Effects of Calcium on the Uptake and Elimination of Cadmium and Zinc in Asiatic Clams

We examined the effects of Ca, one of the major ions contributing to water hardness, on the uptake and elimination of Cd and Zn in the Asiatic clam Corbicula fluminea, a common bivalve species in the rivers and streams of Southern China. Over the wide range of dissolved Cd (4.5 to 446 nM) and Zn (15.3 to 1529 nM) concentrations, uptake of both metals increased with increasing dissolved metal concentration, showing Michaelis-Menten–type saturation kinetics. At each tested metal concentration, increased Ca concentration led to a significantly lower metal uptake. When the Ca, Cd, Zn concentrations were expressed as free-ion activities, Cd and Zn uptake data fitted the Michaelis-Menten inhibition model well. We also quantified the effects of the two Ca blockers on metal uptake by the clams. Verapamil significantly inhibited the uptake Cd and Zn, but the inhibitive effect of lanthanum on metal uptake was more evident for Cd than for Zn. Ca did not significantly affect the assimilation efficiency of either metal from ingested phytoplankton, nor did it affect the elimination of the two metals during a 1-month depuration period. Our study has shown that Ca inhibited the uptake of metals from water; such an effect could be predicted based on the free-metal ion activities. Ca did not seem to have a direct effect on metal assimilation from food or metal elimination from the bivalve.     

Influences of metal concentration in phytoplankton and seawater on metal assimilation and elimination in marine copepods

Radiotracer experiments were conducted to examine the influence of the concentration of Cd, Se, and Zn in ingested phytoplankton (dinoflagellate Prorocentrum minimum and diatom Thalassiosira weissflogii) and in ambient seawater on metal assimilation and elimination efficiencies of three marine copepods, Acartia spinicauda, Paracalanus aculeatus, and Calanus sinicus. The assimilation efficiencies (AEs) decreased by 1.7 to 2.0 times, 1.4 to 4.1 times, and 1.3 to 2.2 times in the copepods with an increase in metal concentration in ingested algae by 16 to 84 times, 14 times, and 45 to 153 times, for Cd, Se, and Zn, respectively. However, the physiologic turnover rate constant was relatively independent of the metal concentration in copepods. No evidence was found of any interaction between Cd and Zn in their assimilation by copepods. Assimilation efficiencies of Cd were higher in copepods feeding on the dinoflagellate P. minimum, whereas the AEs of Zn were higher in copepods feeding on the diatom T. weissflogii. Differences in metal distribution in algal cytoplasm at different ambient metal concentrations may be partially responsible for the observed influence of metal concentration in algal cells on metal assimilation in copepods. However, metal desorption within the gut of the copepod may have little influence on metal assimilation, as a result of the short gut residence time of food particles and the neutral gut pH. Our study also indicated that the ingestion rate of copepods was reduced by a higher concentration of Cd and Se, but was not affected by Zn concentration in the food particles. Consequently, partial regulation of metal trophic transfer in response to increasing metal contamination may be achieved by a change in metal assimilation efficiency and the ingestion activity of the copepod, but not by changes in metal turnover rates from the animals.     

Kinetic uptake of bioavailable cadmium,selenium, and zinc by Daphnia magna

Kinetic uptake of Cd, Se(IV), and Zn by Daphnia magna from the dissolved phase was determined using radiotracer techniques in moderately hard water. The metal influx rate and distribution in the soft tissue and the exoskeleton of the daphnids as influenced by metal concentration, inorganic ligands including pH, Ca2+ and SO4(2-), and body size were quantified. When the metal concentrations were <180 nM for Cd and <769 nM for Zn, the concentration factor in daphnids increased linearly within the 12 h of exposure. At a higher concentration, apparent steady state was reached after 3 h of exposure. Cadmium and Zn distribution in the soft tissues was not affected by the total ambient concentrations, whereas Se distribution in the soft tissue decreased by 7 to 10% with increasing Se concentration from 16 to 643 nM. A linear positive power relationship was found between the influx rates of the metals and the ambient concentrations. The concentration factor for Se, however, decreased significantly with increasing Se concentration in water. The influx rate of metals was inversely related to the body size in a power function. When the pH in ambient water increased from 5.0 to 7.0, the influx rate of Cd, Se, and Zn increased by 2.9, 16.6, and 4.1 times, respectively. The influx rates of Cd, Se, and Zn decreased by 6.9, 8.7, and 4.4 times, respectively, with an increase in Ca2+ concentration from 0.6 to 5.1 mM. In contrast, the uptake rates of all three metals were not significantly affected by the SO4(2-) concentration. The majority of accumulated Se was distributed in the soft tissues after 12 h of exposure, whereas Cd and Zn were about evenly distributed in the soft tissue and exoskeleton. Any changes in pH, Ca2+, and SO4(2-) concentrations did not apparently affect their distributions in the daphnids. Our study provides important kinetic data necessary for delineating the exposure routes and for further development of the biotic ligand model in Daphnia. Using a bioenergetic-based kinetic model, we showed that the dissolved uptake is dominant for Zn accumulation (>50%). For Cd and Se, dietary exposure is dominant when the bioconcentration factors of these metals in phytoplankton are at the high end.     

Biokinetics and tolerance development of toxic metals in Daphnia magna

Daphnia magna is widespread in many freshwater systems of temperate regions and frequently is used to test metal toxicity. Recently, studies have been performed to determine metal biokinetics and development of tolerance in this important zooplankton species. In the present paper, we review the recent progress in these areas and suggest possible directions for future studies. Substantial differences exist in aqueous uptake, dietary assimilation, and elimination of several metals (Cd, Se, Zn, Ag, Hg, and MeHg) by D. magna. The routes of uptake are metal-specific, with Se and MeHg being accumulated predominantly through diet. All metals except Ag can be biomagnified from algae to D. magna, providing that metal concentrations in algae and algal food density are relatively low. Methylmercury is biomagnified in all situations. As a route for metal elimination in D. magna, maternal transfer is especially important for Se, Zn, and MeHg. On the other hand, the effect of single-generation exposure to metals on D. magna is very different from multigeneration exposure, which often results in a significantly higher metal tolerance. Moreover, D. magna easily loses metal tolerance developed through long-term exposure. Recovery from metal stress can temporarily increase the sensitivity of D. magna to metal toxicity. Finally, metallothionein-like protein is responsible for minimizing metal toxicity in D. magna. The results inferred from these studies can be extrapolated to other aquatic invertebrates as well as to other pollutants in the aquatic environment.     

Single and joint effects of Zn and Cd on Porcellio scaber (Crustacea, Isopoda) exposed to artificially contaminated food

This study aimed at determining effects of Zn, Cd and their equitoxic mixtures on metal assimilation and food consumption of the terrestrial isopod Porcellio scaber, in relation to metal availability in the food. Cd was four times less water-extractable than Zn. Cd or Zn extractability was affected neither by metal concentration nor by the presence of the other metal. In single metal exposures, assimilation efficiency (AE) was up to five times higher for Cd than for Zn. In a mixture, AE of Cd significantly increased at low mixture concentrations and decreased at high mixture concentrations. AE of Zn significantly increased at intermediate mixture concentrations. Effects of the Zn and Cd mixture on food consumption were additive (28-day EC_50,total=1.10 TU; EC_{50,water-extractable}=1.18 TU) when based on total and water-extractable concentrations but antagonistic when related to internal metal concentrations in the isopods (EC_50,internal=1.40 TU).     

A kinetic model for the relative contribution of waterborne and dietary cadmium and zinc in the common carp (Cyprinus carpio)

In this work, the uptake and whole-body accumulation of Cd and Zn by the common carp from water, diet, and a combination of both was studied using the radioactive tracers 09Cd and 65Zn. A three-compartment pharmacokinetic model was constructed and revealed metal- and exposure route-dependent accumulation profiles. Under the tested exposure conditions in water (0.1 microM Cd and 1 microM Zn), only 0.1 and 0.07%, respectively, of the total Cd and Zn load that passed the branchial surfaces was accumulated by the carp. In contrast, the carp accumulated 20 and 29% of the total ingested Cd and Zn load (mean concentrations in food of newly accumulated metals were 2.24 +/- 0.29 and 19.91 +/- 2.89 nmol/g wet weight, respectively). However, the contribution of the waterborne metals to the whole-body metal accumulation was higher than the metal uptake from food. The constructed model was used to simulate the effect of variable assimilation efficiency (AE) values on the relative importance of dietary Cd and Zn exposure to the overall metal accumulation in common carp. This simulation was performed under a realistic exposure scenario and with Cd and Zn AE values ranging from 5 to 95%. Dissolved Zn levels were higher and Cd levels lower compared to the laboratory experiments. Levels in the midge larvae were much higher. The results show that under these conditions, even at the lowest Cd and Zn AEs, almost 40% of the total body Cd and Zn concentrations originated from dietary Cd and Zn exposure. Taking into account the Cd and Zn AE of the laboratory experiments, respectively, 31 and 55%, more than 85% of the total body Cd and Zn was taken up from the food.     

