Genetic responses to metal contamination in two clams: Ruditapes decussatus and Ruditapes philippinarum

Comparative transcriptome analysis reveals immunotoxicology induced by three organic UV filters in Manila clam (Ruditapes philippinarum)

Benzophenone-3 (BP-3), 4-methyl-benzylidene camphor (4-MBC) and 2-ethyl-hexyl-4-trimethoxycinnamate (EHMC) are commonly used organic ultraviolet (UV) filters and are frequently detected in water environments. In the present study, we studied the potential adverse impacts of UV filter exposures in Ruditapes philippinarum by investigating transcriptomic profiles and non-specific immune enzyme activities. Transcriptome analysis showed that more genes were differentially regulated in EHMC-treated group, and down-regulated genes (2009) were significantly more than up-regulated ones (410) at day 7. Function annotation revealed that pathways "immune system", "cell growth and death" and "infectious diseases" were significantly enriched. Generally, combined qPCR and biochemical analyses demonstrated that short-term exposure to low dose of UV filters could activate immune responses, whereas the immune system would be restrained after prolonged exposure. Taken together, the present study firstly demonstrated the immunotoxicology induced by BP-3, 4-MBC and EHMC on R. philippinarum, indicating their potential threats to the survival of marine bivalves.

Effects of temperature on caffeine and carbon nanotubes co-exposure in Ruditapes philippinarum

In the marine environment, organisms are exposed to a high and increasing number of different contaminants that can interact among them. In addition, abiotic factors can change the dynamics between contaminants and organisms, thus increasing or even decreasing the toxic effect of a particular compound. In this study, the effects of caffeine (CAF) and functionalized multi-walled carbon nanotubes (f-MWCNTs) induced in the clam Ruditapes philippinarum were evaluated, acting alone and in combination (MIX), under two temperature levels (18 and 21 °C). To assess the impact of such compounds, their interaction and the possible influence of temperature, biochemical and histopathological markers were investigated. The effects of f-MWCNTs and caffeine appear to be clearly negative at the control temperature, with lower protein content in contaminated clams and a significant decrease in their metabolism when both pollutants were acting in combination. Also, at control temperature, clams exposed to pollutants showed increased antioxidant capacity, especially when caffeine was acting alone, although cellular damages were still observed at CAF and f-MWCNTs treatments. Increased biotransformation capacity at 18 °C and MIX treatment may explain lower caffeine concentration observed. At increased temperature differences among treatments were not so evident as at 18 °C, with a similar biological pattern among contaminated and control clams. Higher caffeine accumulation at MIX treatment under warming conditions may result from clams' inefficient biotransformation capacity when exposed to increased temperatures.

Effects of metal contamination on the gene expression profile of two benthic species: Cerastoderma edule and Ruditapes philippinarum

This study aimed to identify new biomarkers for metal exposure in two bivalve species. Suppressive Subtractive Hybridization (SSH) was employed to evaluate the transcriptomic response of Cerastoderma edule and Ruditapes philippinarum to metal pollution. Protein synthesis and catalytic activity were the most affected metabolic processes in C. edule and R. philippinarum, respectively. Also, different genes responded to the effect of contamination in each species. The different response observed in both species reinforces the importance of including more than one bioindicator species in risk assessment studies. These results provide the basis for new studies, which are necessary for further validation of the use of the identified genes as molecular biomarkers for metal exposure.

Effects of hypoxia in the gills of the Manila clam Ruditapes philippinarum using NMR-based metabolomics

Coastal hypoxia affects the survival, behavior, and reproduction of individual local marine organisms, and the abundance, biomass, and biodiversity of coastal ecosystems. In this study, we investigated the chronic effects of hypoxia on the metabolomics in the gills of Ruditapes (R.) philippinarum. The results indicated significant alterations in the metabolite profiles in the gills of the hypoxia-treated clams, in comparison with those maintained under normoxia. The levels of betaine, taurine, glycine, isoleucine, and alanine were significantly reduced, suggesting a disturbance of osmotic balance associated with hypoxia. Meanwhile, metabolites involved in energy metabolism, such as alanine and succinate, were also affected. Dramatic histopathological changes were observed in the gills and hepatopancreases of R. philippinarum grown in hypoxic waters, demonstrating tissue damages apparently caused by long-term exposure to hypoxia. Our findings suggest that hypoxia significantly affects the physiology of R. philippinarum, even at a sub-lethal level, and impedes health of the clams.

The impacts of As accumulation under different pH levels: Comparing Ruditapes decussatus and Ruditapes philippinarum biochemical performance

Marine bivalves have been used to assess environmental As contamination and the effects of seawater acidification when both factors are acting alone, but limited information is available regarding the impacts of both factors acting in combination. The aim of this study was to compare physiological (glycogen) and biochemical (lipid peroxidation, superoxide dismutase, catalase, glutathione-S-transferase and alkaline phosphatase) responses in both native (Ruditapes decussatus) and introduced (R. philippinarum) clams, when exposed to the combined effects of pH (7.8, control; 7.3) and As concentrations (0 and 4mg/L). The combined effect of As and pH on the health risks associated with clam consumption was also analyzed. Results revealed that both species were able to accumulate As under both pH levels, although higher As concentrations where observed under low pH. Thus, predicted pH decrease will potentiate health risks associated with the consumption of such species, since less amount of clams exposed to As is needed for an adult to exceed the provisional tolerable weekly intake (PTWI). Low pH, As exposure and the combination of both factors did not negatively affect the native species, since clams were able to maintain their physiological and biochemical performance among all conditions. On the other hand, R. philippinarum was negatively affected by As exposure at control pH (7.8), inducing biotransformation and antioxidant defense mechanisms against As toxicity. R. philippinarum exposed and non-exposed to As presented similar responses under low pH although at this condition the introduced species accumulated twice the amount of As than R. decussatus.

Clams sensitivity towards As and Hg: A comprehensive assessment of native and exotic species

To assess the environmental impact of As and Hg, bioindicator organisms such as bivalves have been used. Nevertheless, few studies have assessed the impacts of As and Hg in Ruditapes decussatus and Ruditapes philippinarum, which are native and exotic species in Europe, respectively. The main goal of the present study was to assess elements' partitioning and detoxification strategies of R. decussatus and R. philippinarum. Both clams showed a higher capacity to bioconcentrate Hg (BCF 2.29-7.49), when compared to As (0.59-1.09). Furthermore, As accumulation in both species was similar in the soluble and insoluble fractions, while in both species the majority of Hg was found in the insoluble fraction. Clams exposed to As showed different detoxification strategies, since R. decussatus had higher ability to enhance antioxidant enzymes and metallothioneins in order to reduce toxicity, and R.philippinarum increased glutathione S-transferase Ω activity, that catalyzes monomethyl arsenate reduction, the rate-limiting reaction in arsenic biotransformation. When exposed to Hg, R. decussatus presented, higher synthesis of antioxidant enzymes and lower LPO, being able to better tolerate Hg than the exotic species R. philippinarum. Thus under relevant levels of As and Hg contamination our work evidenced the higher ability of R. decussatus to survive and inhabit coastal environments not heavily contaminated by Hg and As.

Proteomic and metabolomic responses of clam Ruditapes philippinarum to arsenic exposure under different salinities

Arsenic (As) contamination is a severe problem in the intertidal zones of the Bohai Sea (China) with wide salinity variation. In the present study, we combined proteomics and metabolomics to characterize the differential responses of arsenic in clam Ruditapes philippinarum under different salinities (31.1, 23.3 and 15.6 psu). Both proteomic and metabolomic responses indicated that varying salinities could significantly affect the toxicological responses of clams to As. Metabolic biomarkers revealed that the environmentally relevant arsenic (20 μg L(-1)) exposure induced disturbance in energy metabolism and/or osmotic regulation under different salinities, whereas protein biomarkers indicated oxidative stress, cellular injury and apoptosis and disturbance in energy metabolism. In addition, the up-regulated proteins including ATP synthase, succinyl-CoA synthetase and nucleoside diphosphate kinase were validated by related metabolites, succinate and ATP, which confirmed the disturbance in energy metabolism in clam gills at low salinity (15.6 psu). These findings provide important insights into toxicological effects of environmental contaminant at molecular levels using combined proteomics and metabolomics.

Evolutionary concepts in ecotoxicology: tracing the genetic background of differential cadmium sensitivities in invertebrate lineages

In many toxicological and ecotoxicological studies and experimental setups, the investigator is mainly interested in traditional parameters such as toxicity data and effects of toxicants on molecular, cellular or physiological functions of individuals, species or statistical populations. It is clear, however, that such approaches focus on the phenotype level of animal species, whilst the genetic and evolutionary background of reactions to environmental toxicants may remain untold. In ecotoxicological risk assessment, moreover, species sensitivities towards pollutants are often regarded as random variables in a statistical approach. Beyond statistics, however, toxicant sensitivity of every species assumes a biological significance, especially if we consider that sensitivity traits have developed in lineages of species with common evolutionary roots. In this article, the genetic and evolutionary background of differential Cd sensitivities among invertebrate populations and species and their potential of adaptation to environmental Cd exposure will be highlighted. Important evolutionary and population genetic concepts such as genome structure and their importance for evolutionary adaptation, population structure of affected individuals, as well as micro and macroevolutionary mechanisms of Cd resistance in invertebrate lineages will be stressed by discussing examples of work from our own laboratory along with a review of relevant literature data and a brief discussion of open questions along with some perspectives for further research. Both, differences and similarities in Cd sensitivity traits of related invertebrate species can only be understood if we consider the underlying evolutionary processes and genetic (or epigenetic) mechanisms. Keeping in mind this perception can help us to better understand and interpret more precisely why the sensitivity of some species or species groups towards a certain toxicant (or metal) may be ranked in the lower or higher range of species sensitivity distributions. Hence, such a perspective will transcend a purely statistical view of the sensitivity distributions concept, and will enhance ecotoxicology in many respects.

Biomonitoring study of an estuarine coastal ecosystem, the Sacca di Goro lagoon, using Ruditapes philippinarum (Mollusca: Bivalvia)

Coastal lagoons are constantly subjected to releases of chemical pollutants, and so organisms may be exposed to such toxicants. This study investigated through a multivariate approach the physiological status of bivalve Ruditapes philippinarum, farmed in Sacca di Goro lagoon. Biomarkers at different levels of biological organization (catalase, superoxide dismutase, genotoxicity, reburrowing behavior) were evaluated at three sites exposed to different environmental conditions. A seasonal trend was observed, and micronucleus frequency was significantly lowest at the relatively pristine reference site. Enzymatic activity toward oxyradicals be quite efficient since variations in responsiveness were not consistent. However, behavioral impairment was observed in reburrowing rates. Sediment concentrations showed low PAH levels and high natural levels of trace metals Cr and Ni. DistLM statistical analysis revealed a non-significant relationship between selected biomarkers and xenobiotics. Therefore other potentially toxic compounds in admixture at low doses may be involved in driving differing spatial distribution of physiological impairment.

A metabolomic investigation on arsenic-induced toxicological effects in the clam Ruditapes philippinarum under different salinities

Arsenic is an important contaminant in the Bohai marine ecosystem due to the anthropogenic activities. In this work, we investigated the toxicological effects of arsenic in Ruditapes philippinarum under different seawater salinities using NMR-based metabolomics. Under normal salinity (31.1 ppt), arsenic decreased the levels of amino acids (glutamate, β-alanine, etc.), and increased the levels of betaine and fumarate. The metabolic biomarkers including decreased threonine, histidine, ATP and fumarate were found in the muscles of arsenic-treated clams under medium salinity (23.3 ppt). However, only elevated ATP and depleted succinate were detected in the arsenic-exposed clam samples under low salinity (15.6 ppt). These differential metabolic biomarkers indicated that arsenic could induce osmotic stress and disturbance in energy metabolism in clam under normal and medium salinities. However, arsenic caused only disturbance in energy metabolism in clam under low salinity. Overall, our results demonstrated that seawater salinity could influence the toxicological effects of arsenic.

Identification and expression profile of a new cytochrome P450 isoform (CYP414A1) in the hepatopancreas of Venerupis (Ruditapes) philippinarum exposed to benzo[a]pyrene, cadmium and copper

With the objective to identify promising molecular biomarkers for marine pollution monitoring, a new cytochrome P450 gene was identified from Venerupis (Ruditapes) philippinarum and classified as a member of a new subfamily, CYP414A1. Phylogenetic analysis showed that CYP414A1 was closely related to members of the CYP2 family. CYP414A1 mRNA expression was significantly induced by 50 μg/L B[a]P at 96 h, while no significant change was found in 5 μg/L B[a]P-exposed samples. For heavy metals exposure, the expression of CYP414A1 was significantly up-regulated by Cd but sharply depressed by Cu exposure. These results suggested that CYP414A1 responded to various xenobiotics stresses, and could be used as a candidate biomarker of heavy metals and B[a]P.

Characterisation and genetic polymorphism of metallothionein gene CgMT4 in experimental families of Pacific oyster Crassostrea gigas displaying summer mortality

Summer mortality events have been observed in Pacific oyster Crassostrea gigas for several decades. This paper examines the selective pressure exerted by summer mortality on the polymorphism of a newly identified oyster metallothionein gene. CgMT4 cDNA and genomic sequences were obtained. CgMT4 was studied in two generations of oysters reared in three sites on the French Atlantic coast, using single strand conformation polymorphism analysis. Four alleles were detected. Individuals carrying genotype MT4-CD seem to have higher susceptibility to summer risk conditions. The MT4 gene could be a potential new genetic marker for susceptibility; further validation studies are recommended.

NMR-based metabolomic investigations on the differential responses in adductor muscles from two pedigrees of Manila clam Ruditapes philippinarum to Cadmium and Zinc

Manila clam Ruditapes philippinarum is one of the most important economic species in shellfishery in China due to its wide geographic distribution and high tolerance to environmental changes (e.g., salinity, temperature). In addition, Manila clam is a good biomonitor/bioindicator in "Mussel Watch Programs" and marine environmental toxicology. However, there are several pedigrees of R. philippinarum distributed in the marine environment in China. No attention has been paid to the biological differences between various pedigrees of Manila clams, which may introduce undesirable biological variation in toxicology studies. In this study, we applied NMR-based metabolomics to detect the biological differences in two main pedigrees (White and Zebra) of R. philippinarum and their differential responses to heavy metal exposures (Cadmium and Zinc) using adductor muscle as a target tissue to define one sensitive pedigree of R. philippinarum as biomonitor for heavy metals. Our results indicated that there were significant metabolic differences in adductor muscle tissues between White and Zebra clams, including higher levels of alanine, glutamine, hypotaurine, phosphocholine and homarine in White clam muscles and higher levels of branched chain amino acids (valine, leucine and isoleucine), succinate and 4-aminobutyrate in Zebra clam muscles, respectively. Differential metabolic responses to heavy metals between White and Zebra clams were also found. Overall, we concluded that White pedigree of clam could be a preferable bioindicator/biomonitor in marine toxicology studies and for marine heavy metals based on the relatively high sensitivity to heavy metals.

Genetic polymorphism and its potential relation to environmental stress in five populations of the European flounder Platichthys flesus, along the French Atlantic coast

In this study, new DNA markers were explored for the flounder Platichthys flesus. cDNA and genomic sequences of the genes encoding the glyceraldehyde-3-phosphate-deshydrogenase (GAPDH), the cytosolic creatine kinase (CK), the prostaglandin D synthase (PGDS) and the betaine homocysteine methyltransferase (BHMT) were characterized. The tumour suppressor p53 gene structure was already described. A PCR-SSCP (Single Strand Conformation Polymorphism) analysis was finally conducted to study the genetic polymorphism of different populations of flounders collected along the French Atlantic coast. Four highly contaminated French estuaries (Seine, Vilaine, Loire and Gironde) were sampled and compared to a reference estuary (Ster) to explore possible selective effect of the environment on specific allelic frequencies. Our results showed that two loci p53 and PGDS, could be potential markers of chemical stress: p53A allele frequency increased in contaminated systems compared to the reference system. In the Vilaine estuary, PGDS polymorphism could be related to pesticide stress.

Genetic impact of the Prestige oil spill in wild populations of a poor dispersal marine snail from intertidal rocky shores

In November 2002, the sinking of the Prestige cargo ship produced an oil spill of 60,000 tons that affected many areas along the Galician coast (in the northwest of Spain). In a number of rocky shore sites, most organisms (particularly marine mollusks) were nearly extinct at a local scale. We tested whether the local bottleneck/extinction that occurred in affected localities caused any detectable reduction of the genetic diversity in the marine snail Littorina saxatilis, an ovoviviparous rocky shore model species characterized by a low dispersal ability, high population density, and wide distribution range. We compared the level of genetic variation and population differentiation between affected (polluted) and control sites located in seven geographical areas (three sites per area, one impacted and two controls, and two replicates per site) one and a half years after the spill. The analysis included molecular marker variation (microsatellite and AFLP loci) and quantitative trait genetic variation for shell variables in embryos extracted from pregnant females. Our results indicate that the affected populations did not show a significant overall reduction in genetic diversity when compared to the controls, suggesting that the species is highly resistant to losing genetic variability as a consequence of a local short-term pollution process in spite of its low dispersal ability and direct development. However, some genetic effects were detected in the polluted populations, particularly for quantitative shell traits and AFLPs, consistent with local adaptations resulting from the fuel contamination.

