Heavy Metal-polluted Sites Research Articles

Sulfate-reducing consortium HQ23 stabilizes metal(loid)s and activates biological N-fixation in mixed heavy metal-contaminated soil

Sulfate-reducing bacteria (SRB) are used in the remediation of mine pollution; however, the mechanism of stabilizing multiple heavy metal(loid)s by the SRB consortium under low oxygen conditions needs further study. Indigenous microflora were extracted from non-ferrous metal-contaminated soil co-inoculated with enriched SRB consortium and assembled as the HQ23 consortium. The presence of Desulfovibrio (SRB) in HQ23 was confirmed by 16S rRNA sequencing and qPCR. The effects of culture media, dissolved oxygen (DO), SO42¯, and pH on the HQ23 growth rate, and the SO42¯-reducing activity were examined. Data indicates that the HQ23 sustained SRB function under low DO conditions (3.67 ± 0.1 mg/L), but the SRB activity was inhibited at high DO content (5.75 ± 0.39 mg/L). The HQ23 can grow from pH 5 to pH 9 and can decrease mobile or bioavailable Cr, Cu, and Zn concentrations in contaminated soil samples. FTIR revealed that Cu and Cr adsorbed to similar binding sites on bacteria, likely decreasing bacterial Cu toxicity. Increased abundances of DSV (marker for Desulfovibrio) and nifH (N-fixation) genes were observed, as well as an accumulation of nitrate-N content in soils suggesting that HQ23 stimulates the biological N-fixation in soils. This study strongly supports the future application of SRB for the bioremediation of heavy metal-polluted sites.

Source and risk assessment of heavy metals in mining-affected areas in Jiangxi Province, China, based on Monte Carlo simulation.

In recent years, heavy metal contamination of soils has become a major concern in China due to the potential risks involved. To assess environmental pollution and human health risks in a typical heavy metal polluted site in Jiangxi Province, a thorough evaluation of the distribution, pollution levels, and sources of heavy metals in soils of the Yangmeijiang River watershed was conducted in this study. Positive matrix factorization and Monte Carlo simulation were used to evaluate the ecological and human health risks of heavy metals. The research findings indicate that heavy metal pollution was the most severe at the depth of 20-40cm in soils, with local heavy metal pollution resulting from mining and sewage irrigation. The high-risk area accounted for 91.11% of the total area. However, the pollution level decreased with time due to sampling effects, rainfall, and control measures. Leaf-vegetables and rice were primarily polluted by Cd and Pb. The main four sources of heavy metals in soils were traffic emission, metal smelting, agricultural activities and natural sources, mining extraction, and electroplating industries. Heavy metals with the highest ecological risk and health risk are Cd and As, respectively. The non-carcinogenic and carcinogenic risks of children were 7.0 and 1.7 times higher than those of adults, respectively. Therefore, children are more likely to be influenced by heavy metals compared to adults. The results obtained by the risk assessments may contribute to the identification of specific sources of heavy metals (e.g., traffic emissions, metal smelting, mining excavation, and electroplating industries). Additionally, the environmental impacts and biotoxicity associated with various heavy metals (e.g., Cd and As) can also be reflected. These outcomes may serve as a scientific basis for the pollution monitoring and remediation in the mining-affected areas.

Synchrotron-based techniques for elemental analysis in soil-plant system under polluted environment

Analytical techniques for elemental analysis in the soil-plant system have significance importance, especially emerging techniques such as synchrotron radiation (SR). Improved techniques allow samples to be examined in a non-invasive manner at high speed and resolution, resulting in better sample data. By applying various analytical techniques based on SR, it is possible to gather different information about the structure of the studied samples. In mining ecology, such techniques are widely used in assessing heavy metal-polluted sites, i.e., overburden dumps and areas around operating and mothballed mines. The present review elaborated insights into different analytical techniques for applying SR in plant-soil samples. The review also compared traditional research techniques with SR-based emerging and improved techniques. The need to use SR techniques for the complex diagnostics of sample structures to study their elemental and phase composition is substantiated. Using an integrated approach with SR, we can study the dynamics and speciation of HMs with carrier phases and uncover the mechanisms underlying the interactions between the adsorption centers of minerals, organic components, and heavy metals. It also improves the efficiency and accuracy of analysis and broadens the range of information obtained, which could lead to a more precise analysis of samples.

