Temporal and spatial trends in heavy metal concentrations in the marine mussel Mytilus edulis from the Western Scheldt estuary (The Netherlands)

Since the first North Sea Conference (1984, Bremen), all countries bordering the North Sea made commitments to reduce discharge of hazardous substances into the North Sea. From Belgium and The Netherlands, large reductions (upto 90) in heavy metal emissions from land-based sources have been reported between 1985 and 2000. Recently, some studies in the Western Scheldt estuary (WS) have shown that total metal concentration in the water, sediments and suspended particles have decreased compared to levels in the 70s. However, data on aquatic organisms is still very limited and it is therefore difficult to confirm whether the reductions in pollution input and generally improving water quality in the WS have a corresponding impact on the levels of heavy metals in aquatic organisms. The current study measured metal concentrations in the soft tissues of mussels, Mytilus edulis (known to be good indicators of environmental metal pollution) during the period 1996–2002. Spatial (salinity and pollution gradients), temporal and seasonal variations were also studied. Results showed a down-stream decreasing trend for the metals studied (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) during all sampling campaigns. There was also a significant seasonal effect on tissue metal concentrations, with a peak observed around spring in both WS and the nearby less polluted Eastern Scheldt (ES). On temporal trends, a clear drop of metals in mussels was observed in the early 80s coinciding with the start of the efforts to reduce chemical pollution input into the North Sea. Since those early reductions, metal concentrations in mussels generally remained unchanged upto mid 90s. However, in recent times metal concentration in mussels have increased significantly, for example Cd in 2002 was almost 10 times the values in 1983 and similar to levels seen during the peak in the 70s. Other metals also increased in the 90s also reaching levels seen in the 70s. As there is no indication of recent increase in metal input into the estuary, we suggest that increased metal concentrations observed in mussels in recent years especially in the upper estuary are most likely a result of changes in physical and chemical speciation and metal bioavailability. Such changes may be caused by changes in some water quality parameters in the estuary (i.e. increased dissolved oxygen, concentration of organic matter), resulting in conditions that favour releases of sediment-bound metals into the water column. The relationship between metal content and season showed very similar annual profiles in the polluted WS and less polluted ES. Thus, seasonal variations in metal concentrations appear to be largely controlled by biological processes, while total body burdens are dependent on environmental levels and bioavailability.     

Modelling of water and sediment quality in the Scheldt estuary

In 1987 the Tidal Waters Division (Ministry of Transport, Public Works and Water Management, The Netherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt). In the framework of this project, a water quality model for the Scheldt estuary was made. The model aims to establish the relation between pollution inputs and the concentration of pollutants in the abiotic compartments and to establish the fate of wasted substances. The model includes oxygen, nitrogen and trace metals. Due to the low oxygen content of the upper Scheldt estuary, a new model approach for trace metals had to be developed, taking into account precipitated metal sulphides. The calibration, verification and sensitivity analysis of the model provided a good understanding of the chemistry of the estuary. Afterwards the model was used to support policy making by computing how the water and sediment quality in the estuary respond to reduced inputs of waste.     

Dissolved and particulate trace metal geochemistry in the scheldt estuary,S. W. Netherlands (water column and sediments)

