Uptake of Hg from203Hg-labeled mercury compounds by wheat and beans grown on an oxisol

Summary Studies were conducted to evaluate the uptake of mercury by wheat (Triticum aestivum L. runar) and beans (Phaseolus vulgaris L. marshal) growth on an oxisol with different levels of 2-methoxyethylmercury chloride (Aretan) and mercuric chloride. Dry matter and grain yields of wheat were little affected by either Aretan or mercuric chloride, although Aretan at 50 mg Hg/kg soil delayed germination by four to five days. Germination of beans grown with both compounds at the 50 mg Hg/kg soil failed completely, even after repeated sowing. Yields were somewhat, though not significantly, decreased by mercury chloride up to 5 mg Hg/kg soil.The concentration of Hg in wheat straw and grain increased significantly with increased levels of Aretan and HgCl₂ application, with more Hg taken up by the plants grown with HgCl₂ than with those grown with Aretan. Translocation of Hg to grain was greater in the plants grown with HgCl₂.The concentration of Hg in bean straw, but not grain, increased significantly with increasing levels of Aretan and HgCl₂ application, and was greater in plants grown with HgCl₂. Translocation to grain was low, with little difference between plants grown with Aretan or HgCl₂.     

Isolation and Characterization of Mercury Resistant Bacillus sp. from Soils with an Extensive History as Substrates for Mercury Extraction in Mexico

Metal phytoextraction assisted by bacteria plays an important role in bioremediation systems. In this work, mercury-resistant bacterial strains were isolated from soils with high levels of mercury (San Joaquin, Queretaro State, Mexico) and identified as Bacillus sp. based on the 16S rDNA gene sequence analysis. The bacterial strains were found to exhibit different multiple mercury-resistance and carbon source utilization characteristics. The mercury reduction ability was tested through a volatilization assay. The bacterial isolates were also evaluated for their ability to promote growth and mercury uptake in tomato plants. In a roll towel assay, the maximum vigor index of tomato plants was obtained with the inoculation of Bacillus sp. A2, A12, B11, B15 and C1, while in a pot assay, the maximum vigor index was obtained with the inoculation of Bacillus sp. A6, A7 and B20, compared with un-inoculated controls in the presence of HgCl₂. Maximum Hg accumulation in the roots and shoots of tomato plants was obtained only with Bacillus sp. A7 in the roll towel assay, whereas in the pot assay, maximum accumulation was obtained with Bacillus sp. A12 compared with un-inoculated controls. Our results show that mercury accumulation in tissue is enhanced by these plant growth promoting bacterial strains, which recommends their possible use as microbe-assisted phytoremediation systems in mercury-polluted soils.     

Ecological strategy for soil contaminated with mercury

Aims

The paper presents results from plot experiments aimed at the development of an ecological strategy for soil contaminated with mercury. Meadow grass (Poa pratensis) was tested on mercury contaminated soil in a former chlor-alkali plant (CAP) in southern Poland for its phytoremediation potential.

Methods

The stabilisation potential of the plants was investigated on plots without additives and after the addition of granular sulphur. Biomass production, uptake and distribution of mercury by plants, as well as leachates and rhizosphere microorganisms were investigated, along with the growth and vitality of plants during one growing season.

Results

The analysed plants grew easily on mercury contaminated soil, accumulating lower amounts of mercury, especially in the roots, from soil with additive of granular sulphur (0.5 % w/w) and sustained a rich microbial population in the rhizosphere. After amendment application the reduction of Hg evaporation was observed.

Conclusions

The obtained results demonstrate the potential of using Poa pratensis and sulphur for remediation of mercury contaminated soil and reduction of the Hg evaporation from soil. In the presented study, methods of Hg reduction on “hot spots” were proposed, with a special focus on environmental protection. This approach provides a simple remediation tool for large areas heavily contaminated with mercury.

     

Toxicity of organic and inorganic mercury species in differentiated human neurons and human astrocytes

Organic mercury (Hg) species exert their toxicity primarily in the central nervous system. The food relevant Hg species methylmercury (MeHg) has been frequently studied regarding its neurotoxic effects in vitro and in vivo. Neurotoxicity of thiomersal, which is used as a preservative in medical preparations, is to date less characterised. Due to dealkylation of organic Hg or oxidation of elemental Hg, inorganic Hg is present in the brain albeit these species are not able to readily cross the blood brain barrier. This study compared for the first time toxic effects of organic MeHg chloride (MeHgCl) and thiomersal as well as inorganic mercury chloride (HgCl₂) in differentiated human neurons (LUHMES) and human astrocytes (CCF-STTG1). The three Hg species differ in their degree and mechanism of toxicity in those two types of brain cells. Generally, neurons are more susceptible to Hg species induced cytotoxicity as compared to astrocytes. This might be due to the massive cellular mercury uptake in the differentiated neurons. The organic compounds exerted stronger cytotoxic effects as compared to inorganic HgCl₂. In contrast to HgCl₂ exposure, organic Hg compounds seem to induce the apoptotic cascade in neurons following low-level exposure. No indicators for apoptosis were identified for both inorganic and organic mercury species in astrocytes. Our studies clearly demonstrate species-specific toxic mechanisms. A mixed exposure towards all Hg species in the brain can be assumed. Thus, prospectively coexposure studies as well as cocultures of neurons and astrocytes could provide additional information in the investigation of Hg induced neurotoxicity.     

A Critical Review of the Arsenic Uptake Mechanisms and Phytoremediation Potential of Pteris vittata

The discovery of the arsenic hyperaccumulator, Pteris vittata (Chinese brake fern), has contributed to the promotion of its application as a means of phytoremediation for arsenic removal from contaminated soils and water. Understanding the mechanisms involved in arsenic tolerance and accumulation of this plant provides valuable tools to improve the phytoremediation efficiency. In this review, the current knowledge about the physiological and molecular mechanisms of arsenic tolerance and accumulation in P. vittata is summarized, and an attempt has been made to clarify some of the unresolved questions related to these mechanisms. In addition, the capacity of P. vittata for remediation of arsenic-contaminated soils is evaluated under field conditions for the first time, and possible solutions to improve the remediation capacity of Pteris vittata are also discussed.     

Effects of Se on the uptake of essential elements in Pteris vittata L.

Selenium has been proven to be an antioxidant in plants at low dosages. To understand better the mechanisms of Se toxicity and benefit to plants, more investigations about effects of Se on the uptake of essential elements in plants would be desirable. In this study, hydroponic (nutrient solution culture) and pot (soil culture) experiments were simultaneously conducted to investigate the effects of Se on the uptake and distribution of essential elements in Pteris vittata. L (Chinese brake fern), an arsenic (As)-hyperaccumulator and a selenium (Se)-accumulator. Chinese brake fern took up much more Se in nutrient solution culture than in soil culture, with the highest Se content recorded as 1,573 mg kg^?1 in the roots, demonstrating remarkable tolerance to Se. In soil culture, Chinese brake fern also accumulated high content of Se, with the highest content measured as 81 mg kg^?1 and 233 mg kg^?1, in the fronds and roots, respectively. In soil culture, the addition of Se suppressed the uptake of most measured elements, including magnesium (Mg), potassium (K), phosphorus (P), iron (Fe), copper (Cu) and zinc (Zn). In nutrient solution culture, when the Se content in the tissues of Chinese brake fern was relatively low, the supplementation of Se suppressed the uptake of most essential elements; however, with the increase of Se content, stimulation effects of Se on the uptake of Ca, Mg, K were observed. An initial decrease followed by a rapid increase of Fe content in the fronds of Chinese brake fern was found with Se addition and tissue Se content increasing in nutrient solution culture, suggesting antagonistic and synergic roles of Se on these elements under low and high Se exposure, respectively. We suggest that Ca, Mg, K may be involved in the tolerance mechanism of Se, and that the regulation of Fe accumulation by Se in the fronds might be partially due to the dual effects of Se on Chinese brake fern.     

Solubility and accumulation of metals in Chinese brake fern, vetiver and rostrate sesbania using chelating agents

Greenhouse experiments were conducted to study the effects of chelating agents on the growth and metal accumulation of Chinese brake fern (Pteris vittata L.), vetiver (Vetiveria zizanioides L.), and rostrate sesbania (Sesbania rostrata L.) in soil contaminated with arsenic (As), Cu, Pb, and Zn. Among the five chelating agents used [ethylenediaminetriacetic acid (EDTA), hydroxyethylenediaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA), oxalic acid (OA), and phytic acid (PA)], OA was the best to mobilize As, EDTA to mobilize Cu and Pb, and HEDTA to mobilize Zn from soil, respectively. The biomass of vetiver was the highest, followed by rostrate sesbania. All chelating agents inhibited the growth of Chinese brake fern and rostrate sesbania, but HEDTA significantly increased the aboveground biomass of vetiver. Dry weights of both Chinese brake fern and rostrate sesbania decreased with increasing EDTA concentrations amended in the soil, especially in treatments with high EDTA concentrations. EDTA and HEDTA enhanced Cu, Zn, and Pb, but lowered As accumulation in all three plant species, except for As in vetiver, while OA significantly enhanced As accumulation in the aboveground part of vetiver. Concentrations of Cu, Zn, and Pb in the aboveground parts of plants increased significantly with the increase of EDTA concentrations and treatment time. In addition to As, Chinese brake fern also accumulated the highest Cu, Pb, and Zn in its aboveground parts among the three plant species grown in metal-contaminated soil with EDTA/HEDTA treatments. This species, therefore, can be used to simultaneously clean up As, Cu, Pb, and Zn from contaminated soils with the aid of EDTA or HEDTA.     

Uptake and Distribution of Mercury within Higher Plants

The uptake and distribution of inorganic mercury (HgCl₂) within higher plants (Pisum sativum and Mentha spicata) was examined using solution culture and radiotracer techniques. Plants were found to tolerate an external level of 1 mgHg/kg of solution but both physiological and biochemical processes were affected at 5 mgHg/kg and 10 mgHg/kg. The uptake of Hg into plants grown in hydroponic solution was a function of external concentration. Over the concentration range considered the accumulation of Hg in the roots was linear on a log-log basis although the uptake of the element into the shoots appeared to be two-phased. The distribution of Hg in plants was asymmetrical with much greater amounts of the element in the roots than the shoots. Although the level of Hg increased generally in plant tissues with increasing external levels, the proportion retained in the roots, relative to the shoots, was constant (approximately 95%). Two binding characteristics of the Hg within plant tissue were detected. A major proportion of Hg was tightly bound, being unaffected by treatment with ethanol and hydrochloric acid. The remaining Hg in the tissue was removed by either water or hydrochloric acid treatment. Cell fractionation indicated that the major binding component of Hg in plant tissues was the cell wall.     

Effects of arsenate and phosphate on their accumulation by an arsenic-hyperaccumulator Pteris vittata L.

Arsenate and phosphate interactions are important for better understanding their uptake and accumulation by plant due to their similarities in chemical behaviors. The present study examined the effects of arsenate and phosphate on plant biomass and uptake of arsenate and phosphate by Chinese brake (Pteris vittata L.), a newly-discovered arsenic hyperaccumulator. The plants were grown for 20 weeks in a soil, which received the combinations of 670, 2670, or 5340 mol kg^–1 arsenate and 800, 1600, or 3200 mol kg^–1 phosphate, respectively. Interactions between arsenate and phosphate influenced their availability in the soil, and thus plant growth and uptake of arsenate and phosphate. At low and medium arsenate levels (670 and 2670 mol kg^–1), phosphate had slight effects on arsenate uptake by and growth of Chinese brake. However, phosphate substantially increased plant biomass and arsenate accumulation by alleviating arsenate phytotoxicity at high arsenate levels (5340 mol kg^–1). Moderate doses of arsenate increased plant phosphate uptake, but decreased phosphate concentrations at high doses because of its phytotoxicity. Based on our results, the minimum P/As molar ratios should be at least 1.2 in soil solution or 1.0 in fern fronds for the growth of Chinese brake. Our findings suggest that phosphate application may be an important strategy for efficient use of Chinese brake to phytoremediate arsenic contaminated soils. Further study is needed on the mechanisms of interactive effects of arsenate and phosphate on Chinese brake in hydroponic systems.     

