A biological test system for the evaluation of the phytotoxicity of metal-contaminated soils

Overall plant responses to Cd and Pb metal stress in maize: Growth pattern, ultrastructure, and photosynthetic activity

This study provides a full description of the responses of the crop energy plant Zea mays to stress induced by Cd and Pb, in view of a possible extensive use in phytoattenuation of metal-polluted soils. In this perspective, (i) the uptake capability in root and shoot, (ii) the changes in growth pattern and cytological traits, and (iii) the photosynthetic efficiency based on photochemistry and the level of key proteins were investigated in hydroponic cultures. Both metals were uptaken by maize, with a translocation factor higher for Cd than Pb, but only Cd-treated plants showed a reduced growth compared to control (i.e., a lower leaf number and a reduced plant height), with a biomass loss up to 40%, at the highest concentration of metal (10^-3 M). The observation of cytological traits highlighted ultrastructural damages in the chloroplasts of Cd-treated plants. A decline of Rubisco and D1 was observed in plants under Cd stress, while a relevant increase of the same proteins was found in Pb-treated plants, along with an increase of chlorophyll content. Fluorescent emission measurements indicated that both metals induced an increase of NPQ, but only Cd at the highest concentration determined a significant decline of F_v/F_m. These results indicate a different response of Z. mays to individual metals, with Pb triggering a compensative response and Cd inducing severe morpho-physiological alterations at all investigated levels. Therefore, Z. mays could be successfully exploited in phytoattenuation of Pb-polluted soil, but only at very low concentrations of Cd to avoid severe plant damages and biomass loss.

Cd, Pb, and Zn mobility and (bio)availability in contaminated soils from a former smelting site amended with biochar

Biochar is a potential candidate for the remediation of metal(loid)-contaminated soils. However, the mechanisms of contaminant-biochar retention and release depend on the amount of soil contaminants and physicochemical characteristics, as well as the durability of the biochar contaminant complex, which may be related to the pyrolysis process parameters. The objective of the present study was to evaluate, in a former contaminated smelting site, the impact of two doses of wood biochar (2 and 5% w/w) on metal immobilization and/or phytoavailability and their effectiveness in promoting plant growth in mesocosm experiments. Different soil mixtures were investigated. The main physicochemical parameters and the Cd, Pb, and Zn contents were determined in soil and in soil pore water. Additionally, the growth, dry weight, and metal concentrations were analyzed in the different dwarf bean plant (Phaseolus vulgaris L.) organs tested. Results showed that the addition of biochar at two doses (2 and 5%) improved soil conditions by increasing soil pH, electrical conductivity, and water holding capacity. Furthermore, the application of biochar (5%) to metal-contaminated soil reduced Cd, Pb, and Zn mobility and availability, and hence their accumulation in the different P. vulgaris L. organs. In conclusion, the data clearly demonstrated that biochar application can be effectively used for Cd, Pb, and Zn immobilization, thereby reducing their bioavailability and phytotoxicity.

Cadmium Tolerance of Perennial Ryegrass Induced by Aspergillus aculeatus

Cadmium (Cd) pollution is becoming increasingly prevalent, posing a global environmental hazard due to its negative effects on plants growth and human health. Phytoremediation is a green technology that involves uptake of Cd from the soil by a combination of plants and associated microbes. The objective of this study was to investigate the role of Aspergillus aculeatus in perennial ryegrass Cd tolerance. This fungus produced indole-3-acetic acid, siderophores, and 1-aminocyclopropane-1-carboxylate deaminase. Physiological traits including growth rate, turf quality and chlorophyll content were measured to evaluate the physiological responses of perennial ryegrass to Cd stress. These physiological traits were improved after inoculated with A. aculeatus. Inoculation of A. aculeatus actively reduced DTPA-Cd concentration in the soil and Cd translocation to plant shoots. Chlorophyll a fluorescence transient and the C/N ratio in shoots were elevated by A. aculeatus, which implied that the fungus could protect the photosystem II against Cd stress and increase the photosynthetic efficiency. These results suggested that A. aculeatus is beneficial in improving Cd tolerance of perennial ryegrass and reducing Cd-induced injuries, thus, it has promising potential for application of phytostabilization in Cd contaminated soil.

Aided phytostabilisation reduces metal toxicity, improves soil fertility and enhances microbial activity in Cu-rich mine tailings

(Aided) phytostabilisation has been proposed as a suitable technique to decrease the environmental risks associated with metal(loid)-enriched mine tailings. Field scale evaluations are needed for demonstrating their effectiveness in the medium- to long-term. A field trial was implemented in spring 2011 in Cu-rich mine tailings in the NW of Spain. The tailings were amended with composted municipal solid wastes and planted with Salix spp., Populus nigra L. or Agrostis capillaris L. cv. Highland. Plant growth, nutritive status and metal accumulation, and soil physico- and bio-chemical properties, were monitored over three years (four years for plant growth). The total bacterial community, α- and β-Proteobacteria, Actinobacteria and Streptomycetaceae were studied by DGGE of 16s rDNA fragments. Compost amendment improved soil properties such as pH, CEC and fertility, and decreased soil Cu availability, leading to the establishment of a healthy vegetation cover. Both compost-amendment and plant root activity stimulated soil enzyme activities and induced important shifts in the bacterial community structure over time. The woody plant, S. viminalis, and the grassy species, A. capillaris, showed the best results in terms of plant growth and biomass production. The beneficial effects of the phytostabilisation process were maintained at least three years after treatment.

Effect of Zn stresses on physiology, growth, Zn accumulation, and chlorophyll of Phyllostachys pubescen s

The effects of Zinc (Zn) on lipid peroxidation, antioxidative enzymes, growth, Zn accumulation, and leaf chlorophyll of Phyllostachys pubescens (Pradelle) Mazel ex J.Houz. were investigated in two greenhouse experiments. Hydroponics experiment with Zn application of 0, 20, 100, and 400 μM revealed that lower concentration of Zn in solution led to increased malondialdehyde (MDA) and proline contents but inhibited SOD activity in all treatments. P. pubescens had showed strong ability to accumulate Zn in stems and reached maximum level at 100 μM with 7.91-fold increase compared with control. In pot experiment, treatment with Zn ranged from 0, 200, 400, 800, 1,600, to 3,200 mg kg(-1). Application of 800 mg kg(-1) revealed 116, 24.6, and 28.3 times increase in Zn concentration of roots, stems, and leaves, respectively. Growth and chlorophyll contents of plants in pots were better promoted at 400 mg kg(-1) Zn, with 60.5 and 30.9 % enhanced roots and shoot compared with control. The bioaccumulation factor (BAF) was in the sequence of stem > roots > leaves. The translocation factor (TF) of stem was higher than leaves.

Olive mill waste biochar: a promising soil amendment for metal immobilization in contaminated soils

The potential use of biochar from olive mill waste for in situ remediation of metal contaminated soils was evaluated. Biochar was mixed with metal contaminated soil originating from the vicinity of an old zinc smelter. Soil-biochar mixtures were equilibrated for 30 and 90 days. At these time points, Ca(NO3)2 exchangeable metals were determined, and effects of the biochar amendment on soil toxicity were investigated using plants, bacteria, and earthworms. Bean (Phaseolus vulgaris) growth, metal content, antioxidative enzymes activities, and soluble protein contents were determined. Furthermore, effects on soil microbial communities (activity, diversity, richness) were examined using Biolog ECOplates. After 120 days of soil-biochar equilibration, effects on weight and reproduction of Eisenia foetida were evaluated. With increasing biochar application rate and equilibration period, Ca(NO3)2 exchangeable metals decreased, and growth of bean plants improved; leaf metal contents reduced, the activities of antioxidative stress enzymes decreased, and soluble protein contents increased. Soil microbial activity, richness, and diversity were augmented. Earthworm mortality lowered, and their growth and reproduction showed increasing trends.