The geochemistry of dissolved and particulate trace metals has been studied in the water column and the sediments of the Scheldt estuary between 1987 and 1990. A strong seasonal influence on the behaviour of dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phytoplankton activity in the lower estuary (which are both seasonally dependent variables). The dissolved trace metal concentrations in the fresh water end-member are remarkably low during spring and summer, due to metal sulphide precipitation in the anoxic Scheldt river. However, the dissolved concentrations increase rapidly with increasing salinity, due to oxidation of metal sulphides that are present in the suspended matter, accompanied by (e.g. chloro-)complexation of the released metals. Readsorption of Cd and Zn occurs in the lower estuary during the spring phytoplankton bloom. During winter, when the Scheldt river is not completely anoxic, much higher dissolved trace metal concentrations are observed in the fresh water end-member since metal sulphide precipitation in the water column is precluded. Rapid trace metal removal is observed in the low salinity, high turbidity zone, due to adsorption onto suspended matter and freshly precipitated iron and manganese oxyhydroxides. Upon further mixing, desorption is apparent, due to a similar oxidation-complexation mechanism as observed during spring and summer. Pore water infusion may also contribute to the enrichment of dissolved Cd, Cu and Zn in the mid-estuarine region. The trace metal contents of the suspended matter and the sediments show a continuous decrease with increasing salinity. This behaviour is to a very large extent due to physical mixing of contaminated fluvial particulates and relatively unpolluted marine particulates. Desorption of Cd, Cu and Zn can be identified but is of minor importance compared to the conservative mixing process. The distribution of dissolved Cd, Cu and Zn in the pore waters of the mid-estuarine region reflects the impact of early diagenetic processes. Trace metal peaks are observed near the sediment-water interface, and at greater depth in the manganese and iron reduction zones. These peaks are attributed to oxidation of reduced trace metal compounds (e.g. sulphides) and reduction of the (iron and manganese) oxide carrier phases, respectively. At greater depth, the dissolved trace metal concentrations are much lower due to metal sulphide precipitation in the sulphate reduction zone. Analysis of a large sediment dataset indicates severe trace metal pollution of the Scheldt estuary at the end of the fifties. A major reduction of the pollution by As, Cr, Hg, Pb, and Zn has occurred in the seventies, and of Cd and Cu in the eighties. The Ni pollution has increased over the time period considered. In spite of this improvement, the present-day pollution status of the Scheldt estuary is still reason for concern.     

The impact of the Scheldt input on the trace metal distribution in the Belgian coastal area (results of 1981–1983 and 1995–1996)

Offshore fluxes of Cu, Zn, Cd, Pb and Hg were calculated based onresidual flow patterns and salinity gradients along the Belgian coast. Theresidual flow lines along the Belgian coast are more or less parallel to thecoast except in the area where the north-easterly flowing watermass comingfrom the Channel encounters the south-westerly-oriented Scheldt outflow,forming a residual hydrodynamical front. From the steady-state salinitypattern, diffusion coefficients perpendicular to the residual flow werededuced; they ranged from 21 to 108 m² s^-1. Offshore fluxes of dissolved and particulate trace metals based on diffusiveand mixing processes are calculated. The steady state profiles of dissolvedmetals show a dilution effect in the coastal waters, reaching an almostconstant concentration in the marine watermass in the 1981–1983dataset. The ratios of the Scheldt input of trace metals to the totaldissolved offshore flux vary from 38 to 55% (1981–1983),depending on the kind of metal, and from 55 to 91% (1995–1996).The ratio of the Scheldt input to the dissolved metal flow parallel to thecoast, is in both periods (1981–1983 and 1995–1996), smallerthan 1%. The steady-state concentration profiles of particular metalsversus salinity are fairly constant in the coastal-estuarine and marinewatermasses, but decrease very abruptly from the first to the secondwatermass. Assuming a conservative behaviour of the particular metals,offshore fluxes and the resulting concentration increases agree fairly wellwith the observed values. The ratios of the Scheldt input to the particulatetrace metal offshore flux vary between 30 to 46% (1981–1983)and 13 to 37% (1995–1996). The contribution of the Scheldtestuary to the flows parallel to the coast ranges from 1.6 to 2.9%(1981–1983) and from 0.6 to 1.6% (1995). This revised version was published online in July 2006 with corrections to the Cover Date.     