Peroxiredoxin 6 gene: a new physiological and genetic indicator of multiple environmental stress response in Pacific oyster Crassostrea gigas

Peroxiredoxin 6 is a 1-cysteine peroxiredoxin involved in antioxidant processes. We characterised the full-length cDNA and genomic sequence of the gene encoding peroxiredoxin 6 (CgPrx6) in the Pacific oyster, Crassostrea gigas. A phylogenetic analysis of Prx6 sequences showed that the CgPrx6 segregates between vertebrate and invertebrate groups. We analysed the expression of mRNA coding for CgPrx6 using RT-PCR in gills and digestive gland of oysters sampled in different contaminated and reference estuaries of the Atlantic French coast. We also studied CgPrx6 exon 6 polymorphism by PCR-SSCP in the same populations. Our results showed that CgPrx6 gene expression was highly regulated in the estuaries showing differential contamination levels, as expression increased with pollution level. Polymorphism analysis revealed no significant allelic frequency variations between populations. However, heterozygote deficiency seems to occur in the most contaminated estuaries, suggesting a potential selective effect of environmental stress on heterozygote frequency. Finally, the use of CgPrx6 as a possible marker to monitor stress exposure in disturbed ecosystems is discussed.

Oxidative stress in the clam Ruditapes decussatus (Linnaeus, 1758) in relation to polycyclic aromatic hydrocarbon body burden

Seasonal variation of antioxidant enzymes activities (superoxide dismutase, catalase, and glutathione peroxidases) and lipid peroxidation (LPO) were studied in the clam Ruditapes decussatus in relation to body burdens of polycyclic aromatic hydrocarbons (PAHs). Clams were sampled in eight sites from the Ria Formosa lagoon. PAH concentrations were seasonally rather than spatially dependent, being higher in summer (August). Antioxidant enzymes activities and LPO levels in the clam digestive gland were also seasonally dependent. Antioxidant enzymes presented distinct seasonal variations: Mit SOD (superoxide dismutase activity measured in the mitochondrial fraction) was induced in the summer and down-regulated in winter and spring, while Cyt SOD activity (measured in the cytosolic fraction) was highest in autumn and lower in the summer. Neither Mit nor Cyt SOD were related to the clam PAH body burden, suggesting that cells are using other antioxidant systems to eliminate oxyradicals. Catalase (CAT), however, was induced in spring and down-regulated in summer, the inverse of the PAH concentrations in clam tissues. CAT induction in spring appears to be related to the excess of oxyradicals arising from the metabolic activity associated with the reproductive cycle. Conversely, the decrease in CAT activity in the summer may be related to the high water temperatures reached in the Ria Formosa (up to 30 degrees C). Glutathione peroxidases (total fraction - T-GPx and dependent on selenium - Se-GPx) presented a similar seasonal pattern, and were negatively related to PAH concentrations, which may indicate a precarious state of the clams, associated with PAH toxicity. Similarly, LPO was also inversely correlated to the PAH concentrations indicating that increases in PAH concentrations were not causing membrane oxidative damage in R. decussatus digestive gland. The results suggest that antioxidant enzymes in R. decussatus digestive gland are strongly affected by seasonal factors stressing the need of other experiments to clarify the PAHs effect on this clam species.