Functional and structural parameters to assess the ecological status of a metal contaminated area in the tropics

Ecological parameters (soil invertebrates, microbial activity, and plant community) were assessed in a metal contaminated site in an abandoned lead smelter and non-contaminated reference sites, as part of an ecological risk assessment (ERA). Vegetation cover inside the smelter area was lower and presented a more homogenous species composition than outside. A more simplified and less abundant vegetation community within the smelter area also simplified the habitat conditions, which in addition to metal toxicity, impaired the soil microbial and faunal communities. A significant reduction in the feeding activity was observed within the smelter area. Also a significant change in community composition of surface dwelling invertebrates was observed at those sites when compared to sites outside the smelter area. Moreover, basal respiration, microbial biomass C, dehydrogenase and phosphatase activity also decreased in several of these points under the smelter area. As a result, a significant impairment of organic material decomposition in the most contaminated sites was observed. Metal contamination affected the ecological status of the site, leading to a risk for ecosystem functioning and provisioning of ecosystem services like organic matter decomposition and nutrient cycling, even 17 years after the end of smelting activities. Regarding the sensitivity of the ecological parameters assessed, most were able to distinguish sites within the smelter area boundaries from those outside. However, only bait lamina (feeding activity), basal respiration and microbial biomass carbon presented high capacity to distinguish the level of soil contamination, since they were significantly correlated with metal loadings, and thus are promising candidates to be integrated in the Ecological Line of Evidence of an ERA.

Differential responses of microsomal proteins and metabolites in two contrasting cadmium (Cd)-accumulating soybean cultivars under Cd stress

While there are significant genotypic differences in cadmium (Cd) uptake and distribution in soybean cultivars, little attention has been paid to the underlying molecular mechanisms. We adopted a comparative proteomic approach coupled with metabolite analysis to examine Cd uptake and translocation in two contrasting Cd-accumulating soybean cultivars, Enrei and Harosoy, which accumulate higher amount of Cd in the roots and aerial parts, respectively. Proteins extracted from the root microsomal fraction were evaluated by immunoblot analysis using different subcellular marker proteins. Analysis of control and Cd-exposed samples by two-dimensional gel electrophoresis coupled with mass spectrometry revealed a total of 13 and 11 differentially expressed proteins in the Enrei and Harosoy cultivars, respectively. Metabolome profiling identified a total of 32 metabolites, the expression of 18 of which was significantly altered in at least in one cultivar in response to Cd stress. Analysis of the combined proteomic and metabolomic results revealed that proteins and amino acids associate with Cd-chelating pathways are highly active in the Enrei cultivar. In addition, proteins associated with lignin biosynthesis are significantly upregulated in the Enrei cultivar under Cd stress. Our results indicate that in the Enrei cultivar, Cd-chelating agents may bind excess free Cd ion and that translocation of Cd from the roots to the aerial parts might be prevented by increased xylem lignification.

Antioxidant responses and gene expression in perennial ryegrass (Lolium perenne L.) under cadmium stress

Perennial ryegrass (Lolium perenne L.), widely used in temperate climates, is one of turf grasses that enrich cadmium (Cd). The objective of this study was to explore the physiological responses and candidate gene expression in perennial ryegrass to Cd stress. Grasses were subjected to three levels of 0, 0.2, and 0.5 mM Cd for 7 days in the greenhouse. The results indicated that soluble protein content was lower in the Cd-treated perennial ryegrass compared to untreated plants. The Cd-treated perennial ryegrass exhibited a greater level of malondialdehyde and activity of the peroxidase (POD), catalase, and superoxide dismutase (SOD) relative to the control. The Cd stress induced up-regulated expression of FeSOD, MnSOD, Chl Cu/ZnSOD, Cyt Cu/ZnSOD, APX, GPX, GR and POD at 4-24 h after treatment began for perennial ryegrass. Results suggested that the gene transcript profile was related to the enzyme activity under Cd stress.

Influence of fly ash aided phytostabilisation of Pb, Cd and Zn highly contaminated soils on Lolium perenne and Trifolium repens metal transfer and physiological stress

Due to anthropogenic activities, large extends of soils are highly contaminated by Metal Trace Element (MTE). Aided phytostabilisation aims to establish a vegetation cover in order to promote in situ immobilisation of trace elements by combining the use of metal-tolerant plants and inexpensive mineral or organic soil amendments. Eight years after Coal Fly Ash (CFA) soil amendment, MTE bioavailability and uptake by two plants, Lolium perenne and Trifolium repens, were evaluated, as some biological markers reflecting physiological stress. Results showed that the two plant species under study were suitable to reduce the mobility and the availability of these elements. Moreover, the plant growth was better on CFA amended MTE-contaminated soils, and the plant sensitivity to MTE-induced physiological stress, as studied through photosynthetic pigment contents and oxidative damage was lower or similar. In conclusion, these results supported the usefulness of aided phytostabilisation of MTE-highly contaminated soils.

