Effect of extracellular polymeric substances (EPS) on Pb2+ accumulation by Aureobasidium pullulans

In Pb²⁺ accumulation by Aureobasidium pullulans, the time to reach an equilibrium state was not dependent on the initial cell dry weight. The Pb²⁺ accumulation capacity was increased from 56.9 to 215.6 mg Pb²⁺/g cell dry weight as the biomass was stored from 1 to 53 days, and correlated with the amount of excreted extracellular polymeric substances (EPS). It was observed that Pb²⁺ accumulated only on the surface of the intact cells of A. pullulans due to the existence of EPS, whereas Pb²⁺ penetrated into the inner cellular parts of the EPS-extracted cells.     

Microcalorimetric Study on the Toxic Effect of Pb<Superscript>2+</Superscript> to <Emphasis Type="Italic">Tetrahymena</Emphasis>

The toxic effect of Pb²⁺ has been studied in eukaryotic cells by using Tetrahymena as a target. The maximum power (P _m) and the growth rate constant (k) were determined, which showed that values of P _m and k were linked to the concentration (C) of Pb²⁺. The addition of Pb²⁺ caused a decrease of the maximum heat production and growth rate constant, indicating that Tetrahymena growth was inhibited in the presence of Pb²⁺, and Pb²⁺ took part in the metabolism of cells. From micrographs, morphological changes of Tetrahymena were observed with addition of Pb²⁺, indicating that the toxic effect of Pb²⁺ derived from destroying the membrane of surface of Tetrahymena. According to the thermogenic curves and photos of Tetrahymena under different conditions, it is clear that metabolic mechanism of Halobacterium halobium R1 growth has been changed with the addition of Pb²⁺.     

Molecular Neurobiology of Lead (Pb<Superscript>2+</Superscript>): Effects on Synaptic Function

Lead (Pb²⁺) is a ubiquitous environmental neurotoxicant that continues to threaten public health on a global scale. Epidemiological studies have demonstrated detrimental effects of Pb²⁺ on childhood IQ at very low levels of exposure. Recently, a mechanistic understanding of how Pb²⁺ affects brain development has begun to emerge. The cognitive effects of Pb²⁺ exposure are believed to be mediated through its selective inhibition of the N-methyl-d -aspartate receptor (NMDAR). Studies in animal models of developmental Pb²⁺ exposure exhibit altered NMDAR subunit ontogeny and disruption of NMDAR-dependent intracellular signaling. Additional studies have reported that Pb²⁺ exposure inhibits presynaptic calcium (Ca²⁺) channels and affects presynaptic neurotransmission, but a mechanistic link between presynaptic and postsynaptic effects has been missing. Recent work has suggested that the presynaptic and postsynaptic effects of Pb²⁺ exposure are both due to inhibition of the NMDAR by Pb²⁺, and that the presynaptic effects of Pb²⁺ may be mediated by disruption of NMDAR activity-dependent signaling of brain-derived neurotrophic factor (BDNF). These findings provide the basis for the first working model to describe the effects of Pb²⁺ exposure on synaptic function. Here, we review the neurotoxic effects of Pb²⁺ exposure and discuss the known effects of Pb²⁺ exposure in light of these recent findings.     

Pb2+ reduces voltage- andN-methyl-d-aspartate (NMDA)-activated calcium channel currents