Application of in situ cage cultures of phytoplankton for monitoring heavy metal pollution in two Norwegian fjords

A simple apparatus for in situ use of the cage culture technique for growing planktonic algae has been developed and used to follow growth and metal uptake of three diatoms in two Norwegian fjords polluted by heavy metals. Continuous pumping of sea water through a chelating resin and continuous water sampling was used to obtain average values for dissolved and particulate heavy metal content at the various test sites. The metals investigated (zinc, copper, lead, cadmium, and mercury) showed differences in the proportion of dissolved to particulate fractions.The three species of diatoms tested gave systematic growth responses to heavy metal pollution in the moderately polluted fjord; one alga died, one showed reduced growth rate at the more contaminated site compared with those at less polluted sites, while the most tolerant alga was apparently not affected. In the heavily polluted fjord only the most tolerant alga survived, showing decreasing growth rate with increasing pollution.Uptake of heavy metals increased generally with increasing heavy metal content in the sea water. The contents in the algae grown in the most polluted fjord were much higher than those obtained in the less polluted fjord, which were higher than the contents reported for algae from non-contaminated areas.     

Biogeochemical behaviour of Cd, Cu, Pb and Zn in the Scheldt estuary during the period 1981–1983

Cu, Zn, Cd and Pb have been analysed in suspendedmatter and water samples from the Scheldt estuary,collected during five cruises between May 1982 and May1983. In order to evaluate the overall metal behaviourin the estuary, continuous longitudinal profiles ofthe total, the particulate and the dissolved metalconcentrations are discussed in relation to variousspecific physico-chemical parameters. By means ofcorrelation and multi-regression techniques, severalestuarine processes could be identified which dominateand/or control the behaviour of the individual tracemetals in three distinct areas of the Scheldt estuary. The behaviour of the four selected metals was foundto show some common features, but is besides this alsocharacterised by many individual and specificproperties. The total metal concentrations seem toco-vary well with the turbidity in the maximumturbidity zone between salinities 2 and 10 psu. In thedownstream area a dilution profile is observed for thetotal metal concentrations of Cu and Cd as well as forthe dissolved concentrations of all metals. Dissolved metal production rates are high for Cu andCd, but much lower for Zn and Pb, confirming thedifferent longitudinal dissolved metal profiles. ForCu and Cd these dissolved metal profiles arecharacterised by a broad mobilisation area, coveringthe whole middle estuary. Pb on the other hand, showsa more confined mobilisation area, and for Zn no sucharea was found. During winter the dissolved metalproduction rates seem to be controlled, predominantly,by desorption processes. During summer dissolved metalproduction is correlated mainly to the dissolvedoxygen content, suggesting the domination of redoxprocesses solely or in combination with desorptionprocesses. In some cases, co-existent with nearly zerooxygen concentrations, removal of dissolved metals wasobserved, possibly reflecting precipitation reactions. Longitudinal particulate metal profiles all show anegative deviation versus the dilution profile. Cu andCd provide the higher deviations, characterised by anarrow enrichment zone preceding a broad mobilisationarea. Finally, all particulate metal profiles appearedto correlate well with POC, when expressed involumetric units. This revised version was published online in July 2006 with corrections to the Cover Date.     

Helix aspersa as sentinel of development damage for biomonitoring purpose: A validation study

Environmental health has always been threatened by the bioaccumulation of heavy metals in the terrestrial ecosystem, affecting its quality and safety. The aim of this review is to investigate the effects of heavy metal soil contamination, using the land snail Helix aspersa as a bioindicator. H. aspersa, a common species of land snail present in the area, has been used as a bioindicator of metal contamination and represents a promising ecological indicator. Various land snails species have become popular in microcosm studies because they accumulate high concentrations of certain trace metals. They express contamination as a whole through ingestion of polluted foods, such as live plants, microorganisms, soil, and water and also through cutaneous contact and from the polluted air they breathe. Land snails are considered appropriate sentinel species because trace metals tend to accumulate in their digestive gland to allow biomonitoring of metal pollution. Different experiments demonstrate that exposure in areas chronically polluted with metals, especially with lead, often causes changes in reproduction, with a variation in the mineral composition in the snail's eggs and also in its development, due to increased energy expenses associated with detoxi?cation and the excretion process.     