Removal of heavy metals using Iris species: Apotential approach for reclamation of heavy metal-polluted sites and environmental beautification.

Globally, the number of heavy metal (HM)-polluted sites has increased rapidly in recent years, posing a serious threat to agricultural productivity, human health, and environmental safety. Hence, it is necessary to remediate HM-polluted sites to increase cultivatable lands for agricultural productivity, prevent hazardous effects to human health, and promote environmental safety. Removal of HMs using plants (phytoremediation) is a promising method as it is eco-friendly. Recently, ornamental plants have been widely used in phytoremediation programs as they can simultaneously eliminate HMs and are aesthetically pleasing. Among the ornamental plants, Iris species are frequently used; however, their role in HM remediation has not been reviewed yet. Here, the importance of Iris species in the ornamental industry and their different commercial aspects are briefly described. Additionally, the mechanisms of how the plant species absorb and transport the HMs to the above-ground tissues and tolerate HM stress are highlighted. The variation in HM remediation efficiency depending on the plant species, HM type and concentration, use of certain supplements, and experimental conditions are also discussed. Iris species are able to remove other hazards as well, such as pesticides, pharmaceutical compounds, and industrial wastes, from polluted soils or waste-water. Owing to the valuable information presented in this review, we expect more applications of the species in reclaiming polluted sitesand beautifying theenvironment.

Glomus mosseae improved the adaptability of alfalfa (Medicago sativa L.) to the coexistence of cadmium-polluted soils and elevated air temperature.

The coexistence of heavy metal-polluted soils and global warming poses serious threats to plants. Many studies indicate that arbuscular mycorrhizal fungi (AMF) can enhance the resistance of plants to adverse environments such as heavy metals and high temperature. However, few studies are carried out to explore the regulation of AMF on the adaptability of plants to the coexistence of heavy metals and elevated temperature (ET). Here, we investigated the regulation of Glomus mosseae on the adaptability of alfalfa (Medicago sativa L.) to the coexistence of cadmium (Cd)-polluted soils and ET. G. mosseae significantly enhanced total chlorophyll and carbon (C) content in the shoots by 15.6% and 3.0%, respectively, and Cd, nitrogen (N), and phosphorus (P) uptake by the roots by 63.3%, 28.9%, and 85.2%, respectively, under Cd + ET. G. mosseae significantly increased ascorbate peroxidase activity, peroxidase (POD) gene expression, and soluble proteins content in the shoots by 13.4%, 130.3%, and 33.8%, respectively, and significantly decreased ascorbic acid (AsA), phytochelatins (PCs), and malondialdehyde (MDA) contents by 7.4%, 23.2%, and 6.5%, respectively, under ET + Cd. Additionally, G. mosseae colonization led to significant increases in POD (13.0%) and catalase (46.5%) activities, Cu/Zn-superoxide dismutase gene expression (33.5%), and MDA (6.6%), glutathione (22.2%), AsA (10.3%), cysteine (101.0%), PCs (13.8%), soluble sugars (17.5%), and proteins (43.4%) contents in the roots and carotenoids (23.2%) under ET + Cd. Cadmium, C, N, G. mosseae colonization rate, and chlorophyll significantly influenced shoots defenses and Cd, C, N, P, G. mosseae colonization rate, and sulfur significantly affected root defenses. In conclusion, G. mosseae obviously improved the defense capacity of alfalfa under ET + Cd. The results could improve our understanding of the regulation of AMF on the adaptability of plants to the coexistence of heavy metals and global warming and phytoremediation of heavy metal-polluted sites under global warming scenarios.

Characterization of Endophytic Bacteria Isolated from Typha latifolia and Their Effect in Plants Exposed to Either Pb or Cd.

