The biotic ligand model for plants and metals: technical challenges for field application

To improve predictions of phytoavailable metal, the mechanistic bases of bioaccumulation and toxicity of metals to plants can be integrated into a biotic ligand model (BLM). There are a number of significant challenges to the application of the BLM to plants in soils, including reliable measurements of free ion concentrations for the metals of interest in rhizospheric soil solution, as well as other free ions, and concentrations of ligands to which the ions could bind; identification of the simplest model that can adequately predict root accumulation, and the potential for more complex models to add accuracy to the predictions; incorporating the dissociation of labile metal complexes (i.e., nonequilibrium processes) into a BLM, which is an equilibrium model; application of factors in a BLM that adequately describe translocation, in order to estimate metal concentration and speciation in plant shoots. The review concluded that the ability to estimate trace metal speciation in samples of soil solution are not likely to be better than within one order of magnitude of actual, thus this would be an additional source of uncertainty to the predictions of toxicity. Further, regulatory use of the BLM would require mechanistic bases; and, until root ligands associated with toxicity are well characterized, incorporating the ameliorative effects of competitive cations cannot be mechanistically based. As well, a functional BLM for soils with lower metal free ion activities will have to include kinetic data for metal-ligand complexes, as their association/disassociation may constitute a greater metal supply to roots than what would be predicted by the free ion concentration in soil solution. To apply the BLM to trophic transfer where metal concentration in plant shoots is the main focus, a probabilistic approach using experimentally determined root-shoot partitioning of metals might permit estimates of shoot accumulation from root data, to within one or two orders of magnitude.     

Effects of Calcium on the Uptake and Elimination of Cadmium and Zinc in Asiatic Clams

We examined the effects of Ca, one of the major ions contributing to water hardness, on the uptake and elimination of Cd and Zn in the Asiatic clam Corbicula fluminea, a common bivalve species in the rivers and streams of Southern China. Over the wide range of dissolved Cd (4.5 to 446 nM) and Zn (15.3 to 1529 nM) concentrations, uptake of both metals increased with increasing dissolved metal concentration, showing Michaelis-Menten–type saturation kinetics. At each tested metal concentration, increased Ca concentration led to a significantly lower metal uptake. When the Ca, Cd, Zn concentrations were expressed as free-ion activities, Cd and Zn uptake data fitted the Michaelis-Menten inhibition model well. We also quantified the effects of the two Ca blockers on metal uptake by the clams. Verapamil significantly inhibited the uptake Cd and Zn, but the inhibitive effect of lanthanum on metal uptake was more evident for Cd than for Zn. Ca did not significantly affect the assimilation efficiency of either metal from ingested phytoplankton, nor did it affect the elimination of the two metals during a 1-month depuration period. Our study has shown that Ca inhibited the uptake of metals from water; such an effect could be predicted based on the free-metal ion activities. Ca did not seem to have a direct effect on metal assimilation from food or metal elimination from the bivalve.     

Low metal bioavailability in a contaminated urban site

Bioavailability of Cd, Cu, Ni, Pb, and Zn in a metal-enriched railway yard in Montréal, Québec, Canada was assessed using metal speciation, plant uptake, and microbial assays. Metal speciation of extracted soil solutions was estimated using the Windmere Humic Aqueous Model. In soil solutions, free Cd, Ni, and Zn ions represented as much as 80%, 72%, and 62%, respectively, of the total dissolved metals. Copper and Pb were strongly bound by dissolved organic matter, and metal-fulvic acid complexes represented as much as 99% of the total dissolved metals. Three field-collected plant species (dandelion, bladder campion, and chicory) varied in their tendency to accumulate metals in either their leaves or roots. Chicory grown in the greenhouse had significantly higher metal bioconcentration factors than wild chicory. Although the site studied is considered to be contaminated, no metal pool, such as free ions or dissolved or total soil metals, consistently predicted metal uptake by potted chicory. Regression analysis revealed that soil total metal concentrations could adequately predict tissue accumulations of Cd, Ni, Pb, and Zn in bladder campion but only Cd and Zn in dandelion. Data from microbial assays also showed that the soil respiration was not affected by the metal contamination, but that nitrification was inhibited for the most contaminated soils. These results indicate that the metal bioavailability in the railway yard is low, but they also suggest that nitrogen cycling may be affected.     