Root exudates and arsenic accumulation in arsenic hyperaccumulating Pteris vittata and non-hyperaccumulating Nephrolepis exaltata

This study compared the roles of root exudates collected from two fern species, the As hyperaccumulating Chinese Brake fern (Pteris vittata L.) and the As-sensitive Boston fern (Nephrolepis exaltata L.), on As-mobilization of two As minerals (aluminum arsenate and iron arsenate) and a CCA (chromated copper arsenate)-contaminated soil as well as plant As accumulation. Chinese Brake fern exuded 2 times more dissolved organic carbon (DOC) than Boston fern and the difference was more pronounced under As stress. The composition of organic acids in the root exudates for both ferns consisted mainly of phytic acid and oxalic acid. However, Chinese Brake fern produced 0.46 to 1.06 times more phytic acid than Boston fern under As stress, and exuded 3–5 times more oxalic acid than Boston fern in all treatments. Consequently, root exudates from Chinese Brake fern mobilized more As from aluminum arsenate (3–4 times), iron arsenate (4–6 times) and CCA-contaminated soil (6–18 times) than Boston fern. Chinese Brake fern took up more As and translocated more As to the fronds than Boston fern. The molar ratio of P/As in the roots of Chinese Brake fern was greater than in the fronds whereas the reverse was observed in Boston fern. These results suggested that As-mobilization from the soil by the root exudates (enhancing plant uptake), coupled with efficient As translocation to the fronds (keeping a high molar ratio of P/As in the roots), are both important for As hyperaccumulation by Chinese Brake fern.     

Uptake and Bioaccumulation of Pentachlorophenol by Emergent Wetland Plant Phragmites australis (Common Reed) in Cadmium Co-contaminated Soil

Despite many studies on phytoremediation of soils contaminated with either heavy metals or organics, little information is available on the effectiveness of phytoremediation of co-occurring metal and organic pollutants especially by using wetland species. Phragmites australis is a common wetland plant and its potential for phytoremediation of cadmium pentachlorophenol (Cd-PCP) co-contaminated soil was investigated. A greenhouse study was executed to elucidate the effects of Cd (0, 10, and 20 mg kg^?1) without or with PCP (0, 50, and 250 mg kg^?1) on the growth of the wetland plant P. australis and its uptake, accumulation and removal of pollutant from soils. After 75 days, plant biomass was significantly influenced by interaction of Cd and PCP and the effect of Cd on plant growth being stronger than that of PCP. Coexistence of PCP at low level lessened Cd toxicity to plants, resulting in improved plant growth and increased Cd accumulation in plant tissues. The dissipation of PCP in soils was significantly influenced by interactions of Cd, PCP and plant presence or absence. As an evaluation of soil biological activities after remediation soil enzyme was measured.     

Interactive effects of selenium and arsenic on their uptake by Pteris vittata L. under hydroponic conditions

The interactive effects of selenium (Se) and arsenic (As) on plant uptake of Se and As have rarely been documented. In this study, the interactive effects of As and Se on their uptake by Chinese brake fern (Pteris vittata), an As-hyperaccumulator and Se-accumulator, were explored in two hydroponic experiments based on a two-factor, five-level central composite design. At Se levels of less than 2.5 mg L^?1, increasing amounts of As stimulated the uptake of Se in Chinese brake fern roots, possibly because of the beneficial effects of Se. In contrast, at Se concentrations greater than 2.5 mg L^?1, As suppressed the uptake of Se in Chinese brake fern roots. Uptake of As by both fronds and roots of Chinese brake fern was suppressed by the addition of Se, indicating the antagonistic effects of Se on As. In addition, at Se concentrations of less than 2.5 mg L^?1, As stimulated the translocation of Se from roots to fronds; meanwhile, the addition of Se resulted in reduced translocation of As from roots to fronds. These findings demonstrate the interactive effects of As and Se on their uptake by Chinese brake fern.     