Inhibition of root growth and polyamine metabolism in sunflower (Helianthus annuus) seedlings under cadmium and copper stress

Although sunflower is usually regarded as a highly tolerant crop, impairment of root growth at initial stages of plant development may result in poor crop establishment and higher susceptibility to pathogen attack. In order to evaluate if Cd2+ and Cu2+ may impact on sunflower germination and initial root development, a pot experiment under controlled conditions was carried out. Possible involvement of polyamine metabolism in sunflower response to these stressors was also investigated. Although Cd2+ and Cu2+ treatments affect neither seed germination nor radical emergence, sunflower seedlings grown in the presence of these heavy metals showed significant inhibition of root growth, being this inhibition greater for Cd2+. Both metals caused significant increases in proline contents at the highest concentrations tested (0.5 and 1 mM), and these increments were more pronounced for Cd2+ treatments, especially between days 3 and 10. Metals also increased putrescine (Put) contents at all concentrations assayed from the seventh day onward, causing no variations on this polyamine time-course pattern. Spermine and spermidine contents, however, were increased only by 1 mM Cd2+. Arginine decarboxylase seems to have been the enzyme responsible for Put increases under both metal treatments. This work demonstrates that initial root growth of sunflower seedlings may be significantly impaired in Cd2+ or Cu2+ contaminated soils. It also shows that polyamines are key biological compounds, which are probably involved in signaling pathways triggered under stress environmental conditions.

Phytostabilization of a metal contaminated sandy soil. I: Influence of compost and/or inorganic metal immobilizing soil amendments on phytotoxicity and plant availability of metals

In a lysimeter set-up, compost addition to an industrial contaminated soil slightly reduced phytotoxicity to bean seedlings. The "Phytotoxicity Index" (on a scale from 1 to 4) decreased from 3.5 to 2.8. The same treatment also reduced metal accumulation in grasses: mean Zn, Cd and Pb concentrations decreased respectively from 623 to 135, from 6.2 to 1.3 and from 10.7 to <6 mg kg-1 dry weight. When combined with inorganic metal immobilizing amendments, compost had a beneficial effect on plant responses additional to the inorganic amendments alone. Best results were obtained when using compost (C)+cyclonic ashes (CA)+steel shots (SS). The "Phytotoxicity Index" decreased to 1.7, highest diversity of spontaneously colonizing plants occurred, and metal accumulation in grasses reduced to values for uncontaminated soils. Based on the first year evaluation, C+CA+SS showed to be an efficient treatment for amendment assisted phytostabilization of the contaminated Overpelt soil.

Potential use of the plant antioxidant network for environmental exposure assessment of heavy metals in soils

In recent years, awareness has risen that the total soil content of pollutants by itself does not suffice to fully assess the potential ecotoxicological risks involved. Chemical analysis will require to be complemented with biological assays in a multidisciplinary approach towards site specific ecological risk assessment (SS-ERA). This paper evaluates the potential use of the plants' antioxidant response to metal-induced oxidative stress to provide a sensitive biological assay in SS-ERA. To this end, plants of Phaseolus vulgaris were grown for two weeks on 15 soils varying in contamination level. Morphological parameters and enzymatic plant responses were measured upon harvest. Foliar concentrations of the (heavy) metals Al, Cu, Cd, Cr, Fe, Mn, Ni, Pb, Zn were also determined. Metal mobility in the soil was further assessed by determining soil solution and NH4OAc extractable levels. In general more significant correlations were observed between plant responses and foliar metal concentrations or exchangeable/soluble levels than between plant responses and the total soil content. The study demonstrates the potential use of the plants' antioxidant defence mechanisms to assess substrate phytotoxicity for application in SS-ERA protocols. However, the system, based on calculation of a soil Phytotoxicity Index (PI), will require adaptation and fine-tuning to meet the specific needs for this type of environmental monitoring. Large variation was observed in phytotoxicity classification based on the various test parameters. The thresholds for classification of the various morphological and enzymatic response parameters may require adaptation according to parameter stress sensitivity in order to decrease the observed variation. The use of partial PI's (leaves and roots separately) may in addition increase the sensitivity of the system since some metals show specific effects in one of both organs only. Loss of biological functionality of enzymes, as was observed for ICDH in one of the more strongly contaminated soils, may also be recognized as an additional stress symptom when assigning phytotoxicity classification, whereas the current system only considers increasing enzymatic capacities. Other easily distinguishable parameters, which could be added to the current indexation are: failure to germinate and the incapacity to develop roots in the toxic substrate. Additional research will be required to determine the possible application range of soil properties for this biological assay and to further improve its performance in SS-ERA.