Effects of humic acid and competing cations on metal uptake by Lolium perenne

Within the biotic ligand model, which describes relationships between chemical speciation and metal binding at an organism's surface, multicomponent (long-term) metal uptake by plants has seldom been studied. In the present work, we exposed perennial ryegrass to nutrient solutions with two levels of Cd, Cu, Ni, Pb, and Zn (1 and 0.1 microM) and with or without 30 mg/L of humic acid. Iron and Mn concentrations were constant over all treatments. The hypothesis tested was that humic acid lowers the free and labile metal concentration and, therefore, reduces the metal uptake and, finally, the metal content of the plant. The free metal ion concentrations in the nutrient solutions were measured by the Donnan membrane technique and labile metal concentrations by diffusive gradients in thin-films. The metal content of the shoots depends on the metal content of the roots. The metal content of the roots is a function of the adsorption of metals on the root surface. In a multicomponent system at metal concentrations of 1 microM, humic acid decreased Cu, Pb, and Fe adsorption at the root surface, but it increased Cd, Zn, and Mn adsorption at the root surface. Complexation of cations such as Cu, Pb, and Fe with high affinity for (dissolved) organic matter may lead to increased uptake of cations with low affinity for organic matter (Ni, Zn, and Cd) because of competition between cations at the root surface. The results suggest that competition between metal ions can play a major role in multicomponent metal uptake, which has to be taken into account during risk assessments of metal-polluted soils.     

Some fundamental (and often overlooked) considerations underlying the free ion activity and biotic ligand models

Trace metal bioavailability is often evaluated on the basis of steady-state models such as the free ion activity model (FIAM) and the biotic ligand model (BLM). Some of the assumptions underlying these models were verified by examining Pb and Zn uptake by the green microalga Chlorella kesslerii. Transporter bound metal ([M-Rcell]) and free ion concentrations ([M(Z+)]) were related to experimentally determined uptake fluxes (Jint). Although the BLM and FIAM correctly predicted Pb uptake in the absence of competing ions, they failed to predict competitive interactions with Ca2+, likely because of modifications of the algal surface charge and the active nature of Ca2+ transport. Zinc transport is also active; in this case, both the internalization rate constant (kint) and the equilibrium constant for the binding of Zn to the transport sites (K(Zn-Rcell)) varied as a function of [Zn2+] in the bulk solution. For this reason, Zn uptake could not be modeled by the steady-state models either in the presence or absence of competitors (Cd and Ca). Furthermore, the role of Cu on Pb and Zn adsorption and uptake could not be predicted by either model because of secondary effects on the algal physiology and membrane permeability. Finally, a 17 degree C reduction in temperature resulted in a two- to fivefold decrease in membrane permeability of the metals, an observation that also is unaccounted for by either the FIAM or BLM. This paper emphasizes the limitations of the models in well-controlled laboratory systems with the goal of extrapolating the results to complex environmental systems.     

Toxicity of metals to roots of cowpea in relation to their binding strength

Metal phytotoxicity is important in both environmental and agricultural systems. A solution culture study examined the toxicity of 26 metals to roots of cowpea (Vigna unguiculata (L.) Walp.); new data were collected for 15 metals and published data for 11 metals. Metal toxicity, calculated as causing a 50% reduction in root elongation rate, was determined based on either the measured concentration in the bulk solution (EC50(b)) or the calculated activity at the outer surface of the plasma membrane (EA50(0)°). The EC50(b) values ranged from 0.007?μM for Tl to 98,000?μM for K, with the order of rhizotoxicity to cowpea, from most to least toxic, being Tl?=?Ag?>?Cu?>?Hg?=?Ni?=?Ga?=?Ru?=?In?>?Sc?=?Cd?=?Gd?=?La?=?Co?=?Cs?=?Pb?>?Zn?=?Al?=?H?>?Mn?>?Ba?=?Sr?>?Li?>?Mg?>?Ca?=?Na?>?K. The EA50(0)° values suggest that the binding of metals to hard ligands is an important, general, nonspecific mechanism of toxicity, a hypothesis supported by the similar toxicity symptoms to roots of cowpea by many metals. However, additional mechanisms, such as strong binding to soft ligands, substantially increase rhizotoxicity of some metals, especially Tl, Ag, and Cs. Besides direct toxic effects, osmotic effects or reduced activity of Ca(2+) at the outer surface of the root plasma membrane (and resultant Ca deficiency) may decrease short-term root growth.     