The uptake of Cd,Cr, and Zn by the macroalga Enteromorpha crinita and subsequent transfer to the marine herbivorous rabbitfish,Siganus canaliculatus

Marine macroalgae can appreciably accumulate metals out of seawater and are the principle food source for marine herbivorous fish, thus playing an important role in introducing metals into marine food webs. There have been few experimental studies on metal transfer from macroalgae up the trophic chain. In this study, we examined the assimilation efficiencies (AEs) of Cd, Cr, and Zn in the rabbitfish, Siganus canaliculatus, which feeds on the macroalga Enteromorpha crinita. The influences of metal concentration and nutrient conditions in the macroalga, and starvation on the assimilation of metals in fish were investigated. The macroalgae were radiolabeled by direct exposure to radiotracers in the dissolved phase. The uptake of metals in the macroalgae proceeded linearly and was directly proportional to the metal concentration in the ambient seawater. Ammonium and nitrate enrichment significantly increased the accumulation of Cd by the macroalgae, whereas Cr accumulation was not influenced by nutrient enrichment. The metal concentrations in the macroalgae did not have any effect on metal assimilation in the rabbitfish. The measured AEs of the metals were generally higher in starved rabbitfish than in fed fish, probably as a result of the prolonged retention of metals in the gut of fish. Rabbitfish fed macroalgae that had been previously exposed to ammonium or nitrate addition decreased their assimilation of Cd and Zn, but not Cr. Phosphate enrichment did not have any significant effect on metal AE in the rabbitfish. Our study demonstrated that marine herbivorous fish can appreciably assimilate trace metals from marine macroalgae and that trophic transfer should be considered as a source for metal accumulation in fish. Nutrient condition and starvation do have considerable effects on the AEs of Cd and Zn in rabbitfish that are fed macroalgae.     

The Uptake of Cd,Cr, and Zn by the Macroalga <Emphasis Type="Italic">Enteromorpha crinita</Emphasis> and Subsequent Transfer to the Marine Herbivorous Rabbitfish, <Emphasis Type="Italic">Siganus canaliculatus</Emphasis>

Marine macroalgae can appreciably accumulate metals out of seawater and are the principle food source for marine herbivorous fish, thus playing an important role in introducing metals into marine food webs. There have been few experimental studies on metal transfer from macroalgae up the trophic chain. In this study, we examined the assimilation efficiencies (AEs) of Cd, Cr, and Zn in the rabbitfish, Siganus canaliculatus, which feeds on the macroalga Enteromorpha crinita. The influences of metal concentration and nutrient conditions in the macroalga, and starvation on the assimilation of metals in fish were investigated. The macroalgae were radiolabeled by direct exposure to radiotracers in the dissolved phase. The uptake of metals in the macroalgae proceeded linearly and was directly proportional to the metal concentration in the ambient seawater. Ammonium and nitrate enrichment significantly increased the accumulation of Cd by the macroalgae, whereas Cr accumulation was not influenced by nutrient enrichment. The metal concentrations in the macroalgae did not have any effect on metal assimilation in the rabbitfish. The measured AEs of the metals were generally higher in starved rabbitfish than in fed fish, probably as a result of the prolonged retention of metals in the gut of fish. Rabbitfish fed macroalgae that had been previously exposed to ammonium or nitrate addition decreased their assimilation of Cd and Zn, but not Cr. Phosphate enrichment did not have any significant effect on metal AE in the rabbitfish. Our study demonstrated that marine herbivorous fish can appreciably assimilate trace metals from marine macroalgae and that trophic transfer should be considered as a source for metal accumulation in fish. Nutrient condition and starvation do have considerable effects on the AEs of Cd and Zn in rabbitfish that are fed macroalgae. Received: 6 April 2002/Accepted: 19 August 2002     

