On the uptake and release of zinc (65Zn) in the growing alga Selenastrum capricornutum Printz

Time-averaged copper concentrations from continuous exposures predicts pulsed exposure toxicity to the marine diatom, Phaeodactylum tricornutum: Importance of uptake and elimination

Intermittent, fluctuating and pulsed contaminant discharges result in organisms receiving highly variable contaminant exposures. Current water quality guidelines are predominantly derived using data from continuous exposure toxicity tests, and most frequently applied by regulators with the assumption that concentrations from a single sampling event will provide a meaningful approach to assessing potential effects. This study investigated the effect of single and multiple (daily) dissolved copper pulses on the marine diatom, Phaeodactylum tricornutum, including measurements of copper uptake and elimination to investigate the toxic mechanism. Copper pulses of between 0.5 and 24h and continuous exposures with equivalent 72-h time-averaged concentrations (TACs) resulted in similar biomass inhibition of P. tricornutum, with continuous exposures often being marginally more toxic. Rates of cell division generally recovered to control levels within 24h of the copper pulse removal. Upon resuspension in clean seawater, the extracellular copper per cell decreased rapidly, whereas the intracellular copper per cell decreased slowly. Negligible loss of copper from the total algal biomass indicated that P. tricornutum did not have an effective mechanism for eliminating copper from cells, rather the intracellular copper decreased as a result of dilution by cellular division as the algal growth rate recovered. The measurement of copper uptake after 72-h exposure and kinetics of elimination thereafter suggest that continuous exposures are marginally more toxic to P. tricornutum than pulsed copper exposures with equivalent TACs because slow internalization and saturation of algal membrane transport sites results in less copper uptake into pulse-exposed cells than continuously-exposed cells coupled with dilution of internalized copper via cellular division in the post-exposure period. In the case of P. tricornutum, the results indicate that water quality guidelines for copper based on continuous exposure will be conservative when applied to short-term discharges.

Dynamics of Metal Partitioning at the Cell-Solution Interface: Implications for Toxicity Assessment under Growth-Inhibiting Conditions

Metal toxicity toward microorganisms is usually evaluated by determining growth inhibition. To achieve a mechanistic interpretation of such toxic effects, the intricate coupling between cell growth kinetics and metal partitioning dynamics at the cell-solution interface over time must be considered on a quantitative level. A formalism is elaborated to evaluate cell-surface-bound, internalized, and extracellular metal fractions in the limit where metal uptake kinetics is controlled by internalization under noncomplexing medium conditions. Cell growth kinetics is tackled using the continuous logistic equation modified to include growth inhibition by metal accumulation to intracellular or cell surface sites. The theory further includes metal-proton competition for adsorption at cell-surface binding sites, as well as possible variation of cell size during exposure to metal ions. The formalism elucidates the dramatic impacts of initial cell concentration on metal bioavailability and toxicity over time, in agreement with reported algae bioassays. It further highlights that appropriate definition of toxicity endpoints requires careful inspection of the ratio between exposure time scale and time scale of metal depletion from bulk solution. The latter depends on metal internalization-excretion rate constants, microorganism growth, and the extent of metal adsorption on nonspecific, transporter, and growth inhibitory sites. As an application of the theory, Cd toxicity in the algae Pseudokirchneriella subcapitata is interpreted from constrained modeling of cell growth kinetics and of interfacial Cd-partitioning dynamics measured under various exposure conditions.

Impact of an urban multi-metal contamination gradient: metal bioaccumulation and tolerance of river biofilms collected in different seasons

The aim of this study was to investigate the repeatability and seasonal variability of the biological response of river biofilms chronically exposed to a multi-metal pressure in an urban contamination gradient. Biofilms were grown on immersed plastic membranes at three sites on the Seine river upstream (site 1) and downstream (sites 2 and 3) from Paris (France). They were collected in four different seasons (autumn, spring, summer and winter). Biofilm tolerance to Cu, Ni, Pb and Zn was measured using a PICT (Pollution-Induced Community Tolerance) approach with a previously developed short-term toxicity test based on β-glucosidase (heterotrophic) activity. Metal concentrations in the river and also in the biofilm samples (total and non-exchangeable bioaccumulated metals) were also monitored. Biofilm-accumulated metal concentrations reflected the increase of the multi-metal exposure along the urban gradient. These concentrations were strongly correlated with dissolved and particulate organic carbon and with the total metal fraction in the river water, which recalls the significant influence of the environmental parameters on metal uptake processes in river biofilms. Overall, natural biofilms allow monitoring water quality by integrating the variations of a diffuse metal contamination overtime. Tolerance levels globally increased from site 1 to site 3 reflecting the metal pollution gradient measured in the river water collected at the three sites. Cu tolerance tended to increase during warm seasons but no clear seasonal tendency could be found for Ni, Pb and Zn. Furthermore, principal component analysis clearly discriminated samples collected upstream (site 1) from samples collected downstream (sites 2 and 3) along the first principal component which was correlated to the metal gradient. Samples collected in winter were also separated from the others along the second principal component correlated to parameters like water temperature and Total Suspended Solids concentration. This study shows that chronic in situ exposure to environmental metal concentrations has a significant impact on natural biofilms. Biofilm tolerance to metals and biofilm metal bioaccumulation both reflect metal exposure levels although they remain low when compared to Environmental Quality Standards from the European Water Framework Directive. Yet temperature appears as an important environmental variable shaping community structure and response to toxic exposure which shows that the sampling date is an important parameter to consider when using natural river biofilms to assess the impacts of urban pressure.

The relationship between metal toxicity and biotic ligand binding affinities in aquatic and soil organisms: a review

The biotic ligand model (BLM) is a theoretical, potentially mechanistic approach to assess metal bioavailability in soil and aquatic systems. In a BLM, toxicity is linked to the fraction of biotic ligand occupied, which in turn, depends on the various components of the solution, including activity of the metal. Bioavailability is a key factor in determining toxicity and uptake of metals in organisms. In this study, the present status of BLM development for soil and aquatic organisms is summarized. For all species and all metals, toxicity was correlated with the conditional biotic ligand binding constants. For almost all organisms, values for Ag, Cu, and Cd were higher than those for Zn and Ni. The constants derived for aquatic systems seem to be equally valid for soil organisms, but in the case of soils, bioavailability from the soil solution is greatly influenced by the presence of the soil solid phase.

Comparison of aquatic and dietary exposure of heavy metals Cd, Cu, and Zn to benthic ostracod Heterocypris incongruens

The benthic ostracod Heterocypris incongruens is becoming an important tool for the ecotoxicological assessment of contaminated sediments. However, no study has yet explored solid-phase (dietary) exposure to ostracod. The present study examined the effects of metals on H. incongruens through aquatic and dietary exposures. The algal food Chlorella vulgaris was exposed to different concentrations of cadmium (Cd), copper (Cu), and zinc (Zn), and subcellular distributions of these metals in algal cells were determined. Parallel experiments were also performed to determine the toxic effects of dissolved metals on ostracod through aqueous exposure. Significant toxic effects on mortality were observed when ostracod were fed with Cd-contaminated (47-100%) and Cu-contaminated algae (55-100%). With increasing accumulated Zn in the algal cell, ostracod mortality also increased (20-83%). Aquatic exposure of ostracod to different concentrations of dissolved Cd (3.2-339 µg Cd/L) and Cu (260-2600 µg Cu/L) resulted in high observed mortalities (57-100% and 95-100%, respectively). Based on the results of aqueous and dietary exposure tests, it can be concluded that both exposure routes are important in evaluating the toxic effect of Cd on ostracod. Similar results were observed in tests using other metals (i.e., using Cu and Zn), thus emphasizing the importance of considering not only aquatic but also dietary exposure routes when evaluating metal toxicity to ostracod.