Metal influence on metallothionein synthesis in the hydrothermal vent mussel Bathymodiolus thermophilus

The present study reports on the metallothionein expression in the hydrothermal vent mussel Bathymodiolus thermophilus. Metallothioneins (MT) are proteins involved in intracellular metal regulation and conserved throughout the animal kingdom. The hydrothermal vent environment presents peculiarities (high levels of sulfides and metals, low pH, anoxia) that may have driven associated species to develop original evolutionary ways to face these extreme living conditions. Mussels were exposed to different metal solutions at the atmospheric pressure. The MT mRNA levels and MT contents were measured in gills and mantles of each exposed mussel. The intracellular metal distribution was estimated in fractions obtained after the centrifugation of tissue homogenates. A few of the tested metals (Ag, Cu, Cd, Hg and Zn) were able to significantly induce MT mRNA levels. Silver was the only one that produced a significant increase of the MT protein level in both mantle and gills. The gills always presented higher MT protein levels than the mantle did, while their MT mRNA levels were similar. Our data show that MT mRNA and MT protein levels do not follow a clear relationship in the gills and mantle of B. thermophilus and we assume that a posttranscriptional control occurs in these mussels.

Evidence of altered gene flow, mutation rate, and genetic diversity in redbreast sunfish from a pulp-mill-contaminated river

To determine effects of pulp mill effluent on population genetic structure, redbreast sunfish (Lepomis auritus) were collected from several sites along the Pigeon River, NC, as well as from reference sites. Previous studies found effects on molecular, biochemical, physiological, population, and community level endpoints in these populations. The population genetic structure of these fish was determined using the randomly amplified polymorphic DNA (RAPD) technique. The level of genetic diversity was higher in the Pigeon River populations than in the reference populations. Genetic distances among populations could not be explained by drainage patterns and may have been altered by contaminant exposure. Phylogeographic analysis, maximum likelihood analysis, and assignment tests suggested that there were fewer emigrants and more immigrants in the contaminated sites than in the reference sites, suggesting that the contaminated sites may harbor "sinklike" populations. Finally, a "terminal branch amplitype" analysis (neighbor-joining and minimum-spanning trees) and maximum likelihood analysis indicated that there may be an elevated mutation rate in the polluted sites. Thus, the genetic diversity (within and among populations) in the Pigeon River populations may have been affected by altered gene flow and mutational processes as a result of pulp mill effluent discharge.

Stress response in Cu2+ and Cd2+ exposed oysters (Crassostrea gigas): an immunohistochemical approach

Localization of heat shock proteins (HSPs) and metallothioneins (MTs) was investigated in a marine bivalve (Crassostrea gigas) by immunohistochemical methods. Differential protein expression was demonstrated in digestive gland, gonad and gills, using a polyclonal antibody against C. gigas proteins. Application of this technique showed the cellular and tissue immunolabelling specificity of the two proteins. HSPs and MTs were localized in the epithelium of the digestive gland and gills in contact with the palleal compartment. For the first time, localization of MTs was observed in mature gametes of bivalve molluscs. Our results establish a basis for the use of immmunodetection techniques to study the tissue-specific localization of stress proteins in marine bivalves exposed to metal stress.

Allozyme polymorphisms and heavy metal levels in the green-lipped mussel Perna viridis (Linnaeus) collected from contaminated and uncontaminated sites in Malaysia

It has been widely reported that heavy metal contamination in coastal waters can modify the allozyme profiles of marine organisms. Previous studies have recorded elevated metal concentrations in sediments and mussel tissues off Peninsular Malaysia. In the present study, horizontal starch gel electrophoresis was carried out to estimate the levels of allelic variation of the green-lipped mussel, Perna viridis, collected from one contaminated and three relatively uncontaminated sites off Peninsular Malaysia. Fourteen polymorphic loci were observed. In addition, the concentrations of cadmium, copper, lead, mercury and zinc were determined in the sediments and in the soft tissues of the mussels. Mussels from contaminated site, evidenced by high metal pollution indices (MPI) of the sediment and the mussel tissues, showed the highest percentage of polymorphic loci (78.6%), while those collected from the uncontaminated sites had lower MPI of the sediment and mussel tissue, and exhibited lower percentages of polymorphic loci (35.7-57.1%). The population from the contaminated site showed the highest excess of heterozygosity (0.289) when compared to that of the populations from the three uncontaminated sites (0.108-0.149). Allozyme frequencies at the phosphoglucomutase (PGM; E.C. 2.7.5.1) locus also differed between the contaminated and uncontaminated populations. Previous studies have shown that exposure to heavy metals can select or counter-select for particular alleles at this locus. The present results suggest that allozyme polymorphism in P. viridis is a potential biomonitoring tool for heavy metal contamination but further validation is required.