Comparison of metal uptake rate and absorption efficiency in marine bivalves

Recent studies have quantified extensively metal assimilation efficiency from ingested food sources in aquatic invertebrates. Metal absorption efficiency (alpha) from the dissolved phase is analogous to metal assimilation efficiency, but it remains poorly defined and quantified. In this study, the alpha of four trace metals [Cd, Cr(VI), Se(IV), and Zn] was determined in three species of marine bivalves (green mussel [Perna viridis], black mussel [Septifer virgatus], and clam [Ruditapes philippinarum]). Individual bivalves were first measured for their clearance rates, followed by measurements of the metal influx rate, after which the metal alpha and the uptake rate constant (Ku) were then computed. Among the four metals considered, the highest Ku and alpha were found for Zn, followed by Cd > Cr(VI) > Se(IV). The Ku values were comparable between the two mussels but were 1.8- to 3.3-fold lower in the clams. Interspecific difference in metal Ku was strongly related to, but intraspecific difference in Ku was not affected by, the bivalve's clearance rate. Interspecific difference in metal alpha was smaller than the metal Ku and was independent of the clearance rate, whereas the intraspecific difference in metal alpha correlated with the individual variations of the clearance rate. Within each bivalve species, a significant negative correlation was found between the metal alpha and the clearance rate, implying that an individual pumping a greater amount of water was coupled to a lower alpha. Significant correlation between the alpha of four metals was also documented in all three species of bivalves. Thus, metal bioavailability from the aqueous phase was directly related to the physiological conditions of the animals. Both the aqueous chemistry and the physiology of the animals can be important in affecting metal bioavailability from the dissolved phase.     

Sediment geochemical controls on CD, CR, and ZN assmilation by the clam Ruditapes philippinarum

Several important sediment geochemical factors (particle size, contact time, artificial sediment composition, acid volatile sulfide, and pure inorganic particles coated or uncoated with humic acid) were examined for their influences on the assimilation of Cd, Cr, and Zn in the marine clam Ruditapes philippinarum. Metal desorption and phase speciation were concurrently measured for these particles. Differences in particle size did not significantly affect metal assimilation. Increasing contact time with sediments decreased assimilation by the clams of Cr and Zn, but not of Cd. A significant positive relationship was found between the metal assimilation efficiency (AE) and the metal partitioning in the exchangeable fraction of sediments for Cd and Zn. The assimilation of metals was negatively dependent on the metal distribution in the reducible (Fe oxide) fraction, indicating that Fe oxide reduced metal bioavailability to the clams. Metal AEs measured with artificially prepared sediments were comparable to those measured with natural sediments. The AEs were significantly higher in artificial sediment without Fe oxide. Humic acid coating appeared to have no major and consistent influence on metal assimilation by the clams. The AEs measured for pure inorganic particles were much higher than those measured for sediments or artificially prepared sediments, except for the Fe oxide particles, indicating that digestion may behave differently for pure particles. Metals bound to artificially synthesized acid volatile sulfide were much less bioavailable to the clams. The AEs were as low as 4% for Cd and 7% for Zn, whereas the AE for Cr was somewhat similar to those measured for other types of particles. When all particle types (natural sediments, artificial sediment, and pure particles) were considered, a significant relationship was found between Cd AE and its desorption in seawater. This study has demonstrated that several geochemical fractions of the sediments, particularly the exchangeable, Fe oxide, and sulfide fractions, are important in determining metal bioavailability from sediments to clams.     

Assimilation and bioconcentration of Ag and Cd by the marine black bream after waterborne and dietary metal exposure

We determined the aqueous uptake and dietary assimilation of Cd and Ag by the marine black bream Acanthopagrus schlegeli following one to four weeks' exposure (or conditioning) to waterborne or dietary Cd or Ag at different concentrations. The concentrations of metals and metallothioneins (MT) in different tissues also were determined. The viscera contained the highest Ag, Cd, and MT concentrations after metal exposure. After exposure to waterborne metals, the metal and MT concentrations in the gills were higher than those in the remaining tissues (mainly muscles and bones), but this pattern was reversed following exposure to dietary metals. The assimilation efficiencies (AEs) of Cd and Ag ranged from 6 to 24% and 15 to 30%, respectively. The rate constant of uptake from the dissolved phase (ku) of Cd and Ag ranged from 2.2 to 7.5 and 8.0 to 31.7 L kg(-1) d(-l), respectively. In all the exposure experiments, the ku and AE increased with induced MT concentration and tissue metal concentration. Increasing metal accumulation may have been due to the increased available binding sites following the induction of MT in the fish. Furthermore, the MT induced by either Cd or Ag was not specific, but was able to bind with both metals and enhance bioaccumulation. Exposure to dissolved and dietary metals may increase metal accumulation, which potentially may lead to metal toxicity, although the fish may develop a tolerance to metals due to the apparent induction of MT.     