The effects of Zn-contaminated diets on Daphnia magna reproduction may be related to Zn-induced changes of the dietary P content rather than to the dietary Zn content itself

The effect of dietary zinc (Zn) exposure to Daphnia magna fed living algae remains unsure as existing experimental data exhibit considerable inconsistency. In this study, we examined if Zn-induced changes in nutritional quality (i.e., the molar carbon to phosphorus ratio (C:P) and concentrations of essential omega-3-poly-unsaturated fatty acids (ω3-PUFA)) may contribute to the reproductive effects of dietary Zn exposure to D. magna. We prepared 8 different algal diets differing in Zn content, C:P ratio and ω3-PUFA, by varying the culture conditions (i.e., exposure duration) and culture medium (i.e., Zn concentration and mineral composition). These diets were representative for the diets typically used in published dietary metal toxicity bioassays. The algal diets were offered to D. magna during a standard chronic bioassay, using reproduction as endpoint. A generalized linear model (GLM) was used to determine which algal characteristics significantly explained the observed variability in D. magna reproduction. The most parsimonious GLM resulting in the best prediction of the first brood size had the molar C:P ratio as the sole predictor. The 21-day reproduction was also predicted best by the molar C:P ratio, whereas the contribution of other variables (notably Zn and ω3-PUFA content of the diet) to enhanced predictability was only marginal. In addition, our GLM, which only uses C:P as a predictor, could accurately predict reproduction in an independent (previously published) chronic bioassay with dietary Zn and D. magna. Furthermore, this GLM also accurately predicted the observed effects of algal C:P ratio shifts on D. magna reproduction as reported in ecological literature. Our analysis highlights that the reproductive effects of dietary Zn exposure in D. magna, as observed in previous studies, are probably not caused by direct toxicity of Zn in the diet, but may rather be related to Zn-induced shifts of the dietary C:P ratio. Our study thus seems to resolve inconsistencies among results from different previous studies and has important implications for the experimental design of future dietary metal toxicity research.

Distribution of 137Cs in benthic plants along depth profiles in the outer Puck Bay (Baltic Sea)

A study was conducted on three macroalgae species: Polysiphonia fucoides and Furcellaria lumbricalis, the species of the red algae division, and Cladophora glomerata, representing the green algae division, as well as Zostera marina, representing vascular plants. The main aim of the study was to recognize the level of ¹³⁷Cs concentrations in the plants, which could be used as a measurement of bioaccumulation efficiency in the selected macrophytes at varying depths, and in the seasonal resolution of the vegetation period: spring–summer and autumnal. The plants’ biomass clearly showed seasonal variability, as did the ¹³⁷Cs concentrations in the plants. Cesium activity also changed with depth. Seasonal variability in radionuclide content in the plants, as well as the differences in its activity determined along the depth profile, were related mainly to the plant biomass and the dilution effect caused by the biomass increment and reflected the growth dynamics. P. fucoides showed much greater bioaccumulation ability at each depth as compared to C. glomerata, a green algae. Lower concentrations of ¹³⁷Cs were also identified in F. lumbricalis and in Z. marina, mostly as a result of differences in morphology and physiology. P. fucoides can be recommended as a bioindicator for the monitoring of ¹³⁷Cs contamination due to the high efficiency of bioaccumulation and the available biomass along the depth profile, as well as the occurrence throughout the entire vegetation season.

Seasonal changes of 137Cs in benthic plants from the southern Baltic Sea

137Cs activity concentrations were determined in samples of macrophytes Polysiphonia fucoides (red algae) and Zostera marina (vascular plant) collected during the entire vegetation season in the Gulf of Gdańsk in the southern Baltic Sea. The measurements showed considerable seasonality of 137Cs activity in both species; an increase of cesium concentrations was observed from spring to autumn with maximal levels 49.1 ± 1.4 Bq kgd.w.-1 (P. fucoides) and 14.5 ± 1.0 Bq kgd.w.-1 (Z. marina) in late autumn. 137Cs concentrations observed in a given season are the result of a number of processes, the intensity of which can differ depending on external environmental conditions. The effects of these processes can differ and their directions can frequently be opposite to one another. The examined macrophytobenthic plant species could serve as bioindicators of radionuclide pollution for monitoring purposes on condition that the samples of plants are taken within a strictly defined period of the year to give comparable results and to supply realistic information about pollution levels.