Plant-associated bacteria in heavy-metal-contaminated environments could be a biotechnological tool to improve plant growth. The present work aimed to isolate lead- and cadmium-tolerant endophytic bacteria from the roots of Typha latifolia growing in a site contaminated with these heavy metals. Endophytic bacteria were characterized according to Pb and Cd tolerance, plant-growth-promoting rhizobacteria activities, and their effect on T. latifolia seedlings exposed and non-exposed to Pb and Cd. Pb-tolerant isolates were identified as Pseudomonas azotoformans JEP3, P. fluorescens JEP8, and P. gessardii JEP33, while Cd-tolerant bacteria were identified as P. veronii JEC8, JEC9, and JEC11. They all exert biochemical activities, including indole acetic acid synthesis, siderophore production, and phosphate solubilization. Plant-bacteria interaction assays showed that P. azotoformans JEP3, P. fluorescens JEP8, P. gessardii JEP33, and P. veronii JEC8, JEC9, JEC11 promote the growth of T. latifolia seedlings by increasing the root and shoot length, while in plants exposed to either 5 mg/L of Pb or 10 mg/L of Cd, all bacterial isolates increased the shoot length and the number of roots per plant, suggesting that they are plant-growth-promoting rhizobacteria that could contribute to T. latifolia adaptation to the heavy metal polluted site.

Effect of seedling production method on the growth of Pinus sylvestris L. on reclaimed post-industrial sites in Poland

Selection of tree species and proper technology of afforestation of post-industrial sites play a crucial role in new ecosystem stability and success of the restoration. There is still current discussion about the effectiveness of different methods in the production of seedlings for extreme site conditions. This study aimed to analyze the effect of the seedling production method on the growth of 6-year-old Scots pine (Pinus sylvestris L.) trees planted on reclaimed post-industrial sites in southern Poland. We compared four methods of seedling production: bare root system, containerized without and with inoculation of ectomycorrhizal fungi Laccaria bicolor (Maire) P.D. Orton, and Hebeloma crustuliniforme (Bull.) Quél. The experiment was carried out independently on two reclaimed sites: a post-sand-mining site in Bukowno, and a heavy metal polluted site in Miasteczko Śląskie. The effect of the seedling production method on tree growth expressed by height, root collar diameter, above- and belowground biomass of different tree components was analyzed on 240 sampled trees using analysis of variance and general linear model. In addition, the effect of the seedling production method on the root-to-shoot ratio was investigated. Scots pine tree growth was significantly affected by the seedling production method on heavy metal polluted site. The lowest tree growth parameters were observed in trees planted with bare roots, whereas the largest values were in the case of trees planted from containerized seedlings inoculated with H. crustuliniforme. In contrast, on the post-sand-mining site, the treatment effect turned out to be insignificant. The effect on biomass allocation was observed in the case of both sites. Trees prepared with the bare root method differed in greater biomass allocation to the belowground from other treatment groups. The use of containerized seedlings or additionally inoculated with ectomycorrhizal fungi in heavy metal contaminated sites improves the growth of trees and, although it is more costly and labor-intensive compared to the production of seedlings with a bare root system, should be preferred. In the post-sand-mining area, no effect of the seedling production method on tree growth was observed, and the use of bare root seedlings will be equally effective and additionally less time and cost-consuming.

The Utility of Ground Bryophytes in the Assessment of Soil Condition in Heavy Metal-Polluted Grasslands.

Bryophytes are commonly used in biomonitoring heavy metal pollution, whereas the bioindicative value of bryophyte communities is a less known issue. The aim of the present study is to recognize the utility of the bryophyte community’s structure in the assessment of soil condition in heavy metal-polluted, dry grasslands. The study plots are examined with respect to bryophytes; vascular plants; concentrations of Zn, Pb, Cd, and As in the soil; total nitrogen and organic carbon content in the soil; and soil pH. The results show that both bryophyte species richness and composition greatly depend on soil chemical characteristics, including heavy-metal pollution levels and soil pH. Three groups of species are distinguished: (1) species sensitive to pollution growing on acidic soils, (2) nonspecific species inhabiting a wide spectrum of heavy metal-polluted sites, and (3) species preferring polluted and alkaline soils. Our study reveals a gradual replacement of the bryophyte species alongside increasing soil pollution and alkalinity. This proves that bryophytes are highly responsive to soil factors and the changes in bryophyte composition may indicate the soil condition of a certain site. Furthermore, high concentrations of heavy metals in the soil and an alkaline pH positively affect bryophyte species richness. Consequently, such sites could be considered as biodiversity hotspots for terrestrial bryophytes in post-industrial landscapes.

