Cd and Zn uptake kinetics in Daphnia magna in relation to Cd exposure history

The uptake kinetics of Cd and Zn in a freshwater cladoceran Daphnia magna after exposure to different concentrations of Cd for various durations was quantified. The accumulated Cd concentrations increased with ambient Cd concentration and exposure duration. As a detoxification mechanism, metallothioneins (MTs) were induced when the Cd preexposure condition was beyond the noneffect threshold. The MT induction was dependent on both Cd concentration and duration of preexposure. Increasing the Cd exposure concentration to 20 microg L(-1) for 3 d caused a 44% reduction in Cd assimilation efficiency (AE, the fraction assimilated by the animals after digestion) by the daphnids from the dietary phase, but a 2.4-fold increase in Zn AE. Generally, the dissolved metal uptake rate was not significantly affected by the different Cd preexposure regimes, except at a much higher Cd concentration (20 microg L(-1)) when the Zn influx was enhanced. Significant effects from Cd exposure on the ingestion rate of the daphnids were also observed. When the MT synthesis was not coupled with the accumulated Cd tissue burden (e.g., a delay in MT synthesis), apparent Cd toxicity on the feeding behavior and the Cd AE was observed, thus highlighting the importance of MTs in modifying the metal uptake kinetics of D. magna. Overall, daphnids responded to acute Cd exposure by reducing their Cd AE and ingestion, whereas they developed a tolerance to Cd following chronic exposure. The bioavailability of Zn was enhanced as a result of Cd preexposure. This study highlights the important influences of Cd preexposure history on the biokinetics and potential toxicity of Cd and Zn to D. magna.     

Effects of aqueous and dietary preexposure and resulting body burden on silver biokinetics in the green mussel Perma viridis

To determine whether preexposure of green mussel Perna viridis to Ag influenced metal uptake kinetics we compared various physiological indicators of metal uptake kinetics between the control mussels and mussels preexposed to Ag in both diet and water at different levels (up to 5 weeks). In all preexposed mussels, the assimilation of Ag increased by 1.1-3.0 times with increasing Ag body concentration (0.651-19.3 microg g(-1)) as compared with the controls (Ag body concentration of 0.311-0.479 microg g(-1)), whereas the efflux rate constants decreased by 45-88%. There was no significant increase in Ag associated with the metallothionein-like protein (MTLP) fraction following exposure of the mussels to Ag through either the dissolved or food phase. The clearance rates were little affected or depressed byAg preexposure, and the relationship between the Ag influx rate from the dissolved phase and the Ag preexposure was somewhat complicated. The influx rate decreased with increasing Ag body burden at <2.5 microg g(-1), above which it increased with increasing Ag body burden. Our results indicate that the mussels may modify physiological processes to ambient chronic Ag exposure, consequently accumulating more Ag. Ag body concentration in these mussels may therefore increase disproportionally in response to increasing Ag concentration in the ambient environments. Ag preexposure and resulting body burden should be considered carefully when interpreting the observed Ag concentration in biomonitoring animals to evaluate the Ag pollution in seawater.     

Uptake of aqueous and dietary metals by mussel Perna viridis with different Cd exposure histories

The influences of different Cd pre-exposure regimes (route, concentration, and duration of Cd exposure) on the bioavailability of Cd, Ag, Hg, and Zn to the green mussels Perna viridis were quantified in this study. Following pre-exposing the mussels to Cd, we measured the mussel's tissue Cd concentration and clearance rate, as well as the metal dietary assimilation efficiency (AE) and the influx rate from the dissolved phase of the four studied metals. Differences in the route (aqueous and dietary pathways) and the history of pre-exposure (combined Cd concentration and duration) did not significantly affect the subsequent Cd dietary and aqueous uptake. The Cd dietary AEs increased following both the dissolved and dietary Cd pre-exposure. There was a significant correlation between the Cd AE and the accumulated Cd body concentration in the mussels. Dietary assimilation of Hg and Zn also increased slightly (but not significantly) after Cd pre-exposure, but the AEs of Ag remained constant. Except for the significant decrease in the dissolved uptake of Hg, Cd pre-exposure did not apparently affect the uptake of the other three metals from the solution. Metal-metal interactions are likely to be affected by the specificity of metallothionein induction. Our study demonstrated that the Cd body concentration as well as the environmental Cd concentration instead of the history of pre-exposure was more important in affecting the Cd accumulation in the mussels. Such factors need to be considered in interpreting metal body concentrations in biomonitors.     