Experimental study of the combined effects of pH and salinity on the bioaccumulation of inorganic mercury in the crayfish Astacus leptodactylus

ABSTRACT

The combined effects of pH and salinity were investigated on the bioaccumulation of inorganic mercury in the Turkish crayfish Astacus leptodactylus, at the whole organism and organ levels. Effects on the ionic balance were also analysed during the acclimatization phase and after 3 and 15 days of exposure, via the direct route. The experimental protocol, based on a complete factorial design, took into account nine experimental conditions, resulting from the combination of three levels for pH (6, 7.5 and 9) and for salinity (1, 10 and 100 mM Cl-). The results showed an important Hg accumulation in the organs or tissues in direct contact with the surrounding medium, e.g. gills and carapace. The effects of the nine pH and salinity conditions were complex and variable from one organ to another. Correlations were studied between chemical species concentrations in the water (HgCl₂, HgOHCl, HgCl₃-,HgCl₄², Hg(OH)₂) and amounts of metal accumulated in the different organs. A positive correlation was found between the neutral species HgCl₂ and metal accumulated in the gills and in the carapace. Perturbations of the haemolymph ionic concentrations were no significant, except for Na⁺ after 3 days of exposure.     

Phytoextraction of HG by parsley (Petroselinum crispum) and its growth responses

The effect of mercury (Hg) on the growth and survival of parsley (Petroselinum crispum) was explored at various treatments. The plants were grown in pots having Hoagland's solution to which various Hg treatments were applied and placed under greenhouse conditions. The treatments were: no metal applied (control) and six doses of Hg as mercuric chloride for 15 days. Linear trend of Hg accumulation was noted in roots, stems, and leaves with increasing Hg treatments. The maximum Hg concentration in root, stem and leaf was 8.92, 8.27, and 7.88 at Hg treatments of 25 mg l^–1, respectively. On the whole, Hg accumulation in different plant parts was in the following order: leaves > stem > roots. Linear trend was also observed for Bioaccumulation Factor (BF) and Translocation Factor (TF) with increasing Hg concentrations in the growth medium. The highest respective BF_Hg and TF_Hg values were 9.32 and 2.02 for the Hg treatments of 25 and 50 mg l^–1. In spite of the reduced growth in the presence of Hg, the plant has phytoremediation potential. It is recommended that parsley should not be cultivated in Hg contaminated sites in order to avoid dietary toxicity.     

Heavy-metal-contaminated industrial soil: Uptake assessment in native plant species from Brazilian Cerrado

Plants of the Cerrado have shown some potential for restoration and/or phytoremediation projects due to their ability to grow in and tolerate acidic soils rich in metals. The aim of this study is to evaluate the tolerance and accumulation of metals (Cd, Cu, Pb, and Zn) in five native tree species of the Brazilian Cerrado (Copaifera langsdorffii, Eugenia dysenterica, Inga laurina, Cedrela fissilis, Handroanthus impetiginosus) subjected to three experiments with contaminated soils obtained from a zinc processing industry (S₁, S₂, S₃) and control soil (S₀). The experimental design was completely randomized (factorial 5 × 4 × 3) and conducted in a greenhouse environment during a 90-day experimentation time. The plant species behavior was assessed by visual symptoms of toxicity, tolerance index (TI), translocation factor (TF), and bioaccumulation factor (BF). C. fissilis has performed as a Zn accumulator by the higher BFs obtained in the experiments, equal to 3.72, 0.88, and 0.41 for S₁, S₂, and S₃ respectively. This species had some ability of uptake control as a defense mechanism in high stress conditions with the best behavior for phytoremediation and high tolerance to contamination. With economical and technical benefits, this study may support a preliminary analysis necessary for using native tree species in environmental projects.     

Mercury uptake and phytotoxicity in terrestrial plants grown naturally in the Gumuskoy (Kutahya) mining area,Turkey

This study investigated mercury (Hg) uptake and transport from the soil to different plant parts by documenting the distribution and accumulation of Hg in the roots and shoots of 12 terrestrial plant species, all of which grow naturally in surface soils of the Gumuskoy Pb-Ag mining area. Plant samples and their associated soils were collected and analyzed for Hg content by ICP-MS. Mean Hg values in the soils, roots, and shoots of all plants were 6.914, 460, and 206 µg kg^?1, respectively and lower than 1. The mean enrichment factors for the roots (ECR) and shoots (ECS) of these plants were 0.06 and 0.09, respectively and lower than 1. These results show that the roots of the studied plants prevented Hg from reaching the aerial parts of the plants. The mean translocation factor (TLF) was 1.29 and higher than 1. The mean TLF values indicated that all 12 plant species had the ability to transfer Hg from the roots to the shoots but that transfer was more efficient in plants with higher ECR and ECS. Therefore, these plants could be useful for the biomonitoring of environmental pollution and for rehabilitating areas contaminated by Hg.     

Arsenic Uptake and Translocation by Plants in Pot and Field Experiments

A work undertaken by pot and field experiments to assess the suitability of poplars and ferns for the in-situ, phytoextraction, of a dumping site with residues from the roasting process of arseno-pyrite is reported. The main characteristic of this site is the high content of both the As metalloid and heavy metals (e.g., Al, Fe, Cu, Co, Cr, Pb). Two poplar clones (Populus deltoides ‘Dvina’ and Populus x canadensis ‘Orion’) and Pteris vittata (Chinese brake fern) were planted in the contaminated soil both ex situ in pots and in situ. Plant survival, As accumulation in plant tissues, leaf content of pigments, soluble proteins, activity of catalase and SH-groups in both roots and leaves were evaluated during a 24-month study period. Both poplar and fern plants exhibited an increase in the activity of catalase and SH group contents when grown in the presence of pyrite ashes. The results showed that the co-planting system (arsenic-hyperaccumulator fern Pteris vittata and Populus clones) was suitable for phytoextraction of multi-contaminated dumping sites. Agronomic measures such as irrigation, soil tillage and amendments also seem to be necessary for the successful establishment of poplar trees and ferns in contaminated soils in order to enhance plant growth through the improvement of soil conditions.     

Rhizosphere bacteria enhance the accumulation of selenium and mercury in wetland plants

The role of rhizosphere bacteria in facilitating Se and Hg accumulation in two wetland plants, saltmarsh bulrush (Scirpus robustus Pursh) and rabbitfoot grass (Polypogon monspeliensis (L.) Desf.), was studied. Ampicillin-amended plants (i.e., with inhibited rhizosphere bacteria) supplied with Na₂SeO₄ or HgCl₂ had significantly lower concentrations of Se and Hg, respectively, in roots than plants without ampicillin. These results were confirmed by inoculating axenic saltmarsh bulrush plants with bacteria isolated from the rhizosphere of plants collected from the field; these plants accumulated significantly more Se and Hg compared to axenic controls. Therefore, rhizosphere bacteria can increase the efficiency of Se and Hg phytoremediation by promoting the accumulation of Se and Hg in tissues of wetland plants. Received: 9 April 1999 / Accepted: 11 May 1999     

Effectiveness of (PhSe)2 in protect against the HgCl2 toxicity

This work investigated the preventive effect of diphenyl diselenide [(PhSe)₂] on renal and hepatic toxicity biomarkers and oxidative parameters in adult mice exposed to mercury chloride (HgCl₂). Selenium (Se) and mercury (Hg) determination was also carried out. Mice received a daily oral dose of (PhSe)₂ (5.0 mg/kg/day) or canola oil for five consecutive days. During the following five days, the animals were treated with a daily subcutaneous dose of HgCl₂ (5.0 mg/kg/day) or saline (0.9%). Twenty-four hours after the last HgCl₂ administration, the animals were sacrificed and biological material was obtained. Concerning toxicity biomarkers, Hg exposure inhibited blood δ-aminolevulinic acid dehydratase (δ-ALA-D), serum alanine aminotransferase (ALT) activity and also increased serum creatinine levels. (PhSe)₂ partially prevented blood δ-ALA-D inhibition and totally prevented the serum creatinine increase. Regarding the oxidative parameters, Hg decreased kidney TBARS levels and increased kidney non-protein thiol levels, while (PhSe)₂ pre-treatment partially protected the kidney thiol levels increase. Animals exposed to HgCl₂ presented Hg content accumulation in blood, kidney and liver. The (PhSe)₂ pre-treatment increased Hg accumulation in kidney and decreased in blood. These results show that (PhSe)₂ can be efficient in protecting against these toxic effects presented by this Hg exposure model.     

Mercury toxicity in plants

Mercury poisoning has become a problem of current interest as a result of environmental pollution on a global scale. Natural emissions of mercury form two-thirds of the input; manmade releases form about one-third. Considerable amounts of mercury may be added to agricultural land with sludge, fertilizers, lime, and manures. The most important sources of contaminating agricultural soil have been the use of organic mercurials as a seed-coat dressing to prevent fungal diseases in seeds. In general, the effect of treatment on germination is favorable when recommended dosages are used. Injury to the seed increases in direct proportion to increasing rates of application. The availability of soil mercury to plants is low, and there is a tendency for mercury to accumulate in roots, indicating that the roots serve as a barrier to mercury uptake. Mercury concentration in aboveground parts of plants appears to depend largely on foliar uptake of Hg⁰ volatilized from the soil. Uptake of mercury has been found to be plant specific in bryophytes, lichens, wetland plants, woody plants, and crop plants. Factors affecting plant uptake include soil or sediment organic content, carbon exchange capacity, oxide and carbonate content, redox potential, formulation used, and total metal content. In general, mercury uptake in plants could be related to pollution level. With lower levels of mercury pollution, the amounts in crops are below the permissible levels. Aquatic plants have shown to be bioaccumulators of mercury. Mercury concentrations in the plants (stems and leaves) are always greater when the metal is introduced in organic form. In freshwater aquatic vascular plants, differences in uptake rate depend on the species of plant, seasonal growthrate changes, and the metal ion being absorbed. Some of the mercury emitted from the source into the atmosphere is absorbed by plant leaves and migrates to humus through fallen leaves. Mercury-vapor uptake by leaves of the C₃ speciesoats, barley, and wheat is five times greater than that by leaves of the C₄ species corn, sorghum, and crabgrass. Such differential uptake by C₃ and C₄ species is largely attributable to internal resistance to mercury-vapor binding. Airborne mercury thus seems to contribute significantly to the mercury content of crops and thereby to its intake by humans as food. Accumulation, toxicity response, and mercury distribution differ between plants exposed through shoots or through roots, even when internal mercury concentrations in the treated plants are similar. Throughfall and litterfall play a significant role in the cycling and deposition of mercury. The possible causal mechanisms of mercury toxicity are changes in the permeability of the cell membrane, reactions of sulphydryl (-SH) groups with cations, affinity for reacting with phosphate groups and active groups of ADP or ATP, and replacement of essential ions, mainly major cations. In general, inorganic forms are thought to be more available to plants than are organic ones. Plants can be exposed to mercurials either by direct administration as antifungal agents, mainly to crop plants through seed treatment or foliar spray, or by accident. The end points screened are seed germination, seedling growth, relative growth of roots and shoots, and, in some case, studies of leaf-area index, internode development, and other anatomical characters. Accidental exposures occur through soil, water, and air pollution. The level of toxicity is usually tested under laboratory conditions using a wide range of concentrations and different periods of exposure. Additional parameters include biochemical assays and genetical studies. The absorption of organic and inorganic mercury from soil by plants is low, and there is a barrier to mercury translocation from plant roots to tops. Thus, large increases in mercury levels in soil produce only modest increases in mercury levels in plants by direct uptake from soil. Injuries to cereal seeds caused by organic mercurials has been characterized by abnormal germination and hypertrophy of the roots and coleoptile. Mercury affects both light and dark reactions of photosynthesis. Substitution of the central atom of chlorophyll, magnesium, by mercury in vivo prevents photosynthetic light harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis. The reaction varies with light intensity. A concentration and time-dependent protective effect of GSH seems to be mediated by the restricted uptake of the metal involving cytoplasmic