Summary 1. While intracellular calcium concentrations are closely regulated, two types of ion channels in neurons allow calcium influx: both voltage-activated and NMDA-activated channels are significantly permeable to calcium. In this study we compare the effects of lead (Pb²⁺) on currents carried through voltage-activated calcium channels and NMDA-activated channels.2. Pb²⁺ reduces voltage-activated calcium channel currents elicited by a voltage jump from –80 to 0 mV at 0.1 to 1 µM, with an IC₅₀ of 0.64 µM and a Hill slope of 1.22. This effect was partially reversible and not voltage dependent. Sodium and potassium currents were relatively unaffected at Pb²⁺ concentrations sufficient to block calcium channel currents by more than 80%. Pb²⁺ is, thus, a potent, reversible and selective blocker of voltage-dependent calcium channel currents.3. A fast reversible and slow irreversible blocking action of Pb²⁺ was found on NMDA-activated currents. When Pb²⁺ was applied simultaneously with aspartate and glycine (Asp/Gly), the inward currents were rapidly and reversibly reduced in a dose-dependent manner with a minimum effective concentration below 2 µM and a total blockade (>80%) with 100 µM Pb²⁺. The IC₅₀ was 45 µM and the Hill coefficient 1.1. Preincubation with 50 µM Pb²⁺ resulted in a greater reduction in the response to Asp/Gly/Pb²⁺. This effect was reversed within 2 to 5 sec of wash. The lack of voltage dependence suggests that Pb²⁺ does not block the channel but rather alters the binding of agonists. Prolonged superfusion of a cell with the Asp/Gly/Pb²⁺-containing external solution resulted in a slow and irreversible decrease in the Asp/Gly activated current. No clear threshold concentration is found for this slow and irreversible effect of Pb²⁺. This slow action might be more important for neurotoxic effects of Pb²⁺.     

Biosorption of Cd, Cu, Pb, and Zn from aqueous solutions by the fruiting bodies of jelly fungi (Tremella fuciformis and Auricularia polytricha)

In this study, dried and humid fruiting bodies of Tremella fuciformis and Auricularia polytricha were examined as cost-effective biosorbents in treatment of heavy metals (Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺) in aqueous solution. The humid T. fuciformis showed the highest capacity to adsorb the four metals in the multi-metal solutions. The Pb²⁺ adsorption rates were 85.5%, 97.8%, 84.8%, and 91.0% by dried T. fuciformis, humid T. fuciformis, dried A. polytricha, and humid A. polytricha, respectively. The adsorption amount of Pb²⁺ by dried and humid T. fuciformis in Cd²⁺ + Pb²⁺, Cu²⁺ + Pb²⁺, Pb²⁺ + Zn²⁺, Cd²⁺ + Cu²⁺ + Pb²⁺, and Cd²⁺ + Zn²⁺ + Pb²⁺ solutions were not lower than that in Pb²⁺ solutions. The results suggested that in humid T. fuciformis, Cd²⁺, Cu²⁺, and Zn²⁺ promoted the Pb²⁺ adsorption by the biomass. In the multi-metal solutions of Cd²⁺ + Cu²⁺ + Pb²⁺ + Zn²⁺, the adsorption amount and rates of the metals by all the test biosorbents were in the order of Pb²⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺. Compared with the pseudo first-order model, the pseudo second-order model described the adsorption kinetics much better, indicating a two-step biosorption process. The present study confirmed that fruiting bodies of the jelly fungi should be useful for the treatment of wastewater containing Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺.     

Toxic effects of Pb2+ on the growth and mineral nutrition of signal grass (Brachiaria decumbens) and Rhodes grass (Chloris gayana)

Although grasses are commonly used to revegetate sites contaminated with lead (Pb), little is known regarding the Pb-tolerance of many of these species. Using dilute solution culture to mimic the soil solution, the growth of signal grass (Brachiaria decumbens Stapf cv. Basilisk) and Rhodes grass (Chloris gayana Kunth cv. Pioneer) was related to the mean activity of Pb²⁺ {Pb²⁺} in solution. There was a 50% reduction in fresh mass of signal grass shoots at 5 μM {Pb²⁺} and at 3 μM {Pb²⁺} for the roots. Rhodes grass was considerably more sensitive to Pb in solution, with shoot and root fresh mass being reduced by 50% at 0.5 μM {Pb²⁺}. The higher tolerance of signal grass to Pb appeared to result from the internal detoxification of Pb, rather than from the exclusion of Pb from the root. At toxic {Pb²⁺}, an interveinal chlorosis developed in the shoots of signal grass (possibly a Pb-induced Mn deficiency), whilst in Rhodes grass, Pb²⁺ caused a bending of the root tips and the formation of a swelling immediately behind some of the root apices. Root hair growth did not appear to be reduced by Pb²⁺ in solution, being prolific at all {Pb²⁺} in both species.     