Understanding the differences in Cd and Zn bioaccumulation and subcellular storage among different populations of marine clams

The marine clams Mactra veneriformis were collected from three different locations in a contaminated bay in Northern China. Another species of clams Ruditapes philippinarum was collected from the same contaminated bay as well as from a relatively clean site in Hong Kong. The indices of Cd and Zn bioaccumulation (assimilation efficiency, dissolved uptake rate, and efflux rate), tissue concentration, subcellular distribution, metallothionein (MT) content, and clearance rate of the clams were subsequently quantified in these populations in the laboratory. In the two species of clams, the population with a higher Cd tissue concentration assimilated Cd and Zn more efficiently, in correlation with an increase in the Cd associated with the metallothionein-like protein (MTLP) fraction. The subcellular partitioning of Zn was similar among the different populations. The dissolved uptake rates of Cd and Zn were not influenced by the different tissue concentrations of metals in the clams. However, the clam R. philippinarum from the contaminated site reduced their Zn uptake rate constants in response to increasing Zn concentration in the water. Differences in Cd and Zn tissue concentrations had little influence on the metal efflux rate constant and the clams' clearance rate. Our results indicate that the higher Cd and Zn tissue concentrations observed in these two species may be partially caused by the high levels of metal assimilation. Populations living in contaminated environments may be able to modify their physiological and biochemical responses to metal stress, which can subsequently alter trace metal bioaccumulation to aquatic animals. The relative significance of dietary uptake and the potential trophic transfer of metals in the contaminated areas may be substantially different from those in the clean environments.     

Differences in dissolved cadmium and zinc uptake among stream insects: mechanistic explanations

This study examined the extent to which dissolved Cd and Zn uptake rates vary in several aquatic insect taxa commonly used as indicators of ecological health. We further attempted to explain the mechanisms underlying observed differences. By comparing dissolved Cd and Zn uptake rates in several aquatic insect species, we demonstrated that species vary widely in these processes. Dissolved uptake rates were not related to gross morphological features such as body size or gill size--features that influence water permeability and therefore have ionoregulatory importance. However, finer morphological features, specifically, the relative numbers of ionoregulatory cells (chloride cells), appeared to be related to dissolved metal uptake rates. This observation was supported by Michaelis-Menten type kinetics experiments, which showed that dissolved Cd uptake rates were driven by the numbers of Cd transporters and not by the affinities of those transporters to Cd. Calcium concentrations in exposure media similarly affected Cd and Zn uptake rates in the caddisfly Hydropsyche californica. Dissolved Cd and Zn uptake rates strongly co-varied among species, suggesting that these metals are transported by similar mechanisms.     

A biodynamic understanding of dietborne metal uptake by a freshwater invertebrate

Aquatic organisms accumulate metals from dissolved and particulate phases. Dietborne metal uptake likely prevails in nature, but the physiological processes governing metal bioaccumulation from diet are not fully understood. We characterize dietborne copper, cadmium, and nickel uptake by a freshwater gastropod (Lymnaea stagnalis) both in terms of biodynamics and membrane transport characteristics. We use enriched stable isotopes to trace newly accumulated metals from diet, determine food ingestion rate (IR) and estimate metal assimilation efficiency (AE). Upon 18-h exposure, dietborne metal influx was linear over a range encompassing most environmental concentrations. Dietary metal uptake rate constants (k(uf)) ranged from 0.104 to 0.162 g g(-1) day(-1), and appeared to be an expression of transmembrane transport characteristics. Although k(uf) values were 1000-times lower than uptake rate constants from solution, biodynamic modeling showed that diet is the major Cd, Cu, and Ni source in nature. AE varied slightly among metals and exposure concentrations (84-95%). Suppression of Cd and Cu influxes upon exposure to extreme concentrations coincided with a 10-fold decrease in food IR, suggesting that feeding inhibition could act as an end point for dietary metal toxicity in L. stagnalis.     

Dynamic model for the accumulation of cadmium and zinc from water and sediment by the aquatic oligochaete, Tubifex tubifex

In aquatic environments, organisms are exposed to and accumulate metals via waterborne and dietary routes including ingested sediment. A key element in understanding metal uptake and accumulation is information concerning the relative importance of the routes of uptake and the kinetics of the processes. In this work the bioaccumulation of the essential element zinc and the nonessential element cadmium were studied from the aqueous and sediment phase, in the cosmopolitan oligochaete Tubifex tubifex, using the radiotracers 109Cd and 65Zn. A compartmental kinetic model was constructed and parametrized by fitting the model to metal body concentrations. Using the pharmacokinetic modeling approach and taking into account the distribution of the metal between water and sediment, the different routes were quantitatively separated. Under the experimental conditions, the sediment phase accounted for 9.8% of the cadmium and 52% of the zinc uptake. These values are based on the uptake of the radiotracers spiked sediments and therefore likely represent maximal values since it was shown that under the specific conditions this was the most mobile metal fraction. This difference was largely explained by the large difference in assimilation efficiency between cadmium and zinc. Simulations of different conditions showed that both dissolved and sediment-associated metal can be important sources of metal exposure for the worms and that the relative importance strongly depends on the metal and exposure conditions including the lability of the metals in the sediment phase.     

Role of titanium dioxide nanoparticles in the elevated uptake and retention of cadmium and zinc in Daphnia magna

Titanium dioxide nanoparticles (nano-TiO(2)) are now widely applied in consumer products, and the dispersion of nano-TiO(2) may adsorb metals and modify their behavior and bioavailability in the aquatic environment. In the present study, the aqueous uptake, dietary assimilation efficiency (AE), and efflux rate constant (k(e)) of two toxic metals (cadmium-Cd, and zinc-Zn) adsorbed on nano-TiO(2) in a freshwater zooplankton Daphnia magna were quantified. The biokinetics was then compared to daphnids that were exposed only to dissolved metals as controls. The aqueous uptake of Cd and Zn involved an initial rapid uptake and then an apparent saturation, and the uptake of metals was accompanied by an ingestion of nano-TiO(2). The AEs of Cd and Zn adsorbed on nano-TiO(2) were 24.6 ± 2.4-44.5 ± 3.7% and 30.4 ± 3.4-51.8 ± 5.0%, respectively, and decreased with increasing concentrations of nano-TiO(2). Furthermore, the difference between the AEs of Cd and Zn indicated that the desorption of Cd and Zn from nano-TiO(2) may have occurred within the gut of daphnids. With the use of algae as carrier, the AEs of Cd and Zn adsorbed on nano-TiO(2) were significantly higher than those of Cd and Zn directly from nano-TiO(2). The efflux rate constants of Cd and Zn adsorbed on nano-TiO(2) in the zooplankton were significantly lower than those of Cd and Zn not adsorbed on nano-TiO(2). Our study shows that the uptake and retention of toxic metals is enhanced when they are adsorbed on nano-TiO(2), and suggests more attention be paid to the potential influences of nano-TiO(2) on the bioavailability and toxicity of other contaminants.     

Geochemistry of Cd,Cr, and Zn in highly contaminated sediments and its influences on assimilation by marine bivalves

We tested the controls of metal geochemistry in sediments collected from an extremely contaminated Chinese bay on metal assimilation by marine mussels and clams. Metal speciation in the contaminated sediments, quantified by the Tessier operational extraction method, was significantly dependent on metal concentrations in the sediments. The fractions of Cd in the easily exchangeable and carbonate phases increased, while the reducible and residue phases decreased with increasing Cd concentration. The majority (72-91%) of Cr was associated with the residue component with the remainder of Cr in the organic matter and reducible phases. Zn in carbonate phase increased, whereas in the organic matter and residue phases it decreased with increasing Zn concentration. The bioavailability of Cd, Cr, and Zn to marine green mussels (Perna viridis) and clams (Ruditapes philippinarum) was quantified using radiotracer spiked technique with concurrent measurements of speciation of spiked metals. There was a significant correlation between the Cd assimilation efficiency (AE) by both mussels and clams and Cd partitioning in the easily exchangeable and reducible phases. In contrast to previous studies, a negative correlation was found between the Cd AE and its total concentration in sediment, likely caused by the saturation of Cd binding sites in the gut or by its antagonistic interaction with a very high Zn concentration in these collected sediments. In contrast, there was no significant correlation between the AEs of Cr or Zn and any of their geochemical phases or their concentrations. The metal AEs were further quantified by experimentally manipulating different concentrations and ratios of acid volatile sulfide (AVS) and simultaneously extractable metals (SEM). There was no statistically significant relationship between the AEs of the three metals and the concentrations of AVS and SEM or [SEM-AVS]. Geochemical controls on metal assimilation from contaminated sediment are therefore only relatively apparent for Cd. The influences of metal speciation on metal bioavailability can be confounded by the degree to which sediments are contaminated with metals.     

Co-transport of metal complexes by the green mussel Perna viridis

We examined the uptake of ligand-bound metals (Cd and Zn) by the green mussel Perna viridis using defined artificial seawater. Different free ion concentrations (1 pM to 10 microM) in uptake solutions were created by adding different amounts of total metals (Cd 0.1 nM to 0.1 mM; Zn 0.5 nM to 0.05 mM) and ligands (EDTA, NTA, citric acid). Our results showed that Cd and Zn uptake could not be fully explained by the free Cd and Zn concentrations in the presence of different ligands, indicating that metal-ligand complexes were at least partially available for uptake by the mussels. Total Zn concentrations appeared to be a better predictor of metal uptake than the free Zn ion concentrations in the presence of different ligands. Uptake of lipophilic organic metal complexes was substantially greater than the hydrophilic metal complexes, even though the free ion concentration was comparable or lower. Moreover, the radiolabeled ligand compounds were directly accumulated by the mussels. The accumulation of metal complexes may explain the increased metal uptake with increasing ligand and total metal concentration, even though the free ion metal concentration was constant. Overall, our experimental results indicated that free metal ion cannot fully explain metal uptake since metal complex species were also available to the mussels to some extent, apparently through a co-transport process.     

Use of transplanted Zebra mussels (Dreissena polymorpha) to assess the bioavailability of microcontaminants in Flemish surface waters

Zebra mussels (Dreissena polymorpha) were translocated in cages to 56 water bodies in Flanders (Belgium) during summer 2001. After six weeks, concentrations of polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), p,p'-DDE, and trace metals were measured in the transplanted mussels. It was investigated whether total dissolved water and sediment pollutant levels or bioaccumulation factors (BAFs) and biota-sediment accumulation factors (BSAFs) were predictive for mussel tissue levels. The sample sites covered a broad range both in terms of the type and concentration of the pollutants, and this was reflected in large differences in tissue concentrations of all pollutants among the sites. The highest pollutant levels in mussels were among the highest reported in the literature. For Cd and Zn levels up to 33 and 1994 microg/g dry wt. respectively were found. The lowest levels were comparable to those from uncontaminated sites in Europe and the U.S. For Cd and Zn respectively 51 and 75% of the variation in tissue levels was described. For both metals, dissolved and particulate metal contributed to the variation in accumulation. For other pollutants, relationships between tissue concentration and water or sediment concentration were weak or nonsignificant. Then the measured environmental factors (dissolved calcium, pH, oxygen, organic carbon and clay content in the sediment) were taken into account applying multiple regression analysis, and no increase in the described variation of pollutant accumulation was observed. The BAF and BSAF for all pollutants varied up to 1000-fold even after TOC-normalization. Clear negative relationships were found between BAFs/ BSAFs and environmental levels. However, even at constant environmental concentrations a 10- to 100-fold variation in BAFs/BSAFs was observed. This study illustrated the need for biological monitoring since neither environmental     

Uptake of dissolved Ag,Cd, and Co by the clam,Macoma balthica: relative importance of overlying water,oxic pore water,and burrow water

