CADMIUM AND ZINC BIOAVAILABILITY TO SELENASTRUM CAPRICORNUTUM (CHLOROPHYCEAE): ACCIDENTAL METAL UPTAKE AND TOXICITY IN THE PRESENCE OF CITRATE

Back Accumulation of Diffusive Gradients in Thin-Films Devices with a Stack of Resin Discs To Assess Availability of Metal Cations to Biota in Natural Waters

Determining species, concentrations, and physicochemical parameters in natural waters is key to improve our understanding of the functioning of these ecosystems. Diffusive Gradients in Thin-films (DGT) devices with different thicknesses of the resin or of the diffusive disc can be used to collect independent information on relevant parameters. In particular, DGT devices with a stack of two resin discs offer a simple way to determine dissociation rate constants of metal complexes from the accumulation of the target metal in the back resin disc. In this work, simple approximate expressions for the determination of the dissociation rate constant are reported and applied to a model Ni nitrilotriacetic complex as well as to Zn complexes in the Mediterranean Osor stream. Once the physicochemical parameters are known, one can plot the labile fraction of the metal complexes in terms of the thickness of the diffusion domain. These plots reveal a strong dependence on the nature of complexes as well as on the characteristics of the diffusion domain, and they are of high interest as predictors of availability to biota whose uptake is limited by diffusion.

Enhanced transport of heavy metal ions by low-molecular-weight organic acids in saturated porous media: Link complex stability constants to heavy metal mobility

Environment-ubiquitous low-molecular-weight organic acids (LMWOAs) can interact with heavy metal ions and thus affect their mobility in subsurface aquifers. Herein, the effects of LMWOAs (including acetic acid, tartaric acid, malonic acid, oxalic acid, and citric acid) on the mobility of heavy metal ions (including Cd²⁺, Zn²⁺, Ni²⁺, Mn²⁺, and Co²⁺) in porous media were investigated to reveal the role of the stability constants of metal-LMWOA complexes in the mobility of heavy metal ions in porous media. The results showed that the mobility of different metal ions followed the order of Cd²⁺ < Zn²⁺ < Ni²⁺ < Mn²⁺ < Co²⁺ despite of LMWOAs-free or LMWOAs-addition. For each heavy metal, all the organic acids enhanced its transport by forming stable non-adsorbing metal-LMWOA complexes and the enhanced ability followed the order of citric acid > oxalic acid > malonic acid > tartaric acid > acetic acid. An interesting finding was that there was a significantly positive correlation between the enhanced abilities of LMWOAs to metal mobility and the complex stability constants (log K) (R² = 0.801-0.961, p < 0.05), indicating that the complex stability of metal-LMWOA was the dominant factor responsible for the enhanced transport of heavy metal ions. Meanwhile, the linear slope indicated the intensity of enhancement of LMWOAs on heavy metal mobility was heavy metal type-dependent. This study proposed that the complex stability of metal-LMWOA could be an indicator to quantify and predict the impact of LMWOAs on the mobility of heavy metals.

An untargeted metabolomic approach for the putative characterization of metabolites from Scenedesmus obliquus in response to cadmium stress

Cadmium (Cd) is a widespread contaminant in aquatic systems and has a variety of toxicological implications on freshwater microorganisms. In this study, the green algae Scenedesmus obliquus was exposed to increasing Cd concentrations that inhibited growth by 20% (12.6 μmol L^-1), 30% (39.8 μmol L^-1) and 40% (83.2 μmol L^-1) and the metabolite profiles of released and cellular biomolecules were explored using an untargeted direct infusion high resolution Fourier transform ion cyclotron resonance mass spectrometry approach. In Cd untreated cultures, intrinsic differences in composition existed between released biomolecules and freeze-dried cells. Based on putatively characterized compound groups, a greater proportion of Cys-GSH isomers and carboxyamides were present in exudates whereas sugar isomers and phosphonic acids comprised most cellular metabolites. In cultures exposed to 83.2 μmol L^-1 Cd, an overall shift in metabolomic response across both released biomolecules and cellular components resulted in an increase of lipid-based esters, and Cys-GSH isomers. These two important metabolites are used in antioxidant defense mechanisms and reactive oxygen species prevention during cellular stress. The diversity of metabolites also decreased as Cd concentrations increased when compared to untreated cultures, suggesting that overall metabolites specialize upon metal stress. We show systemic shifