Stability of Ca2+-, Cd2+-, Cu2+-[illite-humic] complexes and pH influence

Decomposition of fresh plant residues in soil is expected to produce humic fractions varying in molecular size. It was hypothesized that metal adsorption by soil, to some degree, will depend on humic acid content and molecular size. The latter is expected to vary in number and type of functional groups. In this study, illite-humic complexes were used to evaluate Ca²⁺, Cd²⁺, and Cu²⁺ adsorption and how this adsorption was affected by humic acids, differing in molecular size, under various pH values. Potentiometric titration using ion-selective electrodes with a stop-and-go procedure was employed to evaluate metal-[illite-humic] complex formation. The results showed that illite-humic complexes exhibited at least two types of metal-ion adsorption sites (low and high affinity) and molecular size of humic fractions had a large potential influence on total metal adsorption but a relatively smaller influence on metal-complex stability. Relative strength of metal-ion-[illite-humic] complexes followed the order of Cu²⁺>Cd²⁺>Ca²⁺ and were affected by pH, especially for low metal-ion affinity sites. Magnitude of metal-[illite-humic] stability constants, depending on molecular size of humic fraction and pH, varied on a log-scale from 3.52 to 4.21 for Ca²⁺, 4.38 to 5.18 for Cd²⁺and from 5.23 to 5.83 for Cu²⁺. There was an approximate 5-fold difference in these stability constants between the three different sizes of humic fractions. The larger the humic fraction, the lower the metal-[illite-humic] stability constant. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

UPTAKE OF CADMIUM BY FRESHWATER GREEN ALGAE: EFFECTS OF PH AND AQUATIC HUMIC SUBSTANCES1

The effects of humic substances and low pH on short‐term Cd uptake by Pseudokirchneriella subcapitata (Korshikov) Hindak and Chlamydomonas reinhardtii Dang were investigated under defined exposure conditions. The uptake experiments were run in the presence of either a synthetic organic ligand (nitrilotriacetate) or natural organic ligands (Suwannee River fulvic or humic acid). An ion‐exchange method was used to measure the free Cd²⁺ concentrations in the exposure solutions. At pH 5, measured free Cd²⁺ concentrations agreed with estimations made using the geochemical equilibrium model WHAM, but at pH 7 the model overestimated complexation by both Suwannee River fulvic and humic acids compared with the ion‐exchange measurements. Consistent with the metal internalization step being rate limiting for overall short‐term uptake, intracellular Cd uptake was linear for exposure times less than 20 min at pH 5 or pH 7 for both algal species. After taking into account complexation of Cd in solution, Suwannee River humic substances had no additional effects on cadmium uptake at pH 7, as would be predicted by the free ion model. This absence of effects other than complexation persisted at pH 5, where the tendency of humic substances to adsorb to the algal cell surface is favored. Changes in pH strongly influenced Cd uptake, with the intracellular flux of Cd being at least 20 times lower at pH 5 than at pH 7 for P. subcapitata. Our results support models such as the free ion model or the biotic ligand model, in which humic substances act indirectly on Cd uptake by reducing the bioavailability of Cd by complexation in solution.     

Effects of Metal Ions on the Conformation and Activity of Acutolysin D from Agkistrodon Acutus Venom

Acutolysin D, isolated from the venom of Agkistrodon acutus, possesses marked haemorrhagic and proteolytic activities. The molecular weight and the absorption coefficients (A ^1% ₂₈₀) of acutolyisn D have been determined to be 47,850 ± 8 amu and 9.3 by mass spectrometer and UV spectrum, respectively. The effects of metal ions on the conformation and activity of acutolysin D have been studied by following fluorescence, circular dichroism and biological activity measurements. Acutolysin D contains two Ca²⁺-binding sites and two Zn²⁺-binding sites determined by atomic absorption spectrophotometer. Zn²⁺ is essential for the enzyme activities of acutolysin D, however, the presence of 1 mM Zn²⁺ significantly decreases its caseinolytic activity and intrinsic fluorescence intensity at pH 9.0 due to Zn(OH)₂ precipitate formation. Ca²⁺ is important for the structural integrity of acutolysin D, and the presence of 1 mM Ca²⁺ markedly enhances its caseinolytic activity. Interestingly, the caseinolytic activity which is inhibited partly by Cu²⁺, Co²⁺, Mn²⁺ or Tb³⁺ and inhibited completely by Cd²⁺, is enhanced by Mg²⁺. The fluorescence intensity of the protein decreases in the presence of Cu²⁺, Co²⁺, Cd²⁺ or Mn²⁺, but neither for Ca²⁺, Mg²⁺ nor for Tb³⁺. Zn²⁺, Ca²⁺, Mg²⁺, Cu²⁺, Mn²⁺, Co²⁺ and Tb³⁺ have slight effects on its secondary structure contents. In addition, Cd²⁺ causes a marked increase of antiparallel β-sheet content from 45.5% to 60.2%.     

Properties of poultry litter humic acid fractions and their metal-complexes

Summary Molecularly homogenous fractions of humic acid extracted from poultry litter were characterized by elemental and functional group analysis, molecular weight determination, U.V. and infrared spectroscopy. The divalent and trivalent metal complexes prepared from different fractions of humic acid were characterized by infrared spectroscopy. The molecular weight of molecularly homogenous fractions of poultry litter humic acid ranged from 2545 to 40219. High amounts of functional groups in low molecular weight of humic acid fraction has been indicated by infrared spectra and by chemical analysis. The presence of chromophores C=C and C=O and auxochromes C−OH, C−NH were indicated by infrared and U.V. spectra of these humic acid fractions. Stable complex formation of Fe³⁺, Cu²⁺, and Zn²⁺ with −OH, −NH₂ and −COOH ligands of humic acid fractions involved electrovalent and coordinate-covalent bonds. Intensity of absorption bands of molecularly homogenous fractions of humic acid in I.R. spectra is differing depending upon the functional groups content of humic acid fractions. Journal paper No. 5. Department of Soil Science, R.A.U., T.C.A., Pusa-Dholi Campus, Dholi-843121, Muzaffarpur, Bihar, India.     

A study on natural and synthetic humic acids and their complexing ability towards cadmium

The natural and synthetic humic acids were characterised by potentiometric titrations, viscosity and surface tension measurements, as well as visible spectometry The results have been correlated with coiling-decoiling behaviour and aliphatic–aromatic balance of these acids. The stability constant of complexes formed by these humic acids with Cd²⁺ ions in aqueous phase was evaluated by the ion-exchange method. Results tend to suggest that humic phenolic –OH group was involved in the formation of Cd²⁺–humic complex, leading to it the given stability in a manner as for the analogous metal–oxine complexation. The hydrophobic moiety of the synthetic humic acid may also provide a cage-type conformation around Cd²⁺ ion, imparting to the Cd²⁺–humic complex the desired stability.     

Trace metals complexation behavior with root exudates induced by salinity from a mangrove plant Avicennia marina (Forsk.) Vierh

This study investigated the characteristics of exudates from mangrove plant Avicennia marina seedling roots under 0, 200 and 600?mM NaCl treatments and their complexation behavior with trace metals using excitation emission matrix (EEM) fluorescence spectrometry. Two fulvic-like fluorescence peaks, namely peak A (Em = 440?nm, Ex = 250?nm, UV fulvic-like compounds) and peak B (Em = 440?nm, Ex = 340?nm, visible fulvic-like compounds) were identified. The fluorescence intensities of peak A and peak B were enhanced by increasing salinity. Furthermore, the fluorescence of both peaks could be quenched by the ions of copper (Cu²⁺), manganese (Mn²⁺) and cadmium (Cd²⁺). Conditional stability constant (logK_a) exhibited that binding capacity of both peak A and peak B with trace metals are Cu²⁺?>?Mn²⁺?>?Cd²⁺ in the range from 2.21 to 4.01. Besides, Hill coefficient (n) >1 for Cu²⁺ but n?<?1 for Mn²⁺ and Cd²⁺. The results of high n and high logK_a for Cu²⁺ rather than Mn²⁺ and Cd²⁺ indicate that the fulvic-like compounds in root exudates of A. marina have maximum potential for Cu²⁺ complexation compared to Mn²⁺ and Cd²⁺, suggesting the fulvic acids in root exudates of A. marina have strong complexation with Cu²⁺ rather than Mn²⁺ and Cd²⁺.     

Heavy metal interference with growth hormone signalling in trout hepatoma cells RTH-149

We have studied the effects of heavy metals (Hg²⁺, Cu²⁺, Cd²⁺) on growth hormone (GH) activation of tyrosine kinase and Ca²⁺ signaling in the trout (Oncorhynchus mykiss) hepatoma cell line RTH-149. Molecular cloning techniques using primer designed on Oncorhynchus spp. growth hormone receptor (GHR) genes allowed to isolate a highly homologous cDNA fragment from RTH-149 mRNA. Thereafter, cells were analysed by Western blotting or, alternatively, with Ca²⁺ imaging using fura-2/AM. Exposure of cells to ovine GH alone produced a stimulation of the JAK2/STAT5 pathway and intracellular free Ca²⁺ variations similar to what has been observed in mammalian models. Cell pre-exposure to Cu²⁺, Hg²⁺ or Cd²⁺ affected cell response to GH by enhancing (Cu²⁺) or inhibiting (Cd²⁺) the phosphorylation of JAK2 and STAT5. Heavy metals induced the activation of the MAP kinase p38, and pre-exposure to Hg²⁺ or Cu²⁺ followed by GH enhanced the effect of metal alone. Image analysis of fura2-loaded cells indicated that pre-treatment with Hg²⁺ prior to GH produced a considerable increase of the [Ca²⁺]_i variation produced by either element, while using Cu²⁺ or Cd²⁺ the result was similar but much weaker. Data suggest that heavy metals interfere with GH as follows: Hg²⁺ is nearly ineffective on JAK/STAT and strongly synergistic on Ca²⁺ signaling; Cu²⁺ is activatory on JAK/STAT and slightly activatory on Ca²⁺; Cd²⁺ is strongly inhibitory on JAK/STAT and slightly activatory on Ca²⁺; heavy metals could partially activate STAT via p38 independently from GH interaction.Published online: March 2005     

Effects of metal ions on activity of plasmin

The effects of some metal ions on amidolytic and fibrinogenolytic activities of highly purified human plasmin were investigated in vitro. In the presence of Zn²⁺, Cu²⁺, Cd²⁺, and Au⁺ in the incubation mixture at the concentrations of 1×10⁻⁵−1×10⁻³ M, the anidolytic plasmin activity was strongly inhibited, whereas Ca²⁺ and Mg²⁺ at the same concentrations were not effective. The analysis of the kinetic study has shown that Zn²⁺ or Cu²⁺ acts as mixed-type inhibitors of plasmin activity. The inhibition of amidolytic plasmin activity by Zn²⁺ and Cu²⁺ was reduced in the presence of EDTA, histidine, or albumin. Incubation of plasmin with Zn²⁺ or Cu²⁺ (at the concentration of 5×10⁻⁴ M) resulted in complete loss of its proteolytic action on fibrinogen, whereas Cd²⁺ and Au⁺ under the same conditions only partially inhibited this process.     

Cation loss during accumulation of heavy metal cations by Saccharomyces cerevisiae

Summary Cu²⁺ accumulation byS. cerevisiae resulted in rapid release of 70% of cellular K⁺, followed by a slower release of approximately 60% of cellular Mg²⁺, but little loss of Ca²⁺. Co²⁺ was accumulated in smaller quantities and caused a smaller loss of physiological cations than either Cu²⁺ or Cd²⁺. Mg²⁺ release during copper accumulation was maximal at pH 6. Mg²⁺ release during Cu²⁺ accumulation increased with temperature and salinity of the suspension.     

Bacterial Degradation of EDTA

Degradation of EDTA (ethylenediaminetetraacetic acid) or metal–EDTA complexes by cell suspensions of the bacterial strain DSM 9103 was studied. The activity of EDTA degradation was the highest in the phase of active cell growth and decreased considerably in the stationary phase, after substrate depletion in the medium. Exponential-phase cells were incubated in HEPES buffer (pH 7.0) with 1 mM of uncomplexed EDTA or EDTA complexes with Mg²⁺, Ca²⁺, Mn²⁺, Pb²⁺, Co²⁺, Cd²⁺, Zn²⁺, Cu²⁺, or Fe³⁺. The metal–EDTA complexes (Me–EDTA) studied could be divided into three groups according to their degradability. EDTA complexes with stability constants K below 10¹⁶ (log K < 16), such as Mg–EDTA, Ca–EDTA, and Mn–EDTA, as well as uncomplexed EDTA, were degraded by the cell suspensions at a constant rate to completion within 5–10 h of incubation. Me–EDTA complexes with log K above 16 (Zn–EDTA, Co–EDTA, Pb–EDTA, and Cu–EDTA) were not completely degraded during a 24-h incubation, which was possibly due to the toxic effect of the metal ions released. No degradation of Cd–EDTA or Fe(III)–EDTA by cell suspensions of strain DSM 9103 was observed under the conditions studied.     

Contributions of cell wall and metal-binding peptide to Cd- and Cu-tolerances in suspension-cultured cells of tomato

Possible roles of cell wall and cytoplasmic peptides in the tolerance of cells to Cu²⁺ and Cd²⁺ ions were studied in suspension-cultured cells of tomato (Lycopersicon esculentum L. cv. Palace). Cu²⁺ and Cd²⁺ ions inhibited growth of wild type cells at concentrations more than 100 and 200 μM, respectively. Tomato cells readily developed tolerance to Cd²⁺ ions up to 1 mM but not to Cu²⁺ ions, after repeated subculturings in the presence of the respective ions. Such a metal-specific adaptation of cells was not due to the difference in the total uptakes between Cd²⁺ and Cu²⁺ ions by cells. Wild-type cells accumulated Cd²⁺ preferentially into the cytoplasmic peptide fraction and Cu²⁺ into the cell-wall fraction, when grown under the subtoxic metal conditions. Under excess metal conditions, Cd-tolerant cells produced greater amounts of Cd-binding peptides in the cytoplasm and retained lesser amounts of Cd²⁺ ions in the cell wall than did wild-type cells. In contrast, tomato cells grown in the presence of Cu²⁺ ions synthesized no detectable amounts of Cu-binding peptides in the cytoplasm and retained most of the Cu²⁺ in the cell-wall fraction, irrespective of cell lines. These results suggested that the cytoplasmic peptides rather than cell wall properties have a primary role in the response of tomato cells to excess metal environments.     

Effect of Ca2+ on the detoxification of Cd2+ byScenedesmus obliquus cells at low or high temperature

Scenedesmus obliquus was incubated with Cd²⁺ in the presence or absence of calcium at low (10°C) or high (40°C) temperature. The Cd²⁺ uptake was affected not only by Ca²⁺ but also by temperature. Growth rate was inhibited by Cd²⁺ especially at low temperature. In all Ca²⁺-containing cultures,S. obliquus exhibited higher rates of growth, dry matter and pigment fractions than in those containing Cd²⁺ alone. Proteins exhibited a similar response. Ca²⁺ in the presence of Cd²⁺ was most efficient where protein contents were mostly doubled. On the other hand Ca²⁺ reduced the solute leakage by the test alga at 10 and 40°C.     

重金属铜、锌、镉复合胁迫对麻疯树幼苗生理生化的影响

该研究以Cu~(2+)、Zn~(2+)、Cd~(2+)单一胁迫为对照,探讨不同浓度的Cu~(2+)、Zn~(2+)、Cd~(2+)复合胁迫对麻疯树幼苗生理生化指标的影响。结果表明:随着Cu~(2+)、Zn~(2+)、Cd~(2+)浓度的增加,麻疯树幼苗叶片中的蛋白质(Pro)、丙二醛(MDA)含量均逐渐增加,其叶片叶绿素含量随着Zn~(2+)胁迫浓度的增加呈现出先降后升的趋势,在中等浓度(100 mg·L-1)的Zn~(2+)胁迫时含量最低、随着Cu~(2+)胁迫浓度的增加叶绿素含量先升高后降低,在Cu~(2+)浓度为200 mg·L-1时含量最高,达到1 200 mg·g-1FW; Cd~(2+)胁迫对叶绿素含量和根系活力无明显影响。根系活力在Zn~(2+)浓度为100 mg·L~(-1)时最强,随着Cu~(2+)浓度的增加而减弱。低浓度的Cu~(2+)、Zn~(2+)、Cd~(2+)对过氧化物酶活性和可溶性糖含量都具有促进作用。Cu~(2+)、Zn~(2+)、Cd~(2+)复合胁迫时对可溶性蛋白、叶绿素和丙二醛含量均无明显影响,随着复合胁迫时浓度的增加,可溶性糖含量和根系活力先增后减。这表明麻疯树对三种重金属的胁迫具有一定的抗性,过高浓度的胁迫会影响麻疯树幼苗生理生化的一些指标,但是麻疯树可以通过自身的防御系统使伤害降到最小。此外,重金属复合胁迫可以在一定程度上减轻单一胁迫对麻疯树幼苗造成的毒害作用。     

Contribution of carbohydrates to the cation-exchange selectivity of aquatic humus from peat-bog water

A sample of soluble humic acid from peat-bog water was a glycoconjugate containing 46% of a glycuronoglycan moiety and 54% of a dark-brown chromophore. These accounted for 37% and 63%, respectively, of the titratable acidity of the polymer. Cation-exchange capacities, and cationic selectivity coefficients relative to magnesium ions (K_Mg^Me), were measured on the humic acid for Pb²⁺, Cu²⁺, Zn²⁺, Ba²⁺, Ca²⁺, and Sr²⁺, and compared with those of extractive-free Sphagnum and other mosses, their chlorite holocelluloses, and two soluble fragments of Sphagnum holocellulose, prepared by acidic and alkaline degradation, respectively. The humic acid showed considerably higher K_Mg^Me values than most of the control materials, the enhancement being especially marked for Pb²⁺, Cu²⁺, and Ca²⁺. Scatchard plots showed that both parts of the glycoconjugate contributed to its selectivity, and that the selectivity of the carbohydrate part was greater in the humic acid than in the holocellulose or its soluble fragments. The results are explained by assuming that there are enhanced possibilities for cross-linking in the colloidal humic-acid complexes.     

Removal of copper using marine brown alga, Undaria Pinnatifida modified with oxime

Summary As oxime is selective for Cu²⁺, oxime groups were introduced to the cell wall of alga by glutaraldehyde. Such modified biomass showed high affinity for Cu²⁺, which resulted in the increase of copper sorption capacity about 4.5 times higher than that of natural alga. For pH range from 2.5 to 3.0, only Cu²⁺ were removed by alga biomass modified with oxime, while other heavy metal ions such as Ca²⁺,Cd²⁺,Pb²⁺ were not adsorbed. By changing pH, selective recovery of Cu²⁺ was achieved.     

Involvement of Ca2+-calmodulin in Cd2+ toxicity during the early phases of radish (Raphanus sativus L.) seed germination

The toxicity of Cd²⁺in vivo during the early phases of radish (Raphanus sativus L.) seed germination and the in vitro Cd²⁺ effect on radish calmodulin (CaM) were studied. Cd²⁺ was taken up in the embryo axes of radish seeds; the increase in fresh weight of embryo axes after 24 h of incubation was inhibited significantly in the presence of 10 mmol m^?3 Cd²⁺ in the external medium, when the Cd²⁺ content in the embryo axes was c. 1.1 μmol g^?1 FW. The reabsorption of K⁺, which characterizes germination, was inhibited by Cd²⁺, suggesting that Cd²⁺ affected metabolic reactivation. The slight effect of Cd²⁺ on the transmembrane electric potential of the cortical cells of the embryo axes excluded a generalized toxicity of Cd²⁺ at the plasma membrane level. After 24 h of incubation, Cd²⁺ induced no increase in total acid-soluble thiols and Cd²⁺-binding peptides able to reduce Cd²⁺ toxicity. Ca²⁺ added to the incubation medium partially reversed the Cd²⁺-induced inhibition of the increase in fresh weight of embryo axes and concomitantly reduced Cd²⁺ uptake. Equilibrium dialysis experiments indicated that Cd²⁺ bound to CaM and competed with Ca²⁺ in this binding. Cd²⁺ inhibited the activation of Ca²⁺-CaM-dependent calf-brain phosphodiesterase, inhibiting the Ca²⁺-CaM active complex. Cd²⁺ reduced the binding of CaM to the Ca²⁺-CaM binding enzymes present in the soluble fraction of the embryo axes of radish seeds. The possibility that Cd²⁺ toxicity in radish seed germination is mediated by the action of Cd²⁺ on Ca²⁺-CaM is discussed in relation to the in vivo and in vitro effects of Cd²⁺.     

CAX1 suppresses Cd‐induced generation of reactive oxygen species in Arabidopsis halleri

The molecular analysis of metal hyperaccumulation in species such as Arabidopsis halleri offers the chance to gain insights into metal homeostasis and into the evolution of adaptation to extreme habitats. A prerequisite of metal hyperaccumulation is metal hypertolerance. Genetic analysis of a backcross population derived from Arabidopsis lyrata × A. halleri crosses revealed three quantitative trait loci for Cd hypertolerance. A candidate gene for Cdtol2 is AhCAX1, encoding a vacuolar Ca²⁺/H⁺ antiporter. We developed a method for the transformation of vegetatively propagated A. halleri plants and generated AhCAX1‐silenced lines. Upon Cd²⁺ exposure, several‐fold higher accumulation of reactive oxygen species (ROS) was detectable in roots of AhCAX1‐silenced plants. In accordance with the dependence of Cdtol2 on external Ca²⁺ concentration, this phenotype was exclusively observed in low Ca²⁺ conditions. The effects of external Ca²⁺ on Cd accumulation cannot explain the phenotype as they were not influenced by the genotype. Our data strongly support the hypothesis that higher expression of CAX1 in A. halleri relative to other Arabidopsis species represents a Cd hypertolerance factor. We propose a function of AhCAX1 in preventing a positive feedback loop of Cd‐elicited ROS production triggering further Ca²⁺‐dependent ROS accumulation.     

Photooxidation of wetland and riverine dissolved organic matter: altered copper complexation and organic composition

In natural waters, the uptake of transition metals such as copper (Cu) by aquatic biota depends on the activity of the free cupric ion ({Cu²⁺}) rather than on total Cu concentration. Thus, an important ecological function of dissolved organic matter (DOM) in aquatic ecosystems is Cu–DOM complexation, which greatly decreases the {Cu²⁺}. However, Cu bioavailability is greatly modified by source and environmental history of DOM because DOM affinity for Cu varies by orders of magnitude among DOM sources; moreover, DOM is photochemically unstable. During 72-h irradiation experiments at intensities approximating sunlight with DOM from a palustrine wetland and a third-order river, we investigated photooxidative effects on DOM complexation of Cu as well as spectral and chemical changes in DOM that might explain altered Cu complexation. Irradiation decreased Cu complexation by riverine DOM, but unexpectedly increased Cu complexation by wetland DOM, resulting in 150% greater {Cu²⁺} in riverine DOM at the same dissolved organic carbon concentrations. The specific ultraviolet absorption (SUVa) and humic substances tracked photochemical changes in the conditional stability constants of Cu–DOM complexes, suggesting that the aromaticity of DOM influences its affinity for Cu. Carbonyl concentration in ¹³C nuclear magnetic resonance spectra (¹³C-NMR) covaried directly with Cu binding-site densities in DOM. However, no aspect of Cu–DOM complexation consistently covaried with fluorophores (i.e., the fluorescence index) or low molecular weight organic acids. Our results suggest that global increases in UV radiation will affect Cu–DOM complexation and subsequent Cu toxicity depending on light regime as well as DOM source. Handling editor: K. Martens     

Effects of heavy metals on seed germination and early seedling growth of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Seed is a developmental stage that is highly protective against external stresses in the plant life cycle. In this study, we analyzed toxicity of essential (Cu²⁺ and Zn²⁺) and non-essential heavy metals (Hg²⁺, Pb²⁺ and Cd²⁺) on seed germination and seedling growth in the model species Arabidopsis. Our results show that seedling growth is more sensitive to heavy metals (Hg²⁺, Pb²⁺, Cu²⁺ and Zn²⁺) in comparison to seed germination, while Cd²⁺ is the exception that inhibited both of these processes at similar concentrations. To examine if toxicity of heavy metals is altered developmentally during germination, we incubated seeds with Hg²⁺ or Cd²⁺ only for a restricted period during germination. Hg²⁺ displayed relatively strong toxicity at period II (12–24 h after imbibition), while Cd²⁺ was more effective to inhibit germination at period I (0–12 h after imbibition) rather than at period II. The observed differences are likely to be due in part to selective uptake of different ions by the intact seed, because isolated embryos (without seed coat and endosperm) are more sensitive to both Hg²⁺ and Cd²⁺ at period I. We assessed interactive toxicity between heavy metals and non-toxic cations, and found that Ca²⁺ was able to partially restore the inhibition of seedling growth by Pb²⁺ and Zn²⁺.     

Calcium signaling mediates the response to cadmium toxicity in Saccharomyces cerevisiae cells

The involvement of Ca²⁺ in the response to high Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Hg²⁺ was investigated in Saccharomyces cerevisiae. The yeast cells responded through a sharp increase in cytosolic Ca²⁺ when exposed to Cd²⁺, and to a lesser extent to Cu²⁺, but not to Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, or Hg²⁺. The response to high Cd²⁺ depended mainly on external Ca²⁺ (transported through the Cch1p/Mid1p channel) but also on vacuolar Ca²⁺ (released into the cytosol through the Yvc1p channel). The adaptation to high Cd²⁺ was influenced by perturbations in Ca²⁺ homeostasis. Thus, the tolerance to Cd²⁺ often correlated with sharp Cd²⁺-induced cytosolic Ca²⁺ pulses, while the Cd²⁺ sensitivity was accompanied by the incapacity to rapidly restore the low cytosolic Ca²⁺.