Root penetration of sealing layers made of fly ash and sewage sludge

Fly ash and sewage sludge are suggested materials for constructing sealing layers covering mine tailings impoundments. Little is known, however, of their effect on vegetation or resistance to root penetration. We investigate: (i) the ability of different plant species to grow in sealing layers comprising fly ash and sewage sludge, (ii) the impact on plant growth of freshly hardened fly ash compared to aged and leached ash, and (iii) the plant stress response to fly ashes of different properties. A 6-mo greenhouse study using birch (Betula pendula Roth.), Scots pine (Pinus sylvestris L.), Kentucky bluegrass (Poa pratensis L.), and willow (Salix viminalis L.) demonstrated that no roots could grow into a compacted layer consisting only of ash, while a 6:4, ash-sludge mixture admitted roots into the upper part and a 1:9, ash-sludge mixture was totally penetrated (to 15 cm in depth) by roots of willow and Scots pine. Freshly hardened ash prevented root growth more effectively than aged ash did, as was observed in tests using reed canarygrass (Phalaris arundinacea L.) and pea (Pisum sativum L.). Furthermore, extracts of highly alkaline ash were more toxic to pea in a 48-h toxicity test than less alkaline ash was. However, stress responses to diluted ash extracts of lower pH, measured as enzyme capacities in dwarf bean (Phaseolus vulgaris L.), were more related to the metal and ion contents. Root penetration of sealing layers is most effectively prevented if little sewage sludge is added, and if ash of high alkalinity is chosen.

Polyamine metabolism in sunflower plants under long-term cadmium or copper stress

The effect of different doses of cadmium and copper was studied in relation to growth and polyamine (Pas) metabolism in shoots of sunflower plants. Cadmium accumulated to higher levels than copper and shoot length was reduced by 0.5 and 1 mM Cd, but only by 1 mM Cu. At 1 mM of Cd or Cu, Put content increased 270% and 160% with Cd2+ and Cu2+, respectively. Spermidine (Spd) was modified only by 1 mM Cd, while spermine (Spm) declined after seeds germinated, increasing thereafter but only with 1 mM Cd or Cu (273% over the controls for Cd and 230% for Cu at day 16). Both ADC and ODC activities were increased by 1 mM Cd, whereas 1 mM Cu enhanced ADC activity, but reduced ODC activity at every concentration used. The role of Pas as markers of Cd or Cu toxicity is discussed.

Biosensors for detection of mercury in contaminated soils

Biosensors based on whole bacterial cells and on bacterial heavy metal binding protein were used to determine the mercury concentration in soil. The soil samples were collected in a vegetable garden accidentally contaminated with elemental mercury 25 years earlier. Bioavailable mercury was measured using different sensors: a protein-based biosensor, a whole bacterial cell based biosensor, and a plant sensor, i.e. morphological and biochemical responses in primary leaves and roots of bean seedlings grown in the mercury-contaminated soil. For comparison the total mercury concentration of the soil samples was determined by AAS. Whole bacterial cell and protein-based biosensors gave accurate responses proportional to the total amount of mercury in the soil samples. On the contrary, plant sensors were found to be less useful indicators of soil mercury contamination, as determined by plant biomass, mercury content of primary leaves and enzyme activities.

