...