Water quality modelling of the western Scheldt estuary

In 1987 the National Institute for Coastal and Marine Management (theNetherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt).The main goals of the project were to acquire knowledge with respect to thepollution problems in the Scheldt estuary and to apply this knowledge tomanagement issues of the Scheldt on an operational level. The main interestwas to understand quantitatively the relation between inputs of pollutingsubstances and effects on the ecosystem. In the framework of the project, awater quality model for the Scheldt estuary was made. In 1995, the modelarea was expanded to include the Belgian coastal region at the mouth of theestuary. The model calculates the fate of discharged substances andestablishes the relation between pollution inputs and the resulting waterquality, including general water quality (oxygen, pH, alkalinity, major ionchemistry and nutrients) as well as pollutant concentrations. The modelincorporates all chemical processes which affect these concentrations,including the precipitation/dissolution of metal sulphides which to a largeextent controls the fate of trace metals. Based on calibration andverification exercises, it has been shown that the model provides a goodrepresentation of the physical and chemical processes taking place withinthe estuary and can therefore be used to support policy development for theestuary. For example, the model can compute how the water and sedimentquality in the estuary will respond to reduced inputs of waste or to suchhuman activities as dredging. This revised version was published online in July 2006 with corrections to the Cover Date.     

The behaviour of copper and zinc in the Scheldt estuary

The behaviour of copper and zinc in the Scheldt estuary, both in the particulate and the dissolved phase, is discussed for the period 1982–1983. The longitudinal particulate copper and zinc profiles fall below the dilution line, which is attributed to mixing and mobilization processes. In the dissolved phase both metals show a production region in the end member of the estuary, for copper a pronounced mid estuarine maximum is observed. Study of the metal speciation suggested that organic ligands might keep heavy metals in solution, especially in area of intensive production, and promote the subsequent export of these metals downstream the estuary.     

Ecological risk and contamination history of heavy metals in the Andong tidal flat,Hangzhou Bay,China

In this study, we collected two sediment cores (C1 and C2) from the Andong tidal flat, Hangzhou Bay, and studied the temporal variations of heavy metals in the cores. Vertical distributions of heavy metals were almost unchanged in both the cores before 2000. After 2000, however, the heavy metal concentrations increased dramatically, suggesting that the sediments have been affected by enhanced human pollution in the recent decade. In the core C1, the sediments were severely polluted by Pb, moderately to considerably polluted by Cr and Zn, and low to moderately polluted by other heavy metals. The core C2 was relatively unpolluted before 2000 and low to moderately polluted after 2000. Multi-statistical analyses indicated that the core C1 was additionally contaminated by local human activities such as wastewater discharge and the Hangzhou Bay Bridge. The heavy metals in the core C2, however, were largely contributed by the Yangtze River and controlled by sedimentation process. The calculated sedimentary flux (4–8 g m^?2 a^?1) of heavy metals generally increased with time. It was closely related to the wastewater discharge in adjacent areas. This study reconstructed the local heavy metal pollution history and provides important information for environmental protection and policy making.     

Overview of trace metal contamination in the Scheldt estuary and effect of regulatory measures

Seasonally, dissolved and particulate metal concentrations in the Scheldt estuary were assessed over a period of 4 years (1995–1998). High quality data were obtained following stringent analytical protocols for each step: sampling, sample treatment, sample storage and analysis. Of the 5 trace metals, Ni showed the most conservative behaviour, while Cd and Cu were clearly transferred from the particulate to the dissolved phase in the middle estuary. A substantial part of the particulate metals entering the estuarine system are lost through sedimentation. General seasonal patterns are the following: lower concentrations in spring and higher ones in winter (sometimes late fall/early winter) for dissolved metals, while in summer a pronounced rise of the longitudinal concentration profile is observed for the particulate metals. A comparison of the trace metal concentrations (dissolved and particulate) at the mouth of the estuary in 1995–1998 with those from 1981 to 1983, reveal reductions between 30 and 58%. Reductions based on direct emission measurements for almost the same period suggest reductions (dissolved + particulate) between 42 and 64%. Biomagnification (BMF) is the accumulation of a compound through the food-chain. It is in our case expressed as the ratio of the metal concentration in the organism (g g^–1, d.w.)/the metal concentration in total suspended matter (g g^–1, d.w.). Almost all BMF-values of Periwinkle, Nereis diversicolor and Macoma balthica (3 bottom organisms in the Scheldt estuary) are negative meaning that these organisms contain less heavy metals than the particulate suspended matter. For all organisms log BMFs for Pb, respectively Ni, are around –1.8, respectively –0.7. For Cd, Periwinkle shows slight enrichment (0.05) and for Cu even more (0.45), while negative values were observed for Nereis diversicolour and Macoma balthica. The latter organisms are more enriched in Zn (–0.09) than Periwinkle (–0.43).     