Effects of low pH and Pb2+ stress on living cyanobacterium, Phormidium angustissimum West & G.S.West : A test of its feasibility as a living biosorbent

Pb²⁺ adsorption by the living cyanobacterium, Phormidium angustissimum followed the Langmuir adsorption model, with the maximum adsorption capacity (q _max ) of 295.4?±?13.8 mg g^?1. This result suggests that P. angustissimum is a promising living biosorbent to remove Pb²⁺ from wastewaters. Living biosorbents are better able to remove Pb²⁺ from wastewater than dead biosorbents, however there are practical limitations for their use are encountered in extreme conditions such as low pH and high Pb²⁺ concentration. The feasibility of using cyanobacterium, P. angustissimum, as a living biosorbent for the extraction of Pb²⁺ from wastewater was studied by investigating its photosynthestic performance and tolerance under Pb²⁺ (0–5 mg ?L^?1) contamination and low pH (pH?3–7). Decreased photosynthetic performance caused by Pb²⁺ contamination and low pH stress was detected in this study by means of a reduction of the maximum photochemical efficiency of PSII (F_v/F_m). Detoxification mechanisms of P. angustissimum on Pb²⁺ appeared to increase its intracellular polysaccharides (IPS), exocellular polysaccharides (EPS), and protein. Living P. angustissimum could increase the pH of the solution which resulted in Pb²⁺ precipitation. The unique ability of P. angustissimum to remove Pb²⁺ and to grow under toxic conditions, demonstrated herein, indicates that it is a promising living biosorbent for mildly acidic water contaminated with Pb²⁺ in bioremoval systems in the which pH is not lower than 5 and Pb²⁺ is not higher than 5 mg L^?1.     

Cu,Zn-superoxide dismutase is differently regulated by cadmium and lead in roots of soybean seedlings

The cadmium (Cd²⁺) and lead (Pb²⁺)-induced changes in Cu,Zn-SOD gene expression on the level of mRNA accumulation and enzyme activity were analyzed in roots of soybean (Glycine max) seedlings. The Cd²⁺ caused the induction of copper–zinc superoxide dismutase (Cu,Zn-SOD) mRNA accumulation, at each analyzed metal concentration (5–25 mg/l), whereas in Pb²⁺-treated roots this effect was observed only at the medium metal concentrations (50–100 mg/l of Pb²⁺). The analysis of Cu,Zn-SOD activity proved an increase in enzyme activity during Cd²⁺/Pb²⁺ stresses, however in Pb²⁺-treated plants the activity of enzyme was not correlated with respective mRNAs level. Presented data suggest that different metals may act on various level of Cu,Zn-SOD expression in plants exposed to heavy metals stress.     

Removal of heavy metals from aqueous solution by common freshwater filamentous algae

Non-living (dried) biomass of five common filamentous algae belonging to Chlorophyta and Cyanophyta (Cyanobacteria) were screened for their metal ion sorption and removal efficiency in a batch system. A considerably higher magnitude of sorption of Pb²⁺ and Cu²⁺ by all the tested algae suggests the prevalence of Pb²⁺- and Cu²⁺-binding ligands in them. The Langmuir isotherm could more appropriately describe metal sorption by the test algae than the Freundlich isotherm. A 1 g l⁻¹ biomass concentration of Pithophora odeogonia and Spirogyra neglecta, respectively removed 97 and 89% Pb²⁺in 30 min from a solution containing 5 mg l⁻¹ initial concentration of Pb²⁺. Metal ion removal by the test algae decreased with increase in metal concentration in the solution. S. neglecta could remove >70% Pb²⁺ even from a solution containing 75 mg Pb²⁺ l⁻¹. S. neglecta and P. oedogonia could remove more than 75% of Pb²⁺ and Cu²⁺ from a multi-metal solution, and therefore have tremendous potential for removing Pb²⁺and Cu²⁺ from wastewaters containing several metal ions simultaneously. Other test algae, namely, Hydrodictyon reticulatum, Cladophora calliceima and Aulosira fertilissima were relatively less efficient in removing metal ions from solution.     