Augmentation of metal-tolerant bacteria elevates growth and reduces metal toxicity in spinach

Heavy metal contamination poses a serious threat to both the ecosystem and human and requires expensive cleanup costs. Bioremediation based on microorganisms, plants, or other biological systems offers cost-effective and environment friendly metal clean-up methods. Studies on bacterial diversity in heavy metal contaminated sites have demonstrated a high diversity of microorganisms that are adapted to the new environment. Bacteria that are resistant to and grow on metals play an important role in the biogeochemical cycling of those metal ions. In pursuit to identify bacteria that are tolerant to different heavy metals and can have a potential in bioremediation, surveys, and collection of samples from several presumptive heavy metal-polluted sites of India were carried out. A total of 77 bacterial morphotypes were obtained, and based on minimum inhibitory concentrations (MIC) of different heavy metals, that is Pb+2, Ni+2, Cd+2, Cr+3, Hg+2, Cu+2, Zn+2, Co+2, and As+2, thirteen potential bacterial isolates were identified possessing very high and multiple heavy metal tolerance like arsenic (50–1100 mg kg−1), lead (100–2000 mg kg−1), chromium (250–500 mg kg−1), cadmium (50–100 mg kg−1), and other heavy metals. All potential bacteria were morphologically characterized, identified based on the 16 s rRNA gene sequences, and studied for plant growth promoting attributes. Bacterial strains were found to be phosphate solubilizers, siderophore and ammonia producers, and nitrate reducers. Bacillus cereus MB1, Bacillus amyloliquefaciens RD4, Bacillus megaterium MF7, and E. cloacae MC4 were evaluated for alleviation of As, Cr, Ni, and Pb toxicity, respectively in spinach. The inoculation of plants with respective heavy metal-tolerant bacteria under study gave higher records of all estimated growth parameters, total chlorophyll content and antioxidant enzyme, superoxide dismutase, activity and differential response in proline biosynthesis when compared to respective uninoculated heavy metal controls. Overall selected plant growth-promoting heavy metal-tolerant bacterial inoculations were found to promote growth and reduce the respective heavy metal toxicity in spinach plant. Since heavy metal contamination in agricultural lands is becoming serious environmental concern, the heavy metal-tolerant plant growth-promoting strains reported in this study can offer suitable economical and eco-friendly base for development of the bioremediation strategies.

Epipactis tremolsii Seed Diversity in Two Close but Extremely Different Populations: Just a Case of Intraspecific Variability?

Analysis of the seed morphology is a widely used approach in ecological and taxonomic studies. In this context, intraspecific variability with respect to seed morphology (size, weight, and density) was assessed in two close Epipactis tremolsii Pau. populations sharing the same ecological conditions, except for the soil pollution distinguishing one of them. Larger and heavier seeds were found in plants growing on the heavy metal polluted site, while no differences in seed density were detected between seeds produced by plants growing on the contaminated and the control site. Moreover, seed coats and embryos varying together in their dimensions were described in the control population, while coats varying in their size independently from embryos were described in plants growing on the polluted site. Seeds from the two studied populations significantly differed in several parameters suggesting that intraspecific seed variability occurred in the case study.

Analysis of biosorption and biotransformation mechanism of Pseudomonas chengduensis strain MBR under Cd(II) stress from genomic perspective.

Microbial treatment of heavy metal-polluted sites is considered an environmentally friendly bioremediation technology with high potential. This study shows that Pseudomonas chengduensis strain MBR, a bacterium that can potentially be applied in the treatment of heavy metal pollution, is most affected by Cd(II) stress at the beginning of its growth. Up to 100% of total Cd(II) adsorption occurs in the first 48h after treatment of stationary phase cells with Cd(II). A biofilm forms on the cell surface, Cd(II) adsorbs, and is reduced to Cd (0) in the form of nanoscale particles. The genome of strain MBR was sequenced, annotated and analyzed. We identified various genes potentially related to cadmium resistance, transport and metabolism. Analysis of the strain MBR genome is helpful to explore the mechanism of Cd(II) resistance, and can provide new ideas for cadmium pollution control.