The influences of ambient and body calcium on cadmium and zinc accumulation in Daphnia magna

The dissolved uptake rate constant (k(u)), dietary assimilation efficiency (AE), and efflux rate constant (k(e)) of cadmium (Cd) and zinc (Zn) were quantified in a freshwater zooplankton Daphnia magna cultured at different ambient calcium (Ca) concentrations. The animals were first acclimated to different ambient Ca levels for 7 d from birth before the biokinetic measurements at corresponding Ca levels. With increasing ambient Ca level from 0.5 to 200 mg/L, the body Ca content increased from 0.91% (as tissue dry wt) to 3.75%. The k(u) for Cd decreased by nine times; for Zn it decreased by 2.6 times; and the AE decreased from 62 to 19% and from 46 to 24% for Cd and Zn, respectively. In contrast, Ca levels did not affect significantly the efflux rates of Cd and Zn. The effects of ambient and body Ca levels were separated by measuring the biokinetic parameters in both low--(0.5 mg/L) and high--(50 mg/L) Ca environments using the daphnids containing different body Ca levels. Ambient and body Ca levels had synergistic inhibitory effects on the AEs of Cd and Zn; however, the protective effects against dissolved uptake of Cd and Zn were explained fully by the effects of ambient Ca. The body Ca either had no significant effect (for Cd) or stimulative effects (for Zn). Multiphase biokinetic modeling using the measured parameters gave reasonable predictions of the body burdens of Cd and Zn in different Ca environments. Our results better explain the role of ambient and body Ca in the accumulation of Cd and Zn in D. magna.     

Seasonal study on the Cd, Se, and Zn uptake by natural coastal phytoplankton assemblages

We conducted a seasonal study of the uptake of Cd, Se(IV), and Zn by natural phytoplankton assemblages collected from two locations in Hong Kong coastal waters. Marine phytoplankton in Port Shelter was dominated by diatoms and cyanobacteria, while phytoplankton in Tolo Harbour was dominated by dinoflagellates. The natural phytoplankton assemblages were spiked with macronutrients (N and P) and metals, and the accumulation of these metals was followed for 3 to 5 d in the laboratory. The two coastal systems were mostly P limited, with N limitation observed only during the summer in Tolo Harbour. With the additions of these macronutrients, phytoplankton responded in different ways in their accumulation of trace metals. Cadmium concentrations (normalized to chlorophyll a [chl a] concentration) increased by 25 to 38% with the simultaneous addition of N and P but were roughly constant when N alone was added. In N-limited systems, the Cd concentration increased by 25% with N addition. Concentrations of Se were reduced competitively (by 40-45%) with P addition. Zinc concentrations were reduced by 23 to 38% when both N and P were added, largely because of a major increase in biomass (biodilution). Both Cd and Zn uptake rates increased with increasing growth rates. The uptake rate of Se was not related to the growth rate because of the confounding influence of P addition. The particle concentration effect was observed only for Zn, in which the increasing chl a concentration biodiluted the Zn concentration in the phytoplankton. Overall, our results suggest that macronutrients may affect trace metal accumulation in natural phytoplankton because of their controls on cell growth, which may have implications for our understanding of trace metal dynamics in coastal ecosystems during phytoplankton blooms as well as for the prediction of trophic transfer in food chains.     

Influence of metal concentrations in food on metal uptake and accumulation in sawfly larvae

Uptake and accumulation of Cd, Cu, Fe, and Zn by sawfly larvae, Dolerus sp, were studied. Larvae were reared under controlled conditions, allowing their uptake of ingested metals to be calculated after exposure to two different doses of metals. After 10 days of exposure, the concentration of Cd was higher than at the start of exposure, whereas concentration of Cu was higher only in the high dose group. In contrast, the Fe concentration decreased during exposure. Concentrations of Zn in the larvae were the same irrespective of the Zn concentrations in the food. Uptake of the metals, quantified in percent of ingested amounts, was 11% for Cd in both low and high dose groups. The uptake of Zn was 26% at the low dose and 12% at the high dose, while that of Cu was 12% and 19%, respectively. There was a loss in both low and high doses of Fe. The amount of a metal taken up by the larvae was correlated with their growth increment. In conclusion, the concentrations of Cd and Cu in plants influence the concentrations of these metals in plant-eating insects more than the Fe and Zn concentrations.     