protein synthesis. Plant cells contain aquaporins, proteins that facilitate the transport of water, in the vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive, and in AQP1 a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-Pro-Ala motif. At low concentrations mercury has a toxic effect on the degrading capabilities of microorganisms. Sensitivity to the metal can be enhanced by a reduction in pH, and tolerance of mercury by microorganisms has been found to be in the order: total population > nitrogen fixers > nitrifiers. Numerous experiments have been carried out to study the genetic effects of mercury compounds in experimental test systems using a variety of genetic endpoints. The most noticeable and consistent effect is the induction of c-mitosis through disturbance of the spindle activity, resulting in the formation of polyploid and aneuploid cells and c-tumors. Organomercurials have been reported to be 200 times more potent than inorganic mercury. Exposure to inorganic mercury reduces mitotic index in the root-tip cells and increases the frequency of chromosomal aberrations in degrees directly proportional to the concentrations used and to the duration of exposure. The period of recovery after removal of mercury is inversely related to the concentration and duration of exposure. Bacterial plasmids encode resistance systems for toxic metal ions, including Hg²⁺, functioning by energy-dependent efflux of toxic ions through ATPases and chemiosmotic cationproton antiporters. The inducible mercury resistance (mer) operon encodes both a mercuric ion uptake and detoxification enzymes. In gram-negative bacteria a periplasmic protein,MerP, an inner-membrane transport protein,MerT, and a cytoplasmic enzyme, mercuric reductase, theMerA protein, are responsible for the transport of mercuric ions into cells and their reduction to elemental mercury, Hg(II). InThiobacillus ferrooxidans, an acidophilic chemoautotrophic bacterium sensitive to mercury ions, a group of mercury-resistant strains, which volatilize mercury, has been isolated. The entire coding sequence of the mercury-ion resistance gene has been located in a 2.3 kb fragment of chromosomal DNA (encoding 56,000 and 16,000 molecular-weight proteins) from strain E-l 5 ofEscherichia coli. Higher plants andSchizosaccharomyces pombe respond to heavy-metal stress of mercury by synthesizing phytochelatins (PCs) that act as chelators. The strength of Hg(II) binding to glutathione and phytochelatins follows the order: γGlu-Cys-Gly(γGlu-Cys)₂Gly(γGlu-Cys)₃Gly(γGlu-Cys)₄Gly. Suspension cultures of haploid tobacco,Nicotiana tabacum, cells were subjected to ethyl methane sulfonate to raise mercury-tolerant plantlets. HgCl₂-tolerant variants were selected from nitrosoguanidine (NTG)-treated suspension cell cultures of cow pea,Vigna unguiculata, initiated from hypocotyl callus and incubated with 18 ⧎g/ml HgCl₂. Experiments have been carried out to develop mercury-tolerant plants ofHordeum vulgare through previous exposure to low doses of mercury and subsequent planting of the next generation in mercury-contaminated soil. Phytoremediation involves the use of plants to extract, detoxify, and/or sequester environmental pollutants from soil and water. Transgenic plants cleave mercury ions from methylmercury complexes, reduce mercury ions to the metallic form, take up metallic mercury through their roots, and evolve less toxic elemental mercury. Genetically engineered plants contain modified forms of bacterial genes that break down methyl mercury and reduce mercury ions. The first gene successfully inserted into plants wasmerA, which codes for a mercuric ion reductase enzyme, reducing ionic mercury to the less toxic elemental form.MerB codes for an organomercurial lyase protein that cleaves mercury ions from highly toxic methyl mercury compounds. Plants with themerB gene have been shown to detoxify methyl mercury in soil and water. Both genes have been successfully expressed inArabidopsis thaliana, Brassica (mustard),Nicotiana tabacum (tobacco), andLiriodendron tulipifera (tulip poplar). Plants currently being transformed include cattails, wild rice, andSpartina, another wetland plant. The problem of mercury contamination can be reduced appreciably by combining the standard methods of phytoremediation—removal of mercury from polluted areas through scavenger plants—with raising such plants both by routine mutagenesis and by genetic engineering. The different transgenics raised utilizing the two genesmerA andmerB are very hopeful prospects.