Effects of environmental metal pollution on reproduction of a free-living resident songbird, the tree sparrow (Passer montanus)

Anthropogenic metal pollution is known to adversely affect bird reproduction; however, few systematic studies are available on the effects of metal pollution on breeding performance and parental investment in a common resident songbird, the tree sparrow (Passer montanus). We conducted this study in two sites, a long-term heavy metal polluted site (Baiyin [BY]) and a relatively unpolluted site at approximately 110 km distance (Liujiaxia [LJX]), to assess the potential effects of environmental metal contamination on breeding parameters (clutch size, hatching success, fledging success, and growth of nestlings) and parental investment. The results showed smaller clutch size, lower fledging success, and differences in incubation behaviors of tree sparrows in BY than in LJX. Although there was no difference in parental body condition (residual body mass) between the two study sites, the parents responded differently with respect to reproduction due to varying metal levels in their habitats and bodies. Higher Cd levels in the primary feathers of females in BY were associated with lower clutch sizes. Parental investment including incubation duration and feeding rates showed no significant difference between the two sites during the incubation and nestling periods, but the frequencies of incubation visits were higher in BY. Parental behavior during the incubation period was also negatively affected by the parental Pb and Cd levels. Although the nestling growth patterns were relatively similar between the two sites, the nestlings were smaller, had lower body weight, and fledged later and fledging rate was also lower in BY than in LJX. Metal concentrations were higher in nestling organs and feces in BY. Taken together, metal pollution might adversely affect nestling growth condition. Our results suggest a negative response in the reproduction of tree sparrows to long-term environmental metal pollution.

Assessment of heavy metal toxicity using a luminescent bacterial test based on Photobacterium sp. strain MIE

Toxicity evaluation of wastewater, polluted sediment and water streams is a very crucial aspect of environmental pollution monitoring. In this work, a newly developed luminescent bacterial test using a tropical luminescent bacterium, Photobacterium sp. strain MIE was used to assess the toxicity of several heavy metals using a 15-min assay format. The assessment was carried out by exposing strain MIE to different concentrations of heavy metals ranging from 0.001 to 200 mg/L in a DTX microplate 96 wells. The toxicity result based on the inhibitory concentration (IC50) was Hg (0.053 mg/L) > Ag (0.12 mg/L) > Cu (0.85 mg/L) > Ni (12.32 mg/L) > Zn (18.72 mg/L) > Cr (26.02 mg/L). Principal Component Analysis (PCA) and Agglomerative Hierarchical Clustering (AHC) analyses showed the sensitivity (IC50) of strain MIE to several toxic heavy metals are comparable to the commercial luminescent assay, Microtox™ as both clusters together making it a good choice for an alternative near-real-time monitoring of heavy metals. The sensitivity of strain MIE towards heavy metals was proven through field trial works on several heavy metal-polluted sites in Malaysia. Thus, it is a good candidate as an early detection system for heavy metals in aquatic bodies in tropical countries.

Heavy metal resistance in algae and its application for metal nanoparticle synthesis.

The ungenerous release of metals from different industrial, agricultural, and anthropogenic sources has resulted in heavy metal pollution. Metals with a density larger than 5gcm-3 have been termed as heavy metals and have been stated to be potentially toxic to human and animals. Algae are known to be pioneer organisms with the potential to grow under extreme conditions including heavy metal-polluted sites. They have evolved efficient defense strategies to combat the toxic effects exerted by heavy metal ions. Most of the algal strains are reported to accumulate elevated metal ion concentration in cellular organelles. With respect to that, this review focuses on understanding the various strategies used by algal system for heavy metal resistance. Additionally, the application of this metal resistance in biosynthesis of metal nanoparticles and metal oxide nanoparticles has been investigated in details. We thereby conclude that algae serve as an excellent system for understanding metal uptake and accumulation. This thereby assists in the design and development of low-cost approaches for large-scale synthesis of nanoparticles and bioremediation approach, providing ample opportunities for future work.