Metal accumulation and biomarker responses in Daphnia magna following cadmium and zinc exposure

Metal accumulation (quantified as body burden) and the responses of two biomarkers (metallothionein [MT] induction and superoxide dismutase [SOD] activity) in a freshwater cladoceran Daphnia magna were quantified after being exposed to different concentrations of Cd and Zn for 3 d. The Cd and Zn body burdens increased with increasing exposure concentrations. The responses of biomarkers were metal-specific and concentration-dependent. As a detoxification mechanism, MT was induced by exposure to Cd or Zn or Cd + Zn and was dependent on the exposure concentrations. Low concentrations of Cd or Zn exposure increased the SOD activity, which was, however, inhibited at higher metal exposure concentrations. Metallothionein concentrations in daphnids were significantly related to the metal body burden. The SOD activity was inversely related to Cd body burden, while it was independent of the Zn body burden. Survivorship of D. magna was high at lower Cd/MT ratios in the animals but then decreased linearly when the ratio exceeded six. In contrast, the relationship between survivorship and Zn/MT ratio was different due to the essentiality of Zn. Overall, MT induction was a useful indicator of metal pollution in cladocerans.     

Metal-metal interactions of dietary cadmium, copper and zinc in rainbow trout, Oncorhynchus mykiss

The influence of metal-metal interactions on uptake, accumulation, plasma transport and chronic toxicity of dietary Cu, Cd and Zn in rainbow trout (Oncorhynchus mykiss) was explored. Juvenile rainbow trout were fed diets supplemented with (μg/g) 500 Cu, 1000 Zn and 500 Cd singly and as a ternary mixture at 2.5% body weight daily ration for 28 days. Complex interactions among the metals dependent on the tissue/organ, metals ratios and duration of exposure were observed. While Zn did not accumulate, whole-body Cd and Cu concentrations increased following linear and saturation patterns, respectively. Early enhanced whole-body Cu accumulation in fish exposed to the metals mixture was correlated with reduced Cd concentration whereas late enhancement of Cd accumulation corresponded with elevated Cd concentration. This suggests early mutual antagonism and late cooperation between Cd and Cu probably due to interactions at temporally variable metal accumulation sites. At the level of uptake, Cd and Cu were either antagonistic or mutually increased the concentrations of each other depending on the duration of exposure and section of the gut. At the level of transport, enhanced Cd accumulation in plasma was closely correlated with reduced concentrations of both Zn and Cu indicating competitive binding to plasma proteins and/or antagonism at uptake sites. Compared to the Cu alone exposure, Cu concentrations were either lower (gills and carcasses) or higher (liver and kidney) in fish exposed to the metals mixture. On the other hand, Cd accumulation was enhanced in livers and carcasses of fish exposed to the mixture compared to those exposed to Cd alone, while Zn stimulated Cu accumulation in gills. Chronic toxicity was demonstrated by elevated malondialdehyde levels in livers and reduced concentrations of Zn and Cu in plasma. Overall, interactions of Cd, Cu and Zn are not always consistent with the isomorphous competitive binding theory.     

Size-dependence of the potential for metal biomagnification in early life stages of marine fish

We investigated the bioaccumulation of metals (Cd, Se, and Zn) in different juvenile sizes of black sea bream Acanthopagrus schlegeli by applying a biokinetic model. A series of experiments were conducted to determine the physiological kinetic parameters, including uptake rate constant of waterborne metals, ingestion rate, assimilation efficiency of dietary metals, efflux rate constant, and specific growth rate as a function of fish size. Body concentration of metals as a function of body size was then simulated by the kinetic model and compared with the actual measurements. The uptake rate constants decreased with increasing fish size, with an allometric exponent (b) of 0.615 to 0.662. Ingestion rate was also negatively correlated with the fish size (b = -0.604). Assimilation efficiencies of Cd were independent of body size, whereas those of Se and Zn increased with fish size. In contrast, efflux rate constants and growth rate constants for metals were comparable in different sizes of fish. Predicted concentrations of Cd and Zn were comparable to actual measurements and were negatively correlated with fish size. Ingestion rate was the most important parameter accounting for the size-dependent bioaccumulation of metals, followed by dietary assimilation. We further showed that the trophic transfer potentials of Se and Zn were > 1 in smaller fish but then reduced to < 1 in larger ones, suggesting that trophic transfer potentials are highly size dependent in marine fish.     