Local adaptation of microbial communities to heavy metal stress in polluted sediments of Lake Erie

Microbial communities must balance the assimilation of biologically necessary metals with resistance to toxic metal concentrations. To investigate the impact of heavy metal contaminants on microbial communities, we performed two experiments measuring extracellular enzyme activities (EEA) in polluted and unpolluted sediments of Lake Erie. In the first experiment, inoculations with moderate concentrations of copper and zinc appreciably diminished EEA from uncontaminated sites, whereas EEA from contaminated sediments increased or were only negligibly affected. In the second experiment, we compared the effects of three separate metals (i.e. copper, arsenic, and cadmium) on microbial community metabolism in polluted and unpolluted locations. Although copper and arsenic elicited differential effects by inhibiting EEA only in unpolluted sediments, cadmium inhibited EEA in both polluted and unpolluted sediments. Multivariate analyses of EEA from polluted sediments revealed direct associations among hydrolytic enzymes and inverse or absent associations between hydrolases and oxidases; these associations demonstrated resilience to heavy metal stress. In contrast, addition of heavy metals to unpolluted sediments appeared to have a higher impact on the multivariate pattern of EEA associations as revealed by an increase in the number of associations, more inverse relationships, and potential enzymatic trade-offs. The results of this study suggest community-level adaptations through the development of resistance mechanisms to the types and local levels of heavy metals in the environment.     

A box-model of metal flows through the Scheldt estuary (1981–1983 and 1992–1995)

The behaviour of Cu, Zn, Cd and Pb in the Scheldtestuary was investigated in the periods 1981 to 1983and 1995. The Hg behaviour was studied between 1992and 1994. A box model was used to establish therelationship between the trace metal inputs and thefate of these pollutants in the Scheldt estuary.Annual budgets were calculated for particulate anddissolved trace metals. For Hg, seasonal budgets ofthe various species (inorganic Hg, methylmercury,Hg °) could also be calculated. These budgetsallow an evaluation of the consistency of eachindividual flux or process, such as the inputs of thevarious metals into the North Sea, the sedimentationflux of particulate metals in the area of highturbidity, the evasional fluxes and the formationrates of Hg °, etc. in relation to the other ones. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamics of plant metal uptake and metal changes in whole soil and soil particle fractions during repeated phytoextraction

Aims

Phytoextration of metal polluted soils using hyperaccumulators is a promising technology but requires long term successive cropping. This study investigated the dynamics of plant metal uptake and changes in soil metals over a long remediation time.

Methods

A soil slightly polluted with metals (S1) was mixed with highly polluted soil (S4) to give two intermediate pollution levels (S2, S3). The four resulting soils were repeatedly phyto-extracted using nine successive crops of Cd/Zn-hyperaccumulator Sedum plumbizincicola over a period of 4 years.

Results

Shoot Cd concentration decreased with harvest time in all soils but shoot Zn declined in S1 only. Similar shoot Zn concentrations were found in S2, S3 and S4 although these soils differed markedly in metal availability, and their available metals decreased during phytoextraction. A possible explanation is that plant active acquisition ability served to maintain plant metal uptake. Plant uptake resulted in the largest decrease in the acid-soluble metal fraction followed by reducible metals. Oxidisable and residual fractions were less available to plants. The coarse soil particle fractions made the major contribution to metal decline overall than the fine fractions.

Conclusion

Sedum plumbizincicola maintained long term metal uptake and the coarse soil particles played the most important role in phytoextraction.     