Efficient biodegradation of acephate by Pseudomonas pseudoalcaligenes PS-5 in the presence and absence of heavy metal ions [Cu(II) and Fe(III)], and humic acid.

The present study was intended to investigate the biodegradation of acephate in aqueous media in the presence and in the absence of metal ions [Fe(III) and Cu(II)], and humic acid (HA). Biodegradations were performed using Pseudomonas pseudoalcaligenes PS-5 (PS-5) isolated from the heavy metal polluted site. Biodegradations were monitored by UV-Visible, FTIR, and electron spray ionization-mass spectrometry (ESI-MS) analyses. ESI-MS analysis revealed that PS-5 degraded acephate to two metabolites showing intense ions at mass-to-charge ratios (m/z) 62 and 97. The observed kinetic was the pseudo-first order, and half-life periods (t1/2) were 2.79d-1 (of PS-5+acephate), 3.45d-1 [of PS-5+acephate+Fe(III)], 3.16d-1 [of PS-5+acephate+Cu(II)], and 5.54d-1 (of PS-5+acephate+HA). A significant decrease in degradation rate of acephate was noticed in the presence of HA, and the same was confirmed by UV-Visible and TGA analyses. Strong aggregation behavior of acephate with humic acid in aqueous media was the major cause behind the slow degradation rate of acephate . New results on acephate metabolism by strain PS-5 in the presence and in the absence of metal ions [Fe(III) and Cu(II)] and humic acid were obtained. Results confirmed that Pseudomonas pseudoalcaligenes strain PS-5 was capable of mineralization of the acephate without formation of toxic metabolite methamidophos. More significantly, the Pseudomonas pseudoalcaligenes strain PS-5 could be useful as potential biological agents in effective bioremediation campaign for multi-polluted environments.

A comparative analysis of endophytic bacterial communities associated with hyperaccumulators growing in mine soils

Interactions between endophytic bacterial communities and hyperaccumulators in heavy metal-polluted sites are not fully understood. In this study, the diversity of stem-associated endophytic bacterial communities of two hyperaccumulators (Solanum nigrum L. and Phytolacca acinosa Roxb.) growing in mine soils was investigated using molecular-based methods. The denaturing gradient gel electrophoresis (DGGE) analysis showed that the endophytic bacterial community structures were affected by both the level of heavy metal pollution and the plant species. Heavy metal in contaminated soil determined, to a large extent, the composition of the different endophytic bacterial communities in S. nigrum growing across soil series (five sampling spots, and the concentration of Cd is from 0.2 to 35.5 mg/kg). Detailed analysis of endophytic bacterial populations by cloning of 16S rRNA genes amplified from the stems of the two plants at the same site revealed a different composition. A total of 51 taxa at the genus level that included α-, β-, and γ-Proteobacteria (68.8% of the two libraries clones), Bacteroidetes (9.0% of the two libraries clones), Firmicutes (2.0% of the two libraries clones), Actinobacteria (16.4% of the two libraries clones), and unclassified bacteria (3.8% of the two libraries clones) were found in the two clone libraries. The most abundant genus in S. nigrum was Sphingomonas (23.35%), while Pseudomonas prevailed in P. acinosa (21.40%). These results suggest that both heavy metal pollution and plant species contribute to the shaping of the dynamic endophytic bacterial communities associated with stems of hyperaccumulators.

Molecular field analysis of trophic relationships in soil‐dwelling invertebrates to identify mercury, lead and cadmium transmission through forest ecosystems