Bioavailability of trace metals to aquatic microorganisms: importance of chemical, biological and physical processes on biouptake

An important challenge in environmental biogeochemistry is the determination of the bioavailability of toxic and essential trace compounds in natural media. For trace metals, it is now clear that chemical speciation must be taken into account when predicting bioavailability. Over the past 20 years, equilibrium models (free ion activity model (FIAM), biotic ligand model (BLM)) have been increasingly developed to describe metal bioavailability in environmental systems, despite the fact that environmental systems are always dynamic and rarely at equilibrium. In these simple (relatively successful) models, any reduction in the available, reactive species of the metal due to competition, complexation or other reactions will reduce metal bioaccumulation and thus biological effects. Recently, it has become clear that biological, physical and chemical reactions occurring in the immediate proximity of the biological surface also play an important role in controlling trace metal bioavailability through shifts in the limiting biouptake fluxes. Indeed, for microorganisms, examples of biological (transport across membrane), chemical (dissociation kinetics of metal complexes) and physical (diffusion) limitation can be demonstrated. Furthermore, the organism can employ a number of biological internalization strategies to get around limitations that are imposed on it by the physicochemistry of the medium. The use of a single transport site by several metals or the use of several transport sites by a single metal further complicates the prediction of uptake or effects using the simple chemical models. Finally, once inside the microorganism the cell is able to employ a large number of strategies including complexation, compartmentalization, efflux or the production of extracellular ligands to minimize or optimize the reactivity of the metal. The prediction of trace metal bioavailability will thus require multidisciplinary advances in our understanding of the reactions occurring at and near the biological interface. By taking into account medium constraints and biological adaptability, future bioavailability modeling will certainly become more robust.

Impact of zinc acclimation on bioaccumulation and homeostasis in Chlorella kesslerii

Growth curves, cellular Zn contents and cellular protein expression were examined for the green alga, Chlorella kesslerii, as a function of different Zn growth regimes (growth in 16 pM, 1.7 nM or 1.6 microM calculated Zn2+). Zn homeostasis was responsible for observed differences in the capacity of the organism to accumulate Zn. The rapid acclimation that occurred in response to a Zn deficiency was likely due to the production of Zn transport sites. No differences were observed among cellular phytochelatin contents or efflux rate constants, although efflux did play an important role in regulating Zn cellular content. A long-term adaptation to Zn was not thought to occur since bioaccumulation and biological responses were similar for four successive cultures (30-40 days, 16-19 cell cycles) at different [Zn2+]. Among proteins that were influenced by the Zn growth regime, the Rubisco and histone H3 proteins were identified as being induced in the presence of 1.6 microM Zn2+ as compared with 1.7 nM Zn2+. The impact of the Zn preconditioning demonstrated that the concentrations of essential metals in the algal growth media would have an important, if not predominant effect on toxicity or bioaccumulation assessments. Furthermore, the high regulation of Zn transport and intracellular events by the microorganisms will likely preclude the use of simple metal uptake models including the free ion activity model and the biotic ligand model to predict either bioaccumulation or toxicological effects of Zn and perhaps other essential metals.

Reproductive toxicity of dietary zinc to Daphnia magna

Regulatory assessments of metals in freshwaters are mostly based on dissolved metal concentrations, assuming that toxicity is caused by waterborne metal only. Little attention has been directed to the toxicity of dietary metals to freshwater invertebrates. In this study the chronic toxicity of dietary zinc to Daphnia magna was investigated. The green alga Pseudokirchneriella subcapitata was exposed for 64 h to a control and three dissolved zinc concentrations, i.e. 23, 28 and 61 microg L(-1), resulting in internal zinc burdens in the algae of 130, 200, 320 and 490 microg g(-1) dry weight, respectively. These algae were used as a food source in chronic, 21-day bioassays with D. magna in a test medium to which no dissolved zinc was added. None of the treatments resulted in effects on feeding rates or somatic growth of D. magna. In contrast, a significant 40% decrease of total reproduction (number of juveniles per adult) was observed in the 28 and 61 microg L(-1) treatments. Time to first brood was not affected, whereas the mean brood size and the fraction of reproducing parent daphnids were reduced from the second brood onwards and the magnitude of these reductions increased with each subsequent brood. The reduced reproduction was accompanied with an elevated zinc accumulation in the 61 microg L(-1) treatment only, suggesting that total body burden is no good indicator of dietary zinc toxicity. Overall our data suggest that dietary zinc specifically targets reproduction in D. magna through accumulation in particular target sites, possibly cells or tissues where vitellogenin synthesis or processing occur. Further, our data illustrate that the potential importance of the dietary exposure route should be carefully considered and interpreted in regulatory assessments of zinc.

Determination of half-life and photon emission probabilities of (65)Zn

A (65) Zn radioactive solution standardized by an absolute measurement method at Laboratoire National Henri Becquerel (LNHB), was sent to Bureau International des Poids et Mesures (BIPM), in order to include the results in their database. The activity value determined at BIPM was in good agreement with that stated by LNHB (the difference was 0.3%). Additional measurements of (65) Zn activity and half-life were also carried out, by using a 4pigamma ionization chamber; the half-life value proposed by this work: 244.15 (10) days, is consistent to others reported in the literature. The spectra analysis was performed by gamma-ray spectrometry with high-purity germanium detectors, with the aim to improve the accuracy of the photon emission probabilities values of this nuclide. The determined photon emission probabilities are 0.498 for the 1115 ke V gamma-ray, 0.347 and 0.0479 for the X-rays Cu-K(alpha) and Cu-K(beta); the associated uncertainties are 0.4%, 0.9% and 1.1%, respectively.

Algal toxicity tests for environmental risk assessments of metals

Current regulatory methods for assessing the effects of contaminants, and metals in particular, rely mainly on a limited number of standardized test methods and test species (OECD, ISO, ASTM, USEPA). However, these test protocols allow a certain degree of freedom in relation to physicochemical parameters or biological aspects, which may lead to large variability in test results. The current review, based on effects data and theoretical considerations reported in the literature, tried to determine and quantify the effect of variation of these factors on the outcome of metal toxicity tests with algae. Major physicochemical parameters that affect metal toxicity to algae are hardness, pH, preculture conditions, type of test medium, and presence of chelating agents: Literature data also clearly demonstrate the importance of test species or strain selection (inter- and intraspecies sensitivity variability) on the outcome of algal toxicity tests. For Zn, a factor of 8.3 is observed between the NOEC for Selenastrum capricornutum (currently renamed Pseudokirchneriella subcapitata) and Croococcus paris. An intraspecies difference for S. capricornutum of a factor of 60 is observed between various reported EC50S for Cd. Next to differences in physicochemical test conditions, possible adaptation or acclimation to deficient/elevated metal concentrations add to the reported differences: S. capricornutum became three times less sensitive to Zn when acclimated to 65 microg Zn/L compared to cultures in ISO medium. This review has revealed that currently accepted standard protocols used in regulatory frameworks contain a number of major shortcomings on the physicochemical and biological aspects of algal toxicity testing with metals. These shortcomings are summarized in Table 5, together with a number of suggestions that could help to modify and improve standard test protocols for evaluating metal toxicity to algae. Until now, important factors such as pH control during test performance, selection of test medium, test species, and the effects of possible adaptation/acclimation to natural metal concentrations have not been considered, which could have serious implications when the resulting unsuitable or irrelevant toxicity data are subsequently used for setting environmental management policies. These findings also have their consequences when extrapolating laboratory data to the field as the complexity of natural waters currently is not reflected in laboratory standard media. These media contain no dissolved organic matter, have a relatively high pH, and contain large amounts of essential nutrients. In addition, the limited number of laboratory test species do not reflect natural phytoplankton communities. Test procedures for assessing the environmental impact of metal contamination in a specified ecoregion should therefore be based on performing a battery of algal tests with species adapted to and tested under the specific natural conditions of the region.

Toxicity of combined sewer overflows on river phytoplankton: the role of heavy metals

The toxic effect of a combined sewer overflow (CSO) on the phytoplankton community of the river Seine has been studied by means of short-term primary production measurements. As the discharged solids usually do not remain in the water column, only filtered or centrifugated fractions were tested. The collected phytoplankton were grown in the laboratory for 2 days, after addition of N, P and EDTA. Stock cultures in exponential growth were directly tested with heavy metals, but resuspended algal cells were used for effluent testing. The results show an increase of EC50 value for the single metal species in the order Cu<<Zn</=Pb. Free metal contents were calculated with the chemical speciation model Mineql+. The EC50 dilution of the tested CSOs effluent was 50%. This value increased by 30% after addition of 5 10(-6) M EDTA, indicating that the effluent toxicity is only partially due to its metallic compounds.