Lead Accumulation Potential in Acacia victoria

To assess the potential of Pb⁺² accumulation in different parts of Acacia victoria, one year old A. victoria seedlings were exposed to Pb²⁺(NO₃)₂ in 5 different concentrations: 0, 50, 250, 500 and 1000 (mg Pb²⁺ L^?1) for 45 days. Subsequently, Pb²⁺ uptake was quantified in roots, shoots and leaves of the seedlings by Atomic Absorption Spectroscopy (AAS). In addition, some physiological parameters such as biomass production, shoots and roots length, plant appearance, tissue concentrations and chlorophyll content were examined. Tissue concentrations increased as Pb²⁺ concentration increased for A. victoria. The visible toxicity symptoms (chlorosis and necrosis) appeared only to the highest concentration (1000 mg Pb²⁺ L^?1), resulting in photosynthesis decrease, plant height, root length and dry biomass reduction. Almost 70% (up to 3580 mg Kg^?1 of dry tissue) from the Pb²⁺ was accumulated in the entire plant tissues was retained in the roots in the seedlings exposed to 1000 mg Pb²⁺ L^?1. The seedlings accumulated between 403 to 913 mg Kg^?1 of Pb²⁺ in shoots and 286 to 650 mg Kg^?1 of Pb²⁺ in leaves at different treatments. Bioconcentration and translocation factors were determined 5.14 and 0.255, respectively. The results show that A. victoria is suitable for lead-phytostabilization in Pb²⁺-contaminated soil.     

Relations between extraction protocols for activated sludge extracellular polymeric substances (EPS) and complexation properties of Pb and Cd with EPS: Part II. Consequences of EPS extraction methods on Pb2+ and Cd2+ complexation

To study the effect of extraction protocols on extracellular polymeric substances (EPS) metal binding ability, EPS from two activated sludges were extracted by eight extraction protocols: three chemical treatments, four physical treatments and a control. Two pK_a, for each EPS, were determined: pK_a1 may be specific for carboxyl and phosphoric and pK_a2 may be attributed to phenolic and amino functional groups according to EPS composition and IR spectra. EPS pK_a values could be affected by the presence of extraction reagents and/or the modifications of EPS by extraction reagents.Complexation study performed at pH 7 by a polarographic method has always showed a greater affinity of EPS for Pb²⁺ than for Cd²⁺. The complexation properties of EPS extracted by chemical methods were greatly modified. Concerning EPS extracted by physical methods, their complexation properties were close except for EPS obtained by heating. Standardized extraction methods must be established as a function of the aims of the EPS study.     