Toxicity interactions of cadmium, copper, and lead on soil urease and dehydrogenase activity in relation to chemical speciation

The toxicity of the trace metals Cd, Cu, and Pb alone and in combination was assessed by measuring the activity of the soil enzymes dehydrogenase and urease. We assayed the enzymatic response of a forest soil exposed to single metals and metal mixtures in a factorial design. The chemical speciation of the metals was measured in 0.01M KNO3 solution extracts using an ion-selective electrode for Cu2+ and estimated from voltammetric determinations of labile metals for Cd2+ and Pb2+. The toxicity interaction of the metal mixtures was predicted according to the classical Bliss independence and Loewe additivity models. Significant antagonistic effects were observed for the majority of the combinations for both the dehydrogenase and the urease assays. Given the strong interaction upon the chemistry, the toxicity interaction data were analyzed based on resulting free metal activities to tease out the effect of chemistry changes from that of toxicological responses. As a result, a stimulation of enzymatic activities was observed in the mixtures in comparison to the enzymatic activities obtained with the individual metal.     

Effect of Cd or Pb Addition to Cu-Contaminated Soil on Tissue Cu Accumulation in the Earthworm,Dendrobaena veneta

Generally, soil heavy metal contamination consists of a mixture of heavy metals. Soil chemical properties and interaction with other pollutants in soil affect the external heavy metal bioavailability. Moreover, interaction with other pollutants accumulated in organisms may change the toxicity of each pollutant. Therefore, the hypotheses was tested that addition of Cd or Pb to Cu-contaminated soil would lead to an increase in tissue Cu accumulation in the earthworm,Dendrobaena veneta,caused by (i) induction of metallothionein by Cd, or (ii) an increase in Cu concentration in soil solution due to the exchange of adsorbed Cu for Pb. Tissue heavy metal concentrations were determined after exposure in contaminated soils for 3 or 21 days. Considerable amounts of Cu, Cd, and Pb were accumulated, indicating that these heavy metals were available for uptake byD. veneta.Both Cd and Pb, however, did not significantly affect tissue Cu accumulation.     

Heavy metals in wetland plants and soil of Lake Taihu, China

Properties of vertical distribution of soil near water bodies are vary considerably from those of land and may greatly affect the transportation of heavy metals in wetlands. Vertical distributions of heavy metals (Cr, Cd, Cu, Pb, and Zn) in the soil of aquatic-terrestrial ecotone (ATE) of Lake Taihu, China, and in wetland plant tissues were studied. Generally, concentrations of heavy metals decreased with increasing depth in the top 40-cm cores and then increased slightly with increasing depth. This investigation indicated that concentrations of Cd, Cr, Pb, and Zn exceeded the geochemical background values in the Taihu Lake area. Concentrations of Cd at all depth soil columns exceeded the Environment Quality Standard for Soils of China. Correlation analysis showed that concentrations of Zn, Cr, Cd, and Pb correlated significantly with one another, suggesting that they had the same origin. The concentration of Cu was negatively correlated with root biomass, which may explain the lower concentration of copper in the soil cores. The dominant plants of the wetland were Phragmites australis and Ludwigia prostrata, and heavy metal accumulated primarily in the root tissue. The general order was root > rhizome > stem > leaf, whereas in L. prostrata, leaf was the main tissue for Cr accumulation. Both P. australis and L. prostrata had the highest concentration factor (CF) to Cu, and CF was 20.3 and 15.8, respectively. Aquatic-terrestrial ecotone plants are more effective in controlling Cu pollution than other heavy metals. This will be very significant for ATE reestablishment near Cu-polluted sites.     