The facultative deposit-feeding clam Macoma balthica is used as a bioindicator organism for assessing coastal metal contamination. Previous work has evaluated the assimilation of metals from different possible food sources for this clam, but no studies have measured the uptake rates of metals from different dissolved sources. This study specifically compares three different dissolved sources: overlying water (SW), oxic pore water (OPW) from a depth of <1 cm (entrained during surface deposit feeding), and burrow water (BW) (a mixture of anoxic pore water and overlying water). Uptake rates of dissolved Ag, Cd, and Co in M. balthica were measured in short-term laboratory experiments using radiotracers. Clams were exposed to metals in water only for SW and surface OPW treatments. In the BW treatment, metal uptake was compared in clams placed in radiolabeled organic-poor or organic-rich sediment under conditions in which feeding was inhibited. Uptake rate constantsfrom SW for Ag, Cd, and Co were 0.35, 0.033, and 0.035 L g(-1) day(-1), respectively. Lower uptake of dissolved metals from OPW was noted but was only significant for Co. Metal uptake from BW and SW were also comparable; however, the trend showed lower Ag and higher Co uptake from BW. Metal distributions and concentrations in the two radiolabeled sediments were affected by active irrigation of SW into the burrows; dissolved metal concentrations in BW were approximately 30% lower than that in the bulk pore water concentrations. In the organic-rich sediment, Cd and Ag partitioned more in the dissolved phase (<0.2 microm) and Co more in the particulate phase as compared with the organic-poor sediment. A sensitivity analysis using measured rate constants for uptake and a range of metal concentrations from field studies suggested that, under most conditions, uptake of dissolved Ag is primarily from OPW, Co is mostly from BW, and Cd uptake varies depending on its concentration in each compartment. Little Co or Ag is likely to be taken up from SW, whereas 20-50% of Cd may be accumulated from this source. Thus, SW, OPW, and BW are all potential sources of metals for M. balthica, and the relative importance of these sources differs among metals and is dependent on the dissolved metal concentrations in each compartment.     

Modeling the effect of temperature on bioaccumulation of metals by a marine bioindicator organism, Mytilus edulis

To determine whether regional or seasonal variability in water temperatures might affect the bioaccumulation of metals by marine invertebrates, we used a biokinetic/bioenergetic approach to model metal bioaccumulation (Ag, Am, Cd, Co, Se, and Zn) from dietary and dissolved sources by blue mussels, Mytilus edulis, acclimated at 2 and 12 degrees C. Accumulation of metal from the aqueous phase was not affected by temperature. However, Ag, Am, and Zn from diet were respectively accumulated up to 5.7-fold, 5.3-fold, and 2-fold more effectively at 2 degrees C than at 12 degrees C, largely because these three metals were assimilated from food more effectively at the lower temperature. In contrast, bioaccumulation of Cd, Co, Se from diet was not substantially affected by temperature even though efflux constants for these metals were up to 4-fold lower at 2 degrees C than at 12 degrees C. Total bioaccumulation of Ag, Am, and Zn was up to 1.7-3.6-fold higher at 2 degrees C than at 12 degrees C, with the largest differences predicted for high food conditions. Temperature-related variability in bioaccumulation of metals should be considered when interpreting patterns in metal tissue concentrations and when adapting management strategies developed for temperate seas to polar areas.     

Antagonistic interaction of mercury and selenium in a marine fish is dependent on their chemical species

It is well-known that selenium (Se) shows protective effects against mercury (Hg) bioaccumulation and toxicity, but the underlying effects of Se chemical species, concentration, and administration method are poorly known. In this study, we conducted laboratory studies on a marine fish Terapon jurbua to explain why Hg accumulation is reduced in the presence of Se observed in field studies. When Se and Hg were administrated concurrently in the fish diets, different Se species including selenite, selenate, seleno-dl-cystine (SeCys), and seleno-dl-methionine (SeMet) affected Hg bioaccumulation differently. At high concentration in fish diet (20 μg g(-1) normally), selenate and SeCys significantly reduced the dietary Hg(II) assimilation efficiency (AE) from 38% to 26%. After the fish were pre-exposed to dietary selenite or SeMet (7 μg g(-1) normally) for 22 days with significantly elevated Se body concentrations, the Hg(II) AEs were pronouncedly reduced (from 41% to 15-26%), whereas the dissolved uptake rate constant and elimination rate constant were less affected. In contrast to Hg(II), all the MeHg biokinetic parameters remained relatively constant whether Se was administrated simultaneously with the fish diet or when the fish were pre-exposed to Se with elevated body concentrations. Basic biokinetic measurements thus revealed that Se had direct interaction with Hg(II) during dietary assimilation rather than with MeHg and that different Se species had variable effects on Hg assimilation.     