Influence of fungi on growth and survival of Onychiurus armatus (Collembola) in a metal polluted soil

Summary The influence of food quantity and quality on growth and survival of Onychiurus armatus (Tullb.) in metal polluted environments has been investigated in laboratory experiments. The Collembola was reared on five species of fungi isolated from a metal polluted soil close to a brass mill in SE Sweden.Survival of O. armatus was improved when fungal biomass was continuously added in a polluted mor (1,300 ppm Zn and 200 ppm Cu), and when specimens were fed metal polluted fungi for 1, 3 and 7 days a week, only those that were starved had increased mortality. Allometric growth, on the other hand, was significantly reduced when Collembola was given surplus of metal polluted fungi, whereas growth losses caused by metals were offset by protein rich food. Hence, sufficient food quantities alone could overcome mortality losses but not growth retardation in a metal polluted environment.Feeding preference of O. armatus was not determined by the protein content of the fungi although this was beneficial for growth. Metals changed the relative palatability of fungal species, but one of the metal tolerant species, Paecilomyces farinosus, which was also protein rich, remained reasonably attractive for O. armatus also when it was metal polluted. The mechanisms by which growth and survival of O. armatus were promoted by a combination of protein and Zn/Cu rich fungi seemed to be crucial in understanding the fate of a population of this species in a metal polluted soil.     

Effect of the Heavy Metals on Developmental Stages of Ovule,Pollen, and Root Proteins in <Emphasis Type="Italic">Reseda lutea</Emphasis> L. (Resedaceae)

Heavy metals are some of the most important environmental pollutants. Excessive amounts of heavy metals adversely affect plant growth and development. Also, the presence of elevated levels of heavy metal ions triggers a wide range of cellular responses including changes in gene expression and synthesis of metal-detoxifying peptides. The overall objective of this research was to elucidate some microscopic effects of heavy metals on the formation, development, and structure of pollen, ovule, and embryo and also root proteins in Reseda lutea L. For this purpose, the vicinity of Ahangaran lead–zinc mine (Hamedan, Iran) was chosen as a polluted area where amount of some heavy metals was several times higher than the natural soils. Flowers and young buds were collected from non-polluted and polluted plants, fixed in FAA₇₀, and studied during developmental stages by light microscopy. The results showed that heavy metals can cause some abnormalities during the pollen and ovule developmental process. The number of pollen grains was decreased, and their shape was changed. Increasing in thickness of the callosic wall and stabilizing of tapetum layer were observed in polluted plants. Asymmetrical formation of ovular integuments, degradation of egg apparatus, irregular formation of embryo sac, considerable vacuolation of embryonic cells, and degeneration of embryo in the late stage of heart-shaped embryo are the results of heavy metal pollution. For protein studies, young roots were harvested from plants exposed to pollution and non-exposed to pollution at the same time. Root proteins were extracted and studied by electrophoresis. The results revealed that some new proteins were synthesized in polluted samples that probably elevate plant tolerance to heavy metals.     

Concentration and ecological risk of heavy metal in street dusts of Eslamshahr,Iran

This study was done to evaluate heavy metal concentrations in street dust samples, to compare measured concentrations in samples to background concentrations in order to make evaluations for pollution indices, and to describe the quality of street dust in the studied area in relation to pollution. A total of 30 cumulative samples were collected from the streets of Eslamshahr City. Concentrations of heavy metals were determined using an Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). Results determined mean concentrations (mg/kg) of the heavy metals Cd, Cr, Cu, Ni, Pb, and Zn, in collected samples of street dust as 0.34, 35.1, 239, 42.4, 71.3, and 573, respectively. I_geo values for Cd and Cr, Cu, Ni, Pb, and Zn showed level of moderately polluted, unpolluted, moderately to strongly polluted, unpolluted, moderately polluted and moderately to strongly polluted, respectively. The pattern of total metal concentrations in the studied area was ranked as follows: Zn and Cu>Pb>Cd>Ni>Cr. The highest values for the monomial potential ecological risk (E_r) were observed for Cd (114). The mean level of RI for the studied soil samples was 192 (91.3–244), which is classed as presenting a strong potential ecological risk.     