Influence of essential elements on cadmium uptake and toxicity in a unicellular green alga: the protective effect of trace zinc and cobalt concentrations

Within the biotic ligand model (BLM) construct, major cations are considered to be simple competitors for metal binding to uptake sites and may offer some protection against metal-induced toxicity, but the influence of essential trace elements and cell preconditioning to different micronutrient concentrations on metal uptake and toxicity is considered negligible. To test these underlying assumptions, we monitored Cd uptake and toxicity in a green alga (Chlamydomonas reinhardtii) after long-term exposures (60 h) to a range of environmentally realistic free Zn(2+) , Co(2+) , Fe(3+) , Mn(2+) , Ca(2+) , and Cu(2+) concentrations buffered with nitrilotriacetic acid. A 200-fold increase in free [Mn(2+) ] as well as a 100-fold increase in free [Fe(3+) ] did not affect Cd uptake or toxicity, whereas a 50-fold increase in free [Ca(2+) ] effectively offered some protection, as predicted by the BLM. However, a 10-fold increase in free [Cu(2+) ] significantly enhanced Cd toxicity by a factor of approximately 2, whereas a 100-fold increase in free [Zn(2+) ] and [Co(2+) ] from 10(-11) to 10(-9) M significantly decreased Cd uptake and toxicity by more than twofold. These effects did not change with prior algal acclimation to different essential micronutrient concentrations. Low essential trace metal concentrations may strongly affect the uptake and toxicity of Cd in freshwater algae and should be taken into consideration in future developments of the BLM.     

Accumulation of cadmium by durum wheat roots: bases for citrate-mediated exceptions to the free ion model

The accumulation of Cd in durum wheat (Triticum turgidum) roots from hydroponic solutions, with the proportion of total Cd (8.9-445 nM Cd) as Cd2+ varied by the addition of citrate, was determined to test the free-ion model (FIM) of metal bioavailability for higher plants. Calcium, Mg, and K were also varied. Citrate enhanced root-Cd accumulation at higher Cd2+ concentrations but not lower relative to the same Cd2+ concentrations in solutions containing 0 mM citrate. Elevating Ca2+ and Mg2+ concentrations in the citrate solution to the same as those in control solutions alleviated some of the citrate-mediated enhancement but not all. Solutions containing 66% less Ca or Mg than control but the same Cd2+ concentration and no citrate also resulted in increased root Cd. Elevated K+ did not influence Cd accumulation. Regression relationships between root-Cd accumulation and total Cd in solution were similar for the control and pooled amended solutions, whereas they were different for root-Cd accumulation and solution Cd2+. These results contribute to the growing body of evidence that the FIM alone is likely insufficient to predict plant accumulation of metals from soils, although it may be a useful probe for the mechanistic bases of metal bioavailability.     

Impact of metal pools and soil properties on metal accumulation in Folsomia candida (Collembola)

Soil-dwelling organisms are exposed to metals in different ways. Evidence exists for predominant pore water uptake of metals by soft-bodied oligochaete species. In the present research, uptake kinetics of metals and the metalloid As by the semi-soft-bodied springtail Folsomia candida were studied, for which uptake via the pore water is less obvious. Springtails were exposed in 16 field soils and in metal-spiked artificial Organization for Economic Cooperation and Development (Paris, France) soil (OECD soil). Subsequently, accumulation parameters were statistically related to soil metal pools and soil properties. In Cd-spiked OECD soil, internal Cd levels were linearly related to external Cd concentrations, whereas the springtails maintained fixed internal levels of Cu and Zn regardless of spiked concentrations. In the field soils, all body concentrations of the elements As, Cr, and Ni were below detection limit. The essential metals Cu and Zn were presumably regulated, and no influence of soil characteristics could be demonstrated. For Cd and Pb, accumulation patterns were correlated mainly to solid-phase soil characteristics. The presence of these explanatory variables in the multiple correlations suggests that an uptake mechanism that is solely determined by pore water concentrations should not be taken as a universally applicable principle in risk assessments of metals for soil invertebrates. Cadmium in OECD soils was more available for uptake than in the field soils. The difference remained when extractability was taken into account. The results suggest that experiments in OECD soil cannot be used directly in risk assessment for nonessential metals (at least for F. candida), although a reduction of uncertainties in metal risk assessment can be reached by consistent use of body residues rather than external concentrations.     