Effect of dissolved organic matter on the uptake of trace metals by American oysters

To examine the effects of dissolved organic matter on metal bioavailability, uptake of trace metals (Cd, Co, Hg, Cr, Ag, Zn) by American oysters (Crassostrea virginica) was compared between treatments with different dissolved organic carbon (DOC) concentrations and contrasting low molecular weight (LMW, 1 kDa) and high molecular weight (HMW, 1 kDa-0.2 micron) DOC fractions, using radiotracer techniques and short-term exposure experiments. Uptake rate constants (mL g-1 h-1) of metals, in general, increased with increasing DOC concentrations, with an initial decrease at lower DOC concentrations. Oyster dry weight concentration factors (DCF, mL g-1), determined at the end of exposure experiments (8 h), also increased for Cd, Co, Cr, Ag, and Zn, but decreased for Hg, with increasing DOC concentrations. Changes of metal uptake rate constants and DCF values with DOC concentration suggest that metal uptake pathways by American oysters vary from predominantly uptake (by diffusion of neutral) of free ionic, inorganically complexed, and LMW organic ligand complexed metals at very low DOC concentration to direct ingestion and digestion of HMW or colloidally complexed metals at higher DOC concentrations. Measured partition coefficients (Kc) between dissolved and colloidal phases were comparable between metals, ranging from 10(5.12) to 10(5.75) mL g-1. However, DCF values and uptake rate constants differed considerably between metals, with the highest DCF values and uptake rate constants found for B-type metals, e.g., Ag, Hg, Zn, and Cd, and the lowest ones for several intermediate-type metals (e.g., Co, Cr). Metal types and thus the interaction of metals with organic ligands, such as strong complexation of B-type metals with S-containing organic ligands, may play an important role in the bioavailability and toxicity of metals to aquatic organisms. Differences in metal uptake in contrasting LMW and HMW DOC treatments suggest a generally depressed bioavailability of colloidally complexed metals at low DOC concentration (0.5 ppm) but a generally enhanced uptake at higher DOC concentrations.     

Are accumulated sulfide-bound metals metabolically available in the benthic oligochaete Tubifex tubifex?

The present study evaluates the relationship between metal-binding sediment characteristics like acid volatile sulfides (AVS), metal accumulation, and internal metal distribution in the benthic oligochaete Tubifex tubifex and relates this accumulation to the induction of metallothionein-like proteins (MTLPs). In total, 15 Flemish lowland rivers were sampled. Cd, Cu, Zn, Pb, Ni, As, Cr, Co, and Ag concentrations were measured in environmental fractions (water and sediment) and worm tissue (both total and subcellular fractions). Furthermore, total cytosolic MTLP concentrations were measured in the worm tissue. Our results showed that Cd, Pb, Ni, and Cr were mainly stored as biological detoxified metal (BDM) while Cu, Zn, As, and Ag were mostly available in the metal sensitive fraction (MSF). A remarkable difference in the subcellular distribution of accumulated Cd, Ni, and Co between anoxic (SEMMe-AVS<0; mostly stored as BDM) and oxic (SEMMe-AVS>0; mostly stored in the MSF) sediments was noticed. Moreover, a rapid increase in MTLP induction was found when SEMTot-AVS>0. Our results indicate that the accumulated sulfide-bound metals were detoxified and little available to the metabolism of T. tubifex under anoxic conditions.     

Influence of chloride and metals on silver bioavailability to Atlantic salmon (Salmo salar) and Rainbow trout (Oncorhynchus mykiss) yolk-sac fry

The effects of differing water chloride concentrations (0-10 mM) or competing metals [Cu(II), Cd(II), Zn(II), Pb(II), Co(II) (1-10,000 nM)] on Ag(I) uptake in yolk-sac fry of two salmonid species, the Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss), were studied. None of the metals tested were strong competitors of Atlantic salmon yolk-sac fry whole body Ag(I) influx. Inhibition of Ag(I) influx was only seen with a 100-fold excess of Cu(II) or Cd(II) or a 1000-fold excess of Pb(II) or Co(II). At these concentrations, the degree of competition appears to be directly proportional to the conditional stability constant of the competing metal to the gill (metal-gill log K). The range of [Cl-] allowed an assessment of Ag+, AgCl(aq), and AgCl2- bioavailability. The pattern of Ag(I) uptake was similar for each fish species. At <1 mM Cl-, where the [Ag+] dominates, the Ag(I) accumulation rate was constant. Above 1 mM Cl-, where the [AgCl(aq)] is dominant and the [AgCl2-] increases, there was a decline in Ag(I) uptake rate. However, even when very little Ag+ was present (i.e., at 10 mM Cl-) Ag(I) accumulated, albeit at a lower rate. This was suggestive of passive influx by AgCl(aq) and indicated little or no entry of negatively charged silver chloride complexes. The decline in Ag(I) uptake above 1 mM Cl- demonstrated that, if Ag(I) was present as both Ag+ and AgCl(aq), salmonid Ag(I) accumulation was dominated by Ag+ uptake. Therefore, the order of bioavailability of the Ag(I) species was determined as Ag+ > AgCl(aq) > AgCl2-.     

Silver bioaccumulation dynamics in a freshwater invertebrate after aqueous and dietary exposures to nanosized and ionic Ag

We compared silver (Ag) bioavailability and toxicity to a freshwater gastropod after exposure to ionic silver (Ag(+)) and to Ag nanoparticles (Ag NPs) capped with citrate or with humic acid. Silver form, exposure route, and capping agent influence Ag bioaccumulation dynamics in Lymnaea stagnalis. Snails efficiently accumulated Ag from all forms after either aqueous or dietary exposure. For both exposure routes, uptake rates were faster for Ag(+) than for Ag NPs. Snails efficiently assimilated Ag from Ag NPs mixed with diatoms (assimilation efficiency (AE) ranged from 49 to 58%) and from diatoms pre-exposed to Ag(+) (AE of 73%). In the diet, Ag NPs damaged digestion. Snails ate less and inefficiently processed the ingested food, which adversely impacted their growth. Loss rates of Ag were faster after waterborne exposure to Ag NPs than after exposure to dissolved Ag(+). Once Ag was taken up from diet, whether from Ag(+) or Ag NPs, Ag was lost extremely slowly. Large Ag body concentrations are thus expected in L. stagnalis after dietborne exposures, especially to citrate-capped Ag NPs. Ingestion of Ag associated with particulate materials appears as the most important vector of uptake. Nanosilver exposure from food might trigger important environmental risks.     

Uptake and elimination routes of inorganic mercury and methylmercury in Daphnia magna

Mercury (Hg) is an important environmental pollutant due to its highly toxic nature and widespread occurrence in aquatic systems. The biokinetics of Hg in zooplankton have been largely ignored in previous studies. This study examines the assimilation, dissolved uptake, and efflux of inorganic mercury [Hg(II)] and methylmercury (MeHg) in a freshwater cladoceran, Daphnia magna, and models the exposure pathways of Hg(II) and MeHg in the daphnids. The assimilation efficiencies (AEs) of both Hg species decreased significantly with increasing algal carbon concentrations. The dissolved uptake of Hg(II) and MeHg was proportional to the ambient concentration (ranging from environmentally realistic to high concentration over a 3-4 orders of magnitude variation), whereas MeHg had a slightly higher uptake rate constant (0.46 L g(-1) h(-1)) than Hg(II) (0.35 L g(-1) h(-1)). Surprisingly, the efflux rate constants of Hg(ll) and MeHg were rather comparable (0.041 -0.063 day(-1)). The release of both Hg(II) and MeHg via different routes (excretion, egestion, molting, and neonate production) was further examined at different food concentrations. It was found that regeneration into the dissolved phase was important for D. magna to eliminate both Hg species, but maternal transfer of Hg(II) (11-15%) and MeHg (32-41%) to neonates represented another important pathway for the elimination of Hg(II) and MeHg from the mothers. Modeling results suggest that food is an important source for MeHg exposure (47-98%), but water exposure represents 31-96% of Hg(II) accumulation in D. magna, depending on the variation of Hg bioconcentration factor in ingested food. Furthermore, MeHg predominates the bioaccumulation of Hg in D. magna even though MeHg constitutes only a small percentage of the total Hg in the water. The results strongly indicate that maternal transfer of Hg(II) and MeHg in freshwater zooplankton should be considered in manytoxicity testings and risk assessment in aquatic food chains.