Mechanisms Of Metal Accumulation Research Articles

Bioremediation of Heavy Metals from Soil and Aquatic Environment: An Overview of Principles and Criteria of Fundamental Processes

Heavy metals are natural constituents of the environment, but indiscriminate use for human purposes has altered their geochemical cycles and biochemical balance. This results in excess release of heavy metals such as cadmium, copper, lead, nickel, zinc etc. into natural resources like the soil and aquatic environments. Prolonged exposure and higher accumulation of such heavy metals can have deleterious health effects on human life and aquatic biota. The role of microorganisms and plants in biotransformation of heavy metals into nontoxic forms is well-documented, and understanding the molecular mechanism of metal accumulation has numerous biotechnological implications for bioremediation of metal-contaminated sites. In view of this, the present review investigates the abilities of microorganisms and plants in terms of tolerance and degradation of heavy metals. Also, advances in bioremediation technologies and strategies to explore these immense and valuable biological resources for bioremediation are discussed. An assessment of the current status of technology deployment and suggestions for future bioremediation research has also been included. Finally, there is a discussion of the genetic and molecular basis of metal tolerance in microbes, with special reference to the genomics of heavy metal accumulator plants and the identification of functional genes involved in tolerance and detoxification.

Improving the phytoextraction capacity of plants to scavenge metal(loid)-contaminated sites

Plants are able to extract metal(loid) contaminants from the soil or water through their roots and translocate them to harvestable aerial shoots. Of late, this plant potential has been used as a phytotechnology, termed as phytoextraction, for cleaning contaminated sites, and this process has successfully removed elements like As, Cd, Cu, Ni, and Pb, among others. Exploring plants with high metal-accumulation capacity, as well as engineering new hyperaccumulators, is a need of the hour. It is assumed that hyperaccumulators have a >1 shoot:root metal-accumulation ratio, which they achieve by way of (i) overexpression of transport systems for improved sequestration, (ii) tissue-specific protein expression, and (iii) high concentration of metal chelators. Unlike nonhyperaccumulators, the hyperaccumulating species normally bind metal ions to weak oxygen ligands and use strong ligands only for transient binding during transport to storage sites. Adequate understanding of genetics, biochemistry, and molecular biology of metal accumulation is a prelude to developing transgenics with improved phytoremediation capacity. Current research in plant breeding, genomics, and proteomics suggest promising leads to the creation of “remediation” cultivars. Several transporter genes associated with metal uptake, transport, and accumulation have been identified. Efforts are underway to enhance the phytoextraction capacity of relevant species, not only by using chelating agents but also by attempting hybridization, protoplast fusion, as well as genetic engineering through novel gene transfer, overexpression of genes, and (or) reverse gene insertion, to enhance (i) transpiration rate; (ii) uptake, translocation, and metabolism of metals; (iii) activity of enzymes related to rate-limiting steps; and (iv) transformation of accumulated metal to volatile forms, and (or) silencing gene(s) that encode proteases. Genome evolution in hyperaccumulators needs to be understood through a systematic study of ecological and molecular genomics. Sequencing of a complete genome of hyperaccumulators can help in identifying the promising functional noncoding regions in the genome, thus making the experimental analysis more accurate. In addition to the constitutive overexpression of a single gene, simultaneous expression of several genes in specific cellular components has to be focused. Other areas that require expert attention include identification of metal-transporter proteins and the introduction of genes encoding the metal transporters, overexpression of metallothioneins and phytochelatin synthase, and overproduction of nicotianamine and histidine in plants. A comprehensive study of transgenic gene frequency, covering several plant generations growing on polluted as well as nonpolluted soils, may assess the possibility of gene escape into the environment and its transfer to the microorganisms present in the surroundings. This review attempts not only to collect and collate information available on mechanisms of metal accumulation and detoxification in plants and on the factors affecting the tolerance and phytoextraction capacity of plants but also the strategies that have been or can be devised for raising novel plant genotypes with elevated capacity of metal accumulation and toxicity tolerance.

Vermicomposting of Tea Factory Coal Ash: metal accumulation and metallothionein response in Eisenia fetida (Savigny) and Lampito mauritii (Kinberg).

Earthworms can accumulate heavy metals in their intestines to a great extent. Impact of feed materials and duration of metal exposure on natural activity of earthworms are rather unclear; this investigation therefore addresses the impact of metal rich Tea Factory Coal Ash (TFCA) on reproduction, composting and metal accumulation ability of Eisenia fetida and Lampito mauritii. Earthworm count and cocoon production increased significantly during vermicomposting. pH of the vermicomposted mixtures shifted toward neutrality, total organic C decreased substantially and total N enhanced significantly compared to composting. High heavy metal (Mn, Zn, Cu, As) accumulation was recorded in the intestine of both the earthworm species. Moreover, gradual increase in the metal-inducible metallothionein concentration indicated the causal mechanism of metal accumulation in these species. TFCA+cow dung (CD) (1:1) were most favorable feed mixture for E. fetida and TFCA+CD (1:2) were good for L. mauritii in regard to metal accumulation and compost quality.

Bioaccumulation of heavy metals in Spartina.

The Spartina Schreb. genus is composed of C4 perennial grasses in the family Poaceae. They are native to the coasts of the Atlantic Ocean in western and southern Europe, north-west and southern Africa, the Americas and the southern Atlantic Ocean islands. Most species are salt tolerant and colonise coastal or inland saltmarshes. The available literature on heavy metal bioaccumulation by Spartina sp. was compiled and compared. Spartina alterniflora Loisel. and Spartina maritima (Curtis) Fernald were the most commonly researched species of the genus, whereas many species were not represented at all. In contrast, Cu and Zn are the most intensively researched heavy metals. The few studies dealing with the physiological impacts of heavy metals or the mechanisms of metal accumulation, which involve extracellular and intracellular metal chelation, precipitation, compartmentalisation and translocation in the vascular system, were documented. Bioaccumulation of metals in roots and tillers of some species of the Spartina genus (e.g. S. maritima and Spartina densiflora Brongn.) has been described as a feasible method for remediating waters and soils contaminated with heavy metals. One such example is Spartina argentinensis Parodi, which has been found to be a Cr-hyperaccumulator; it can concentrate chromium in its tissues to levels far exceeding those present in the soil.

Trace metal concentrations in the tropical sponge Spheciospongia vagabunda at a sewage outfall: synchrotron X-ray imaging reveals the micron-scale distribution of accumulated metals

Major and trace elements were measured in sponges, seawater and sediment in Darwin Harbour (Australia) to test the hypothesis that metals are elevated in sponges closer to a sewage outfall compared with unimpacted sites. Seawater and sediment at the sewage discharge site contained high, but localised, concentrations of phosphorus (P), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As) and lead (Pb) compared with background sites. Metal concentrations in the sponge Spheciospongia vagabunda were highly elevated compared with other sponges and, although site specific, high metal concentrations were unrelated to the presence of sewage effluent. X-ray fluorescence microprobe imaging was used to investigate the metal distribution pattern in S. vagabunda. High Fe, Ni and Zn concentrations were either localised in circular patches (100–200 μm size) near water canals or in the pinacoderm, or scattered in spots (approximately 10 μm) throughout the tissue. This supports a microflora-mediated metal bioaccumulation hypothesis. In contrast, Co and Mn were highly dispersed and probably associated with aluminium- and iron-oxide rich sediment inclusions. Although the lack of association between sewage effluent and metal accumulation precludes the use of S. vagabunda as a biomonitor, the apparent differential mechanisms of metal accumulation warrants further investigation.

Utilising caging techniques to investigate metal assimilation in Nucella lapillus, Mytilus edulis and Crassostrea gigas at three Irish coastal locations

Pollution by metals has been of increasing concern for a number of decades but at present, the mechanism of metal accumulation in sentinel species is not fully understood and further studies are required for environmental risk assessment of metals in aquatic environments. The use of caging techniques has proven to be useful for assessment of water quality in coastal and estuarine environments. This study investigates the application of caging techniques for monitoring uptake of 20 elements [Li, Na, Mg, Al, P, K, Ca, V, Cr, Mn, Fe, Ni, Co, Cu, As, Sb, Pb, Hg, Cd and Zn] in three marine species namely Nucella lapillus, Mytilus edulis and Crassostrea gigas. Stable isotopes were used to determine predatory effects and also used for modelling metal uptake in test species and to track nutrient assimilation. Metal levels were monitored at three different coastal locations, namely Dublin Bay, Dunmore East and Omey Island over 18 weeks. Significant differences in concentrations of Mn, Co and Zn between mussels and oysters were found. Correlations between cadmium levels in N. lapillus and δ13C and δ15N suggest dietary influences in Cd uptake. Levels of Zn were highest in C. gigas compared to the other two species and levels of Zn were most elevated at the Dunmore East site. Copper levels were more elevated in all test species at both Dublin Bay and Dunmore East. Mercury was raised in all species at Dunmore East compared to the other two sites. Biotic accumulation of metals in the test species demonstrates that caging techniques can provide a valid tool for biomonitoring in metal impacted areas.

Implications of metal accumulation mechanisms to phytoremediation

Trace elements (heavy metals and metalloids) are important environmental pollutants, and many of them are toxic even at very low concentrations. Pollution of the biosphere with trace elements has accelerated dramatically since the Industrial Revolution. Primary sources are the burning of fossil fuels, mining and smelting of metalliferous ores, municipal wastes, agrochemicals, and sewage. In addition, natural mineral deposits containing particularly large quantities of heavy metals are found in many regions. These areas often support characteristic plant species thriving in metal-enriched environments. Whereas many species avoid the uptake of heavy metals from these soils, some of them can accumulate significantly high concentrations of toxic metals, to levels which by far exceed the soil levels. The natural phenomenon of heavy metal tolerance has enhanced the interest of plant ecologists, plant physiologists, and plant biologists to investigate the physiology and genetics of metal tolerance in specialized hyperaccumulator plants such as Arabidopsis halleri and Thlaspi caerulescens. In this review, we describe recent advances in understanding the genetic and molecular basis of metal tolerance in plants with special reference to transcriptomics of heavy metal accumulator plants and the identification of functional genes implied in tolerance and detoxification. Plants are susceptible to heavy metal toxicity and respond to avoid detrimental effects in a variety of different ways. The toxic dose depends on the type of ion, ion concentration, plant species, and stage of plant growth. Tolerance to metals is based on multiple mechanisms such as cell wall binding, active transport of ions into the vacuole, and formation of complexes with organic acids or peptides. One of the most important mechanisms for metal detoxification in plants appears to be chelation of metals by low-molecular-weight proteins such as metallothioneins and peptide ligands, the phytochelatins. For example, glutathione (GSH), a precursor of phytochelatin synthesis, plays a key role not only in metal detoxification but also in protecting plant cells from other environmental stresses including intrinsic oxidative stress reactions. In the last decade, tremendous developments in molecular biology and success of genomics have highly encouraged studies in molecular genetics, mainly transcriptomics, to identify functional genes implied in metal tolerance in plants, largely belonging to the metal homeostasis network. Analyzing the genetics of metal accumulation in these accumulator plants has been greatly enhanced through the wealth of tools and the resources developed for the study of the model plant Arabidopsis thaliana such as transcript profiling platforms, protein and metabolite profiling, tools depending on RNA interference (RNAi), and collections of insertion line mutants. To understand the genetics of metal accumulation and adaptation, the vast arsenal of resources developed in A. thaliana could be extended to one of its closest relatives that display the highest level of adaptation to high metal environments such as A. halleri and T. caerulescens. This review paper deals with the mechanisms of heavy metal accumulation and tolerance in plants. Detailed information has been provided for metal transporters, metal chelation, and oxidative stress in metal-tolerant plants. Advances in phytoremediation technologies and the importance of metal accumulator plants and strategies for exploring these immense and valuable genetic and biological resources for phytoremediation are discussed. A number of species within the Brassicaceae family have been identified as metal accumulators. To understand fully the genetics of metal accumulation, the vast genetic resources developed in A. thaliana must be extended to other metal accumulator species that display traits absent in this model species. A. thaliana microarray chips could be used to identify differentially expressed genes in metal accumulator plants in Brassicaceae. The integration of resources obtained from model and wild species of the Brassicaceae family will be of utmost importance, bringing most of the diverse fields of plant biology together such as functional genomics, population genetics, phylogenetics, and ecology. Further development of phytoremediation requires an integrated multidisciplinary research effort that combines plant biology, genetic engineering, soil chemistry, soil microbiology, as well as agricultural and environmental engineering.

Characteristics of copper tolerance in Yarrowia lipolytica

We discovered that a mutant strain of the dimorphic yeast Yarrowia lipolytica could grow in the yeast form in high concentrations of copper sulfate. The amount of metal accumulated by Y. lipolytica increased with increasing copper concentrations in the medium. Washing with 100 mM EDTA released at least 60% of the total metal from the cells, but about 20-25 micromol/g DW persisted, which represented about 30% of the soluble fraction of cultured cells. The soluble fraction (mainly cytosol) contained only about 10% of the total metal content within cells cultured in medium supplemented with 6 mM copper. We suggest that although a high copper concentration induces an efflux mechanism, the released copper becomes entrapped in the periplasm and in other parts of the cell wall. Washing with EDTA liberated not only copper ions, but also melanin, a brown pigment that can bind metal and which located at the cell wall. These findings indicated that melanin participates in the mechanism of metal accumulation. Culture in medium supplemented with copper obviously enhanced the activities of Cu, Zn-SOD, but not of Mn-SOD.

Metal Accumulation Mechanisms in Ascidians and its Application to Selective Accumulation of Rare Metals in Seawater

Metal Accumulation Mechanisms in Ascidians and its Application to Selective Accumulation of Rare Metals in Seawater

Re-evaluation of metal bioaccumulation and chronic toxicity in Hyalella azteca using saturation curves and the biotic ligand model

Bioaccumulation by Hyalella of all metals studied so far in our laboratory was re-evaluated to determine if the data could be explained satisfactorily using saturation models. Saturation kinetics are predicted by the biotic ligand model (BLM), now widely used in modelling acute toxicity, and are a pre-requisite if the BLM is to be applied to chronic toxicity. Saturation models provided a good fit to all the data. Since these are mechanistically based, they provide additional insights into metal accumulation mechanisms not immediately apparent when using allometric models. For example, maximum Cd accumulation is dependent on the hardness of the water to which Hyalella are acclimated. The BLM may need to be modified when applied to chronic toxicity. Use of saturation models for bioaccumulation, however, also necessitates the need for using saturation models for dose–response relationships in order to produce unambiguous estimates of LC50 values based on water and body concentrations. This affects predictions of toxicity at very low metal concentrations and results in lower predicted toxicity of mixtures when many metals are present at low concentrations.

Enzymically mediated bioprecipitation of uranium by a Citrobacter sp. : a concerted role for exocellular lipopolysaccharide and associated phosphatase in biomineral formation.

A Citrobacter sp. accumulated uranyl ion (UO2(2+)) via precipitation with phosphate ligand liberated by phosphatase activity. The onset and rate of uranyl phosphate deposition were promoted by NH4(+), forming NH(4)UO(2)PO(4), which has a lower solubility product than NaUO(2)PO(4). This acceleration decoupled the rate-limiting chemical crystallization process from the biochemical phosphate ligand generation. This provided a novel approach to monitor the cell-surface-associated changes using atomic-force microscopy in conjunction with transmission electron microscopy and electron-probe X-ray microanalysis, to visualize deposition of uranyl phosphate at the cell surface. Analysis of extracted surface materials by (31)P NMR spectroscopy showed phosphorus resonances at chemical shifts of 0.3 and 2.0 p.p.m., consistent with monophosphate groups of the lipid A backbone of the lipopolysaccharide (LPS). Addition of fUO2(2+) to the extract gave a yellow precipitate which contained uranyl phosphate, while addition of Cd(2+) gave a chemical shift of both resonances to a single new resonance at 3 p.p.m. Acid-phosphatase-mediated crystal growth exocellularly was suggested by the presence of acid phosphatase, localized by immunogold labelling, on the outer membrane and on material exuded from the cells. Metal deposition is proposed to occur via an initial nucleation with phosphate groups localized within the LPS, shown by other workers to be produced exocellularly in association with phosphatase. The crystals are further consolidated with additional, enzymically generated phosphate in close juxtaposition, giving high loads of LPS-bound uranyl phosphate without loss of activity and distinguishing this from simple biosorption, or periplasmic or cellular metal accumulation mechanisms. Accumulation of 'tethered' metal phosphate within the LPS is suggested to prevent fouling of the cell surface by the accumulated precipitate and localization of phosphatase exocellularly is consistent with its possible functions in homeostatis and metal resistance.

Trace metals in oysters and sediments of Botany Bay, Sydney

Trace metal concentrations (Pb, Cd, Cu and Zn) in Sydney rock oysters ( Sacostera commercialis) and sediments (<53 μm fraction) were determined for six sites in the northern regions of Botany Bay. Levels for lead, cadmium, copper and zinc in oysters ranged (in μg g −1) from 1.38 to 15.3, 1.81 to 16.3, 56.1 to 212 and 1806 to 2902, respectively. In sediments, levels ranged (in μg g −1) from 599 to 4081, 3.57 to 91.0, 191 to 1113 and 227 to 1472, respectively. Such values indicated high levels of contamination, especially in the sediments. Indeed, high proportions of the samples displayed metal concentrations that exceeded the prescribed limits for oysters and sediments. No significant correlations in metal concentrations were found between oysters and sediments, suggesting that changes in the sediment metal loading are not solely influencing the levels of bioavailable metal. The results supported the conclusion that different rates and mechanisms of metal accumulation are taking place in the two types of samples. Variability between sites was high, particularly for oysters. Multidimensional scaling identified that the Cooks River (site 3) and La Perouse (site 6) sites were most dissimilar, both to each other as well as to the other four sites. This was a reflection of high contamination in the Cooks River and the generally low levels in the La Perouse reference sample. The configuration was mainly influenced by the sediment parameters, rather than the oyster metal concentrations, indicating the sediment data were better for identifying site similarities. These ordinations provide evidence of the usefulness of multidimensional scaling in elucidating the physico–chemical variability of the sampling sites.

An Evaluation of Fresh Water Sediments Contamination: the Lachine Canal Sediments Case, Montréal, Canada. Part I: Quality Assessment

Due to industrialization over the past 100 years, the sediments at the bottom of the Lachine Canal and Bay have become contaminated. This study investigated the heavy metal accumulation mechanisms within the Lachine Canal sediments and was carried out in three phases. Phase I is covered in the present article, while phases II and III will be discussed in a following paper. During Phase I, 28 parameters were measured on 44 sediment samples collected along the Lachine Canal and Bay. The results revealed the presence of a wide variety of both organic and inorganic contaminants. Heavy metals such as Zn < Pb < Cr < Ni < Cu < Cd (in order of decreasing abundance) in addition to organic contaminants (PAHs < MAHs < PCBs) were found in high concentrations exceeding background concentrations and various quality criteria levels. The total load of both organic and inorganic contaminants was found to be higher in the canal than in the bay zone. Correlation analysis was performed to evaluate sediment assessment values with respect to known adsorptive phases such as % clay fraction, TOC (Total Organic Carbon), and % CaCO3. Although a positive correlation existed between % clay fraction or TOC and the total cumulative load of heavy metals, the correlation could not be assessed as conclusive. Significant correlations were obtained between Pb, Ni, and Zn and the carbonate content of the sediments. Pollution Indices (PI) were used according to the format proposed by Canadian authorities and as such were evaluated as incomplete.

Subcellular distribution of accumulated heavy metals in Saccharomyces cerevisiae and Kluyveromyces marxianus

Kluyveromyces spp. have been found to be more efficient than a CUP1R strain of S. cerevisiae in heavy metal resistance and accumulation. The present study describes the subcellular distribution of the accumulated metals (Ag, Cd, Cu) in S. cerevisiae and K. marxianus. Absorption by insoluble cellular material of the metals appears as the main mechanism of metal accumulation in both organisms.

Bioconcentration of heavy metals by plants

Abstract Certain plants can concentrate essential and non-essential heavy metals in their roots and shoots to levels far exceeding those present in the soil. Metal-accumulating plant species are invariably restricted to metalliferous soils found in different regions around the world. The mechanisms of metal accumulation, which involve extracellular and intracellular metal chelation, precipitation, compartmentalization and translocation in the vascular system, are poorly understood. Interest in these mechanisms has led to the development of phytoremediation—a new technology to use plants to clean up soil and water contaminated with heavy metals.

Radiation-chemical decomposition of heavy metals azides; Silver azide radiolysis kinetics

The complex kinetics study of radiation-chemical silver azide decomposition in anion and cation sublattices has been made using three mutually complemented techniques: nitrogen detection evolving into the environment, nitrogen kept by the crystal lattice after dissolving the irradiated crystal, and chemical analysis of silver separated after dissolving irradiated samples in a variety of solvents. The dissolution of irradiated silver azide was stated to have been complicated by complex secondary processes catalyzed with radiolytic silver particles. The possible model of those processes resulting in silver particle autodevelopment has been suggested. Kinetics mechanisms of metal accumulation and nitrogen kept by the crystal lattice have been considered. The facts regarding silver particle formation on the surface impurity centers located in the deformed crystal areas in the vicinity of surface macrodefects have been deduced. The parameters of formation stages and radiolytic silver particle growth have been estimated. The comparable analysis of silver azide radiolysis kinetics in anion and cation sublattices has been made and the relationship of these processes during silver particle formation has been obtained.

びわ湖柱状たい積物中の重金属元素（マンガン，鉄，ヒ素，カドミウム，鉛，銅，亜鉛，コバルト，ニッケル）の形態分別

In order to investigate the mechanism of metal accumulation in the bottom sediment of Lake Biwa, two 22 cm long sediment cores from the center (about 90 meters in water depth) of the northern part of the lake were sliced into 2 cm sections and analyzed for Mn, Fe, As, Cd, Pb, Cu, Zn, Co and Ni. Partitioning of these heavy metals into mineral and organic fractions was determined with selective chemical leaching techniques for core samples. The total contents of heavy metals in the sediment core samples were classified into two groups on the basis of vertical distribution : (1) Manganese, As, and to a lesser degree Cd, Pb, Cu and Zn were concentrated in the upper 2 cm of the core. (2) Iron, Co and Ni showed no vertical gradient. Manganese and Cd were enriched in both the exchangeable and hydrous Fe/Mn oxide fractions in the upper 2 cm layer of the core samples. Arsenic, Pb, Cu and Zn were concentrated in the hydrous Fe/Mn oxide fraction in the surface sediment layer. The heavy metal contents of sulfide, organic and silicate fractions were relatively constant regardless of depth. The remarkable enrichment of Mn and As in the oxidized upper layer of the sediment is assumed to be mainly due to upward migration of dissolved Mn and As from the deeper, reduced sediment layers and to reprecipitation in the thin oxidized surface layer. High concentrations of Cd, Pb, Cu and Zn in the upper 2 cm are presumably due mainly to upward migration in addition to human activities. These results suggest that hydrous Fe/Mn oxides play an important role in the sedimentation and surface enrichment of heavy metals in the sediment of Lake Biwa.