Comparison of soil solution speciation and diffusive gradients in thin-films measurement as an indicator of copper bioavailability to plants

The toxicity effect concentrations (10% effective concentration [EC10] and 50% effective concentration [EC50]) of total added Cu derived from barley root elongation and tomato growth assays varied widely among 18 European soils. We investigated whether this variation could be explained by the solubility or speciation of Cu in soil solutions or the diffusive gradients in thin-films (DGT) measurement. Solubility and Cu speciation varied greatly among the soils tested. However, the EC10 and EC50 of soil solution Cu or free Cu2+ activity varied even more widely than those based on the total added Cu, indicating that solubility or soil solution speciation alone could not explain intersoil variation in Cu toxicity. Estimated EC10 and EC50 of free Cu2+ activity correlated closely and negatively with soil pH, indicating a protective effect of H+, which is consistent with the biotic ligand model concept. The DGT measurement was found to narrow the intersoil variation in EC50 considerably and to be a better predictor of plant Cu concentrations than either soil solution Cu or free Cu2+ activity. We conclude that plant bioavailability of Cu in soil depends on Cu speciation, interactions with protective ions (particularly H+), and the resupply from the solid phase, and we conclude that the DGT measurement provides a useful indicator of Cu bioavailability in soil.     

Effect of dissolved organic matter source on acute copper toxicity to Daphnia magna

The protective effect of dissolved organic matter (DOM) on metal toxicity to aquatic organisms has been reported by numerous authors. Bioavailability models such as the biotic ligand model (BLM) thus account for this factor to predict metal toxicity to aquatic organisms. Until now, however, few attempts have been made to assess the effect of the DOM source on metal speciation and toxicity and, accordingly, on BLM predictions. The aims of this study were to investigate to what extent DOMs differ in their ability to decrease acute copper toxicity to the cladoceran Daphnia magna and to evaluate if ultraviolet (UV) absorbance measurements may be a simple and effective method to incorporate DOM variability into the acute Cu-BLM for D. magna. Acute toxicity tests were carried out in artificial test water enriched with DOMs isolated from six locations in Europe and North America and in seven natural European surface waters. The acute Cu-BLM for D. magna was then used to estimate the copper complexing capacity of each DOM (expressed as % active fulvic acid, %AFA). A factor of 6 difference was observed between the lowest and the highest copper complexing capacity. A significant linear relationship was observed between the UV-absorbance coefficient at 350 nm (epsilon350) and the %AFA. Linking this relationship to the acute Cu-BLM resulted in a significant improvement of the predictive capacity of this BLM. Without accounting for this relationship, 90% of the predicted 48-h 50% effective concentrations (EC50) were within a factor of 2 of the observed EC50s; taking this relationship into account, 90% of the EC50s were predicted with an error of less than factor 1.3. The present study and other studies seem to indicate that UV absorbance may be a good measure of biologically and toxicologically relevant differences in copper binding behavior of DOM.     

Metals in alcoholic beverages: A review of sources, effects, concentrations, removal, speciation, and analysis

A critical review is offered concerning the different sources, effects, concentrations, removal methods, speciation, and analysis of metals (e.g., Al, Ca, Cd, Cu, Fe, Pb, Mg, Mn, Ni, K, Na, and Zn) present in a wide variety of alcoholic beverages.     

Nondestructive pollution exposure assessment in the European hedgehog (Erinaceus europaeus): I. Relationships between concentrations of metals and arsenic in hair, spines, and soil

Conventional metal exposure assessment in terrestrial mammals is generally based on organ analyses of sacrificed animals. Few studies on mammals use nondestructive methodologies despite the growing ethical concern over the use of destructive sampling. Nondestructive methods involve minimal stress to populations and permit successive biomonitoring of the same populations and individuals. In the present study we assessed metal exposure of hedgehogs (Erinaceus europaeus) by investigating relationships between concentrations of metals (Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) and As in soil samples and in hair and spines of hedgehogs. Samples were collected in seven study sites along a metal pollution gradient, characterized by decreasing total soil Ag, As, Cd, Cu, Ni, and Pb concentrations with increasing distance from a nonferrous metallurgic factory. For a number of elements, soil contamination was related both to distance to the smelter and to habitat. Soil concentrations were positively related to levels in hair and spines for Ag, As, Cd, and Pb and thus to hedgehog exposure. Metal concentrations in soil did not relate to metal concentrations in hair and spines for essential elements (e.g., Cu, Fe, Mn, Ni, and Zn), except Co in hair and soil. Our results demonstrate that, at least for nonessential elements, concentrations in soils can be used to predict contamination of these elements in hedgehogs or vice versa. Furthermore, hedgehog exposure increased toward the smelter and was higher for hedgehogs foraging in grasslands than for animals foraging in the forest. Moreover, we believe that hair and spines are promising tools in terrestrial wildlife exposure assessment studies of metals and As.     