Genetic variation and heavy metal tolerance in the ectomycorrhizal basidiomycete Suillus luteus

Twenty-one isolates of the ectomycorrhizal fungus Suillus luteus were screened for their tolerance to the heavy metals Zn, Cd, Cu and Ni, measured as inhibition of radial growth and biomass production. Two populations from even-aged pine stands were investigated: 10 isolates were obtained from an area polluted with high levels of Zn, Cd and Cu, and 11 isolates were obtained from a control population located in a nearby unpolluted area. RFLP patterns of the internal transcribed spacer region of the isolates confirmed the morphological identification of the carpophores. All isolates were maintained on basic medium without elevated metals to avoid phenotypically acquired metal tolerance. The in vitro Zn and Cd tolerance of the S. luteus isolates from the polluted habitat were significantly higher than the tolerances measured in the isolates from the nonpolluted site. This observation suggests that the elevated soil metal concentrations might be responsible for the evolution of adaptive Zn and Cd tolerance. Tolerance was maintained in an isolate not exposed to elevated metals for 3 yr. The two S. luteus populations did not differ in tolerance to Cu and Ni. The mechanisms for the adaptive Zn and Cd tolerance are not identical as there was no correlation between response to the two metals; the most Zn-tolerant isolate was the most sensitive for Cd in the metal-tolerant population. Zinc did not accumulate in basidiocarp tissue, whereas Cd levels in basidiocarps were significantly higher in the population on the polluted site. Inter-simple sequence-repeat fingerprints showed that 90% of the isolates were from different individuals. The genetic variation in the population from the unpolluted site was considerably larger than that observed at the polluted site.     

Effect of heavy metal pollution on mycorrhizal colonization and function: physiological, ecological and applied aspects

 High concentrations of heavy metals in soil have an adverse effect on micro-organisms and microbial processes. Among soil microorganisms, mycorrhizal fungi are the only ones providing a direct link between soil and roots, and can therefore be of great importance in heavy metal availability and toxicity to plants. This review discusses various aspects of the interactions between heavy metals and mycorrhizal fungi, including the effects of heavy metals on the occurrence of mycorrhizal fungi, heavy metal tolerance in these micro-organisms, and their effect on metal uptake and transfer to plants. Mechanisms involved in metal tolerance, uptake and accumulation by mycorrhizal hyphae and by endo- or ectomycorrhizae are covered. The possible use of mycorrhizal fungi as bioremediation agents in polluted soils or as bioindicators of pollution is also discussed. Accepted: 23 June 1997     

Investigating the Effect of Heavy Metals on Developmental Stages of Anther and Pollen in <Emphasis Type="Italic">Chenopodium botrys</Emphasis> L. (Chenopodiaceae)

Excessive amounts of heavy metals adversely affect plant growth and development. Whereas some regions naturally contain high levels of heavy metals, anthropogenic release of heavy metals into the environment continuously increases soil contamination. Preliminary studies have shown that Chenopodium botrys can grow in some heavy metal contaminated soils and is a high accumulator plant species for Cu and moderately accumulator plant species for Fe, Mn, and Zn, thus, was considered as an important species in this study. Based on that, in this species, we studied the individual effects of heavy metals on the formation, development, and structure of anther and pollen. To achieve this purpose, surrounding area of Hame-Kasi iron and copper mine (Hamedan, Iran) was chosen as a polluted area where the amount of some heavy metals was several times higher than the natural soils. Flowers and young pods were removed from non-polluted and polluted plants, fixed in FAA 70, and subjected to developmental studies. Analysis of anther development in plants from contaminated sites showed general similarities in the pattern of pollen formation with those from non-polluted ones, but also deviation from typical form of major stages of anther and pollen development was seen in plants from polluted ones. Stabilizing of tapetum layer, increasing in tapetum layer numbers, thickening callose wall in the microspore mother cell stage, changing the anther shape, and decreasing the size of anther were the effects of heavy metals. Reduction of pollen number was also seen in the plants collected from polluted area.