Response of stress indicators and growth parameters of Tibouchina pulchra Cogn. exposed to air and soil pollution near the industrial complex of Cubatão, Brazil

The present study was performed in the vicinity of the industrial complex of Cubatão, São Paulo, Brazil, in order to evaluate the response of 'manaca da serra' Tibouchina pulchra Cogn. (Melastomataceae), a common species of secondary Atlantic Rain Forest vegetation, to the impact of complex air pollution. Emphasis was given to changes of biochemical parameters such as ascorbic acid concentration, peroxidase activity, contents of water-soluble thiols, pH of leaf extract and buffering capacity. These plant factors are often used as early indicators of air pollution stress. Field experiments included sampling of leaves from mature trees in areas with different air pollution load (passive monitoring), exposure of saplings cultivated in uniform soil at these areas (active monitoring) and a study on the combined effects of contaminated soil and air pollution. In general, metabolic response of saplings was more accentuated than that of mature trees. Leaf extract pH and buffering capacity showed no or only small alterations in plants exposed to industrial emissions. In contrast, air pollution resulted in a distinct decrease in ascorbic acid contents and an increase in peroxidase activity and thiol concentrations in leaves. Cultivation of saplings in soil types from contaminated regions frequently caused the same modifications or enhanced the effects produced by air pollution. Growth analysis of exposed saplings demonstrated that a change of the relationship between above-ground and below-ground plant parts was the most obvious effect of air pollution and soil contamination. The experiments showed that even T. pulchra, a species considered resistant to air pollution, suffers metabolic disturbances by the present ambient air and soil quality. Although biochemical and physiological alterations were not related to a certain air pollution type, they could be used to estimate the overall pollution load and to map zones with different air quality.

Reclamation of a bare industrial area contaminated by non-ferrous metals: physico-chemical and biological evaluation of the durability of soil treatment and revegetation

In 1990, 3 ha of a highly metal polluted acid sandy soil at the site of a former pyrometallurgical zinc smelter was treated with a combination of beringite and compost; beringite is a substance that has a strong metal immobilization capacity. After soil treatment and sowing of a mixture of metal-tolerant Agrostis capillaris and Festuca rubra, a healthy vegetation cover developed. Five years later, an evaluation was made of soil physico-chemical parameters, potential phytotoxicity, floristic and fungal diversity and mycorrhizal infection of the plant community. Phytotoxicity was shown to be maintained at the low level observed immediately after soil treatment. The water-extractable metal fraction of the treated soil was up to 70 times lower compared to the non-treated soil. The vegetation was still healthy and regenerating by vegetative means and by seed. Diversity of higher plant species and saprophytic fungi was extremely low in the untreated area due to the high soil toxicity and the absence of metal tolerant ecotypes of plants and fungi. On the treated soil, in contrast, the species richness of higher plants was much higher; several perennial forbs which are not noted as metal tolerant had colonized the revegetated area. Most of these species belong to mycotrophic families so that the presence of a mycorrhizal network in the soil promotes their establishment. The ubiquity of the mycorrhizal fungi in the roots showed that a functioning ecosystem was establishing. In non-treated soil, the mycorrhizal infection rates of the roots were consistently lower during the whole growing season.

Reclamation of a bare industrial area contaminated by non-ferrous metals: in situ metal immobilization and revegetation

To reduce the environmental impact of a 135-ha bare industrial area with a highly phytotoxic soil contaminated by non-ferrous metals, a rehabilitation strategy was developed, aimed at the restoration of a vegetation cover. Two different techniques to overcome the high phytotoxicity of the soil were first evaluated on a laboratory scale: reduction of soil phytotoxicity by the addition of a powerful metal immobilizing substance and use of metal-tolerant plants. Since a combination of both approaches proved most promising, this strategy was subsequently utilized in a 3-ha field experiment on the most contaminated location of the industrial area. After soil treatment and sowing of seeds of metal-tolerant grasses, a complete and healthy vegetation cover was quickly established, even at the sites where metal concentrations were extremely high. The reduction of soil phytotoxicity by addition of beringite was immediate and was confirmed 15 months after the treatment. Thirty months after the reclamation activities, the vegetation is still healthy and vegetative and generative plant propagation is abundant.