Effects of Soil Properties and Land Use Types on the Bioaccessibility of Cd,Pb, Cr,and Cu in Dongguan City,China

In order to determine the potential heavy metal contamination in soil across Dongguan City, 124 soil samples from seven land use types were collected, four heavy metals (Cd, Pb, Cr, and Cu) were analyzed. Total Cd, Cr, and Cu contents were significantly higher than the background values for Guangdong Province. Lead bioaccessibility in urban green land was lower than that in industrial and abandoned districts. The bioaccessibility of heavy metals was affected by total metal concentrations, soil properties, and land use types. The results showed that there was a negative correlation between the bioaccessibility of heavy metals (except for Cu) and their total concentrations. Soil pH and organic matter were the main factors affecting the bioaccessibility of Cd, Cr, Pb, and Cu in most land use types. Furthermore, sand, P, and clay also affected Pb, Cr, and Cu bioaccessibility. With the exception of the industrial zone periphery and urban green land, the bioaccessibility of heavy metals was mainly affected by clay.

     

Metal uptake by Lolium perenne in contaminated soils using a four-step approach

Most research dealing with soil (solution) speciation and metal uptake by plants has focused on the relationships between a certain bioavailable fraction in the soil and metal uptake by aboveground parts of the plants. Here, a new approach to interpretation of metal uptake is presented that considers four steps: First, the metal concentration in the soil solution is related to the total metal content of the soil. Second, the metal adsorption to the root surface is related to the metal concentration in the soil solution. Third, the metal content in the roots is related to the adsorption of metal ions to the root surface. Fourth, the metal content in the shoots is related to the metal content in the roots. For grass grown on 10 different soils, it is shown that the metal adsorption to the root surface is pH-dependently related to the free or total metal concentration in the soil solution. The metal content in the roots depends linearly on the metal adsorption at the root surface, whereas the metal content in the shoots depends on the metal content in the roots, either linearly (Zn) or reaching a maximum (Cu, Pb, and Cd). For the Ni content in the shoots as a function of the root content, the relation is pH dependent, probably because of the competition effects of Ca. The pH of the soil has to be taken into account when CaCl2 extractions are used as a basis for risk assessment toward plants.     

Extended biotic ligand model for prediction of mixture toxicity of Cd and Pb using single metal toxicity data

The combined toxic effects of Cd and Pb in the presence of different concentrations of Ca(2+) were predicted using the biotic ligand model (BLM), with the parameters derived from the Cd-only and Pb-only toxicity data. The BLM-based toxic unit (TU) approach and the proposed BLM-based f(mix) approach were used for prediction. The predicted mixture toxic effects using the BLM-based f(mix) approach were closer to the observed mixture effects (root mean square error [RMSE]?= 9.7 at 25 mM Ca(2+) ) than that using the BLM-based TU approach (RMSE?= 25.6 at 25 mM Ca(2+) ). This can be attributed to the fact that the BLM-based TU approach only considers the competition between Ca(2+) and Cd or Pb, whereas the BLM-based f(mix) approach considers both the competition between Cd and Pb and the competition between Ca(2+) and Cd or Pb for biotic ligand sites. In addition, the parameters derived from the Cd-only and Pb-only solutions in the presence of the background Ca(2+) concentration (0.025 mM) could be used to predict the mixture toxic effects at higher Ca(2+) concentrations. The BLM-based f(mix) approach proposed in the current study suggests a new way of predicting the Cd-Pb mixture toxicity from the single metal toxicity data by adopting the f(mix) values.