Cd2+和Pb2+对花翅摇蚊(Chironomus kiiensis)幼虫生长发育的影响及富集效应

河流、湖泊等水生环境中普遍存在的重金属污染破坏水生生态系统并间接威胁人类健康。为探究重金属胁迫下水生昆虫花翅摇蚊（Chironomus kiiensis）生态毒理,测定了重金属Cd²⁺和Pb²⁺胁迫对花翅摇蚊化蛹率和羽化率的影响,检测了摇蚊的口器致畸与富集效应。研究结果表明,Cd²⁺和Pb²⁺影响摇蚊幼虫化蛹和羽化过程,单一重金属离子处理14 d Pb²⁺处理组的化蛹率和羽化率分别为22.22%和8.89%,低于Cd²⁺的化蛹率（25.56%）和羽化率（11.11%）,表现出更强的抑制效应。混合离子1：2和2：1配比处理组化蛹率和羽化率均为11.11%和4.44%,显著低于单一重金属离子胁迫下的化蛹率和羽化率。单一重金属离子及混合离子处理均能导致花翅摇蚊幼虫口器致畸,表现为上颚前齿断裂,中齿和基齿磨损、缺失,下唇板齿部不规则,下唇板边缘齿与中央齿磨损、断裂、增生、缺失。不同重金属离子处理下幼虫口器致畸率不同,并与暴露时间呈正相关,其中1：2配比处理14 d致畸率达到40.61%。重金属离子在摇蚊幼虫体内产生生物富集效应,单一重金属离子处理下的Pb²⁺富集含量7 d至14 d由11.46 mg/kg上升至31.32 mg/kg,不同配比混合离子处理下Pb²⁺富集含量均呈增加趋势,其中1：2配比处理组由15.48 mg/kg上升至42.50 mg/kg,而Cd²⁺在单一重金属与1：1混合离子处理组7 d至14 d的富集含量无显著性变化,2：1配比处理组由14.20 mg/kg下降至9.52 mg/kg,1：2配比由5.85 mg/kg上升至20.99 mg/kg。这些研究结果表明Cd²⁺和Pb²⁺胁迫影响花翅摇蚊幼虫生长发育且口器出现畸型,与重金属在幼虫体内的富集密切相关,为研究重金属对水生生态系统多重效应提供了理论依据。     

Responses of elongation growth rate,turgor pressure and cell wall extensibility of stem cells of Impatiens balsamina to lead,cadmium and zinc

Elongation growth rate of stem cells of Impatiens balsamina was inhibited by the heavy metals Pb²⁺, Cd²⁺ and Zn²⁺ due to their suppression on cell wall extensibility. Effective turgor was also inhibited by Pb²⁺ and Cd²⁺ but it played a secondary role in reducing the stem cell elongation growth rate. The major rate-limiting factor for cell elongation growth was the cell wall extensibility. Furthermore, Cd²⁺ was found to be more toxic than Pb²⁺, while Pb²⁺ was more toxic than Zn²⁺.     

Zinc and lead uptake by mycelium and regenerating protoplasts of Verticillium marquandii

The ability of the filamentous fungus Verticillium marquandii for Zn²⁺ and Pb²⁺ uptake from aqueous solution was studied. The 24-h-old living mycelium bound Zn²⁺ and Pb²⁺ (206.2 and 324.5 mg/g dry weight, respectively) effectively, in contrast to a very low Zn²⁺ uptake by autoclaved mycelium (20.2 mg/g). The most effective results were noted when the metals were introduced as acetates and incubated with mycelium for 24 h in case of Zn²⁺ while Pb²⁺ achieved the maximum level of metal binding after as early as 3 h. The cell wall was the main site of effective Zn²⁺ and Pb²⁺ binding by V. marquandii mycelium (91.0–93.6% of metals were located in cell wall after 24 h of exposure). The metabolic inhibitors: antimycin A and sodium azide had a strong limitation effect on Zn²⁺ uptake by a 24-h-old living mycelium, whereas Pb²⁺ binding did not decrease to a large extent. The freshly obtained protoplasts accumulated Zn²⁺ and Pb²⁺ on a low level in comparison with cells at different stages of cell wall regeneration. The use of regenerating protoplasts showed that resynthesis of cell wall was necessary for high binding of Zn²⁺, whereas Pb²⁺ uptake on the significant level took place during cell wall regeneration. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sorption of heavy metals by four basidiomycetous fungi

Biosorptions of Pb²⁺, Cr⁶⁺, Cd²⁺ and Ni²⁺ were investigated, with special emphasis on the first one, using live and dead fungal mycelia. Of the four fungi, namely Polyporus ostreiformis, Volvariella volvacea, Pleurotus sajor-caju and Phanerochaete chrysosporium, the last one was found to be most effective in Pb²⁺ removal. Total biosorption was effected in 6 days up to the Pb²⁺ concentration of 6 mg/l, with a specific uptake of 1.33 mg Pb²⁺/g dry cell mass. The removal of other three metals varied between 28.8–73.3% from a medium containing 4 mg/l of each of the metals.Laboratory facilities created under M.Tech programme in Biotechnology and Engineering (sponsored by Department of Biotechnology, Govt. of India) were partly utilized for this study.     

The Effect of Copper and Lead Ions on Growth and Lipid Composition of the Fern Matteuccia sthruthiopteris

In the present study, the effect of copper (Cu²⁺) and lead (Pb²⁺) ions on the growth and lipid composition of various parts of the fern, Matteuccia sthruthiopteris, was examined. Plants were incubated in the presence or absence of 1, 10, 100 μM of Cu(NO₃)₂ or Pb(NO₃)₂. Cu²⁺ and Pb²⁺ ions at concentrations of 1 and 10 μM caused an increased growth of the roots and leaves. A higher concentration of Pb²⁺ did not show any effect on growth, whereas that of Cu²⁺ slowed down the growth of the whole plants. The roots accumulated more than 700 μg of Cu²⁺ and 400 μg of Pb²⁺ per 1 g dry weight when the plants were incubated with the higher concentrations of metals, whereas in the leaves the concentration of Cu²⁺ was much lower and did not exceed 12 μg/g dry weight. No accumulation of Pb²⁺ ions by leaves was detected. The lipid composition of photosynthetic leave tissues was shown to be affected by the presence of metal ions in the root medium at either concentration studied. Various changes in lipid classes were noted as responsive reactions of M. sthruthiopteris to the heavy metal ions in nutrient medium. Cu²⁺ ions decreased the content of total lipids, total phospholipids, and individual phosphatidylcholines and phosphatidylethanolamines, whereas Pb²⁺ ions caused a decrease in the content of total lipids and glycolipids. Changes in the lipid composition were more pronounced in the mature leaves than in the scrolls of the studied fern.     

Resonance Scattering Spectral Detection of Trace Pb<Superscript>2+</Superscript> Using Aptamer-Modified AuPd Nanoalloy as Probe

The resonance scattering spectral probe for Pb²⁺ was obtained using aptamer-modified AuPd Nanoalloy. In the pH 7.0 Na₂HPO₄–NaH₂PO₄ buffer solution, the aptamer interacted with AuPd nanoalloy particles to form stable aptamer-AuPd nanoalloy probe for Pb²⁺ that is stable in high concentration of salt. The probe combined with Pb²⁺ ions to form a G-quadruplex and to release AuPd nanoalloy particles that aggregate to form big particles which led the resonance scattering (RS) intensity enhancing. The reaction solution was filtered by 0.15 μm membrane to obtain the filtration containing aptamer-AuPd nanoalloy probe that has strong catalytic effect on the electrodeless nickel particle plating reaction between Ni(II) and PO₂³⁻ that exhibited a strong RS peak at 508 nm. The RS intensity at 508 nm decreased when the Pb²⁺ concentration increased. The decreased intensity (ΔI _508nm) is linear to the concentration of 0.08–42 nM Pb²⁺, with regress equation of DI_508nm = 16.3 c + 1.5 \Delta {I_{{5}0{\rm{8nm}}}} = {16}.{3}\,c + {1}.{5} , correlation coefficient of 0.9965, and detection limit of 0.04 nM Pb²⁺. The RS assay was applied to the analysis of Pb²⁺ in wastewater, with satisfactory results.     

香蒲对不同浓度Pb~(2+)胁迫的生理应答及其细胞超微结构变化

通过室内水培试验,研究了不同浓度Pb2+(0、0.25、0.50、1.00和2.00mmol·L-1)胁迫对东方香蒲根和叶中Pb含量、叶绿素含量、丙二醛(MDA)含量、抗氧化酶(SOD、CAT和POD)活性以及亚细胞结构的影响。结果显示:(1)随着外源Pb2+浓度的增加,Pb在香蒲根和叶中的积累量均显著高于对照,且Pb在根中的含量明显高于叶中,并与外源Pb2+浓度呈显著正相关关系。(2)香蒲叶片中的叶绿素a和叶绿素b含量随着外源Pb2+浓度的增加呈先升后降趋势,均在处理浓度为0.50mmol·L-1时达到峰值。(3)胁迫处理叶片的MDA含量与对照相比变化不显著,但根中MDA含量呈显著下降趋势。(4)叶片中SOD活性在1.00mmol·L-1 Pb2+处理时达到峰值,然后下降,但始终高于对照,CAT和POD活性则均低于对照组;根中SOD活性除1.00mmol·L-1 Pb2+处理组外均显著低于对照组,CAT和POD活性分别在0.25和0.50mmol·L-1 Pb2+处理时达到峰值,然后随处理Pb2+浓度升高而下降。(5)电镜观察发现,Pb2+胁迫使香蒲叶细胞中叶绿体被膜破裂,类囊体膨胀、破损;根和叶细胞中的线粒体被膜均破裂、内腔空泡化,细胞核核膜破损、核仁消失、染色质凝集。研究表明,Pb2+胁迫致使东方香蒲根、叶生理代谢失衡,亚细胞结构出现不可逆的损伤,这为从分子水平研究Pb2+作用的具体机理以及香蒲在重金属污染修复中的应用提供了依据。     

Evaluation of DNA damage and mutagenicity induced by lead in tobacco plants

Tobacco (Nicotiana tabacum L. var. xanthi) seedlings were treated with aqueous solutions of lead nitrate (Pb²⁺) at concentrations ranging from 0.4 mM to 2.4 mM for 24 h and from 25 μM to 200 μM for 7 days. The DNA damage measured by the comet assay was high in the root nuclei, but in the leaf nuclei a slight but significant increase in DNA damage could be demonstrated only after a 7-day treatment with 200 μM Pb²⁺. In tobacco plants growing for 6 weeks in soil polluted with Pb²⁺ severe toxic effects, expressed by the decrease in leaf area, and a slight but significant increase in DNA damage were observed. The tobacco plants with increased levels of DNA damage were severely injured and showed stunted growth, distorted leaves and brown root tips. The frequency of somatic mutations in tobacco plants growing in the Pb²⁺-polluted soil did not significantly increase. Analytical studies by inductively coupled plasma optical emission spectrometry demonstrate that after a 24-h treatment of tobacco with 2.4 mM Pb²⁺, the accumulation of the heavy metal is 40-fold higher in the roots than in the above-ground biomass. Low Pb²⁺ accumulation in the above-ground parts may explain the lower levels or the absence of Pb²⁺-induced DNA damage in leaves.     

Lead toxicity and phosphate deficiency in chlamydomonas

The addition of lead salts to phosphate-containing Chlamydomonas reinhardtii media caused precipitation of Pb₃(PO₄)₂, effectively removing phosphate from solution. The effect of Pb²⁺ on growth of Chlamydomonas in liquid cultures depended strictly on the ratio of the equivalents of Pb²⁺ to phosphate present. When the amount of Pb²⁺ approached equivalency with phosphate, cell growth was initially slow as cells adhered to the surface of the precipitated Pb₃(PO₄)₂. Later, cells grew at a normal rate, spread throughout the solution, and reached the same densities obtained in the absence of Pb²⁺. Cells did not survive when the amount of Pb²⁺ in the culture exceeded the equivalents of phosphate.

Elemental analysis showed that in the presence of equivalent Pb²⁺ and phosphate, considerable Pb²⁺ remained in solution. The concentration of dissolved Pb²⁺ did not vary significantly when the amount of Pb²⁺ added to the culture was increased slightly, from an amount which permitted growth to an amount which completely prevented growth. The concentration of phosphate was decreased to an undetectable level when the amount of Pb²⁺ approached equivalency with phosphate.

In the presence of the chelating agent nitrilotriacetic acid, higher concentrations of Pb²⁺ remained in phosphate-containing media. The chelated Pb²⁺ did not retard the growth of Chlamydomonas.

It appears that Pb²⁺ is not toxic to Chlamydomonas, but kills cells by depriving them of phosphate.