Effects of dietary zinc and cadmium on tissue selenium concentration and glutathione peroxidase activity in rats fed DL-selenomethionine or sodium selenite.

The effects of dietary zinc (Zn) and cadmium (Cd) on tissue selenium (Se) concentration and glutathione peroxidase (GSH-Px) activity were studied in weanling male Wistar rats. One group of rats was fed a purified diet based on casein and sucrose, and the other rats used in a 2 x 2 x 2 factorial arrangement of treatment were fed this diet supplemented with 0.1 mg Se/kg, either as DL-selenomethionine or sodium selenite and plus 100 mg Zn/kg as zinc sulfate or 5 mg Cd/kg as cadmium chloride or both for 4 weeks. Se concentrations in plasma, erythrocytes, muscle, heart, and liver were significantly elevated by Zn. Cd significantly decreased Se concentration in muscle. Addition of Zn to the diets markedly increased (p less than 0.001) hepatic GSH-Px activity. However, Cd in the diets produced a significant increase (p less than 0.001) in erythrocyte GSH-Px activity. These results indicate that Zn level of marginal deficiency (8.6 mg/kg diet) can decrease Se availability and a small excess of Zn increases Se availability for hepatic GSH-Px activity.     

Evaluating the trophic transfer of cadmium, polonium, and methylmercury in an estuarine food chain

We examined the transfer of Cd, methylmercury (MeHg), and Po in an estuarine food chain (from phytoplankton to zooplankton [Daphnia pulex] to killifish [Fundulus heteroclitus] and, finally, to juvenile striped bass [Morone saxatilis]) to better understand both the extent to which these elements may biomagnify and the underlying mechanisms governing this biomagnification. Among the phytoplankton examined (Cyclotella meneghiniana and Chlamdomonas reinhardtii), metal uptake was rapid, and volume concentration factors for all metals reached values between 10(4) and 10(5). The resulting assimilation efficiencies (AEs) of ingested metals in all animals were highest for MeHg, with values greater than 76%. The AEs of Cd were 21 to 33% in D. pulex, 1 to 16% in F. heteroclitus, and 38 to 56% in M. saxatilis. Polonium AEs were 69 to 87% in D. pulex, 25 to 41% in F. heteroclitus, and 9 to 21% in M. saxatilis. Loss rate constants (ke) of metals in D. pulex ranged from 0.04/d for MeHg to 0.39/d for Po; metal ke values for F. heteroclitus ranged from 0.01 to 0.02/d. Using a kinetic model, we showed that the trophic transfer factor, defined as the ratio of metal concentration in predatory animals to metal concentration in prey organisms, was greater than unity for all three metals in D. pulex feeding on phytoplankton, suggesting that these metals may be biomagnified at this trophic step. In killifish feeding on D. pulex, the trophic transfer factor was consistently greater than one for MeHg, consistently less than one for Cd, and from 0.1 to 1.4 for Po, suggesting that both MeHg and Po have the potential to biomagnify at this trophic step.     

Potential Toxic Metal Accumulation in Soil,Forage and Blood Plasma of Buffaloes Sampled from Jhang,Pakistan

This study was conducted to determine the concentration of toxic metals in soil, forage and blood plasma of lactating and non-lactating buffaloes in the district Jhang, Punjab, Pakistan. Soil samples were collected from varying distances from the road side. Plasma separation was achieved by centrifugation. The concentration of arsenic (As), selenium (Se), cadmium (Cd), chromium (Cr), iron (Fe), zinc (Zn), copper (Cu) and cobalt (Co) were determined by using Atomic Absorption Spectrophotometer. The results of the study showed that the mean As, Se and Cd concentrations in soil samples were lower while Cr, Fe, Zn, Cu and Co were higher than the official guidelines. In plasma samples, mean concentration values of Co, Zn, Fe, Cd, Se and As were lower while Cu and Cr were higher than the recommended concentrations. According to the results of the study there was no potential exposure of toxicity in buffaloes of the study area.