Contamination pathways in complex food chains in soil ecosystems can be difficult to elucidate. Molecular analysis of predator gut content can, however, rapidly reveal previously unidentified trophic interactions between invertebrates and thereby uncover pathways of pollutant spread. Here, we measured concentrations of the toxic metals lead, cadmium and mercury in carabid beetle predators and their prey. Invertebrates were sampled at one control and four heavy metal-polluted sites to reveal the impact of diet composition and seasonal variation in prey availability on metal burden in carabids and metal transfer pathways through forest ecosystems. This is the first report, to our knowledge, of carabid diet composition based on PCR analysis of gut contents at the forest community level, rather than in cultivated fields. Extensive screening using group- and species-specific primers revealed that carabids ate primarily earthworms and slugs, as well as smaller numbers of woodlice and springtails. Metal concentrations in carabids correlated with seasonal changes in diet. Mercury accumulated in beetle predators more than in their slug prey. As earthworms, slugs and carabid beetles are the major prey of many birds and mammals, prey-predator transfer and associated toxicity are major risks at mercury-contaminated sites. Carabids may be useful bioindicators for assessing the impact of pollutants on soil ecosystems, as long as species and seasonal factors are taken into account.

Poplar clones of different sizes, grown on a heavy metal polluted site, are associated with microbial populations of varying composition

We performed a field trial to evaluate the response of different poplar clones to heavy metals. We found that poplar plants of the same clone, propagated by cuttings, had a marked variability of survival and growth in different zones of the field that were characterized by very similar physical–chemical prosperities. Since metal uptake and its accumulation by plants can be affected by soil microorganisms, we investigated soil microbial populations that were collected in proximity to the roots of large and small poplar plants. We used microbiological and molecular tools to ascertain whether bacterial strains or species were associated with large, or small poplars, and whether these were different from those present in the bulk (without plants) soil.We found that the culturable fraction of the bacteria differed in the three cases (bulk soil, small or large poplars). While some taxa were always present, two species (Chryseobacterium soldanellicola and Variovorax paradoxus) were only found in the soil where poplars (large or small) were growing, independently from the plant size. Bacterial strains of the genus Flavobacterium were prevalent in the soil with large poplar plants. The existence of different microbial populations in the bulk and in the poplar grown soils was confirmed by the DGGE profiles of the bacterial culturable fractions.Cluster analysis of the DGGE profiles highlighted the clear separation of the culturable fraction from the whole microbial community. The isolation and identification of poplar-associated bacterial strains from the culturable fraction of the microbial community provided the basis for further studies aimed at the combined use of plants and soil microorganisms in the remediation of heavy metal polluted soils.

Transcriptome analysis by cDNA-AFLP of Suillus luteus Cd-tolerant and Cd-sensitive isolates

The ectomycorrhizal basidiomycete Suillus luteus (L.:Fr.), a typical pioneer species which associates with young pine trees colonizing disturbed sites, is a common root symbiont found at heavy metal contaminated sites. Three Cd-sensitive and three Cd-tolerant isolates of S. luteus, isolated respectively from non-polluted and a heavy metal-polluted site in Limburg (Belgium), were used for a transcriptomic analysis. We identified differentially expressed genes by cDNA-AFLP analysis. The possible roles of some of the encoded proteins in heavy metal (Cd) accumulation and tolerance are discussed. Despite the high conservation of coding sequences in S. luteus, a large intraspecific variation in the transcript profiles was observed. This variation was as large in Cd-tolerant as in sensitive isolates and may help this pioneer species to adapt to novel environments.

Proteomic analysis of Arabidopsis halleri shoots in response to the heavy metals cadmium and zinc and rhizosphere microorganisms

Arabidopsis halleri has the rare ability to colonize heavy metal-polluted sites and is an emerging model for research on adaptation and metal hyperaccumulation. The aim of this study was to analyze the effect of plant-microbe interaction on the accumulation of cadmium (Cd) and zinc (Zn) in shoots of an ecotype of A. halleri grown in heavy metal-contaminated soil and to compare the shoot proteome of plants grown solely in the presence of Cd and Zn or in the presence of these two metals and the autochthonous soil rhizosphere-derived microorganisms. The results of this analysis emphasized the role of plant-microbe interaction in shoot metal accumulation. Differences in protein expression pattern, identified by a proteomic approach involving 2-DE and MS, indicated a general upregulation of photosynthesis-related proteins in plants exposed to metals and to metals plus microorganisms, suggesting that metal accumulation in shoots is an energy-demanding process. The analysis also showed that proteins involved in plant defense mechanisms were downregulated indicating that heavy metals accumulation in leaves supplies a protection system and highlights a cross-talk between heavy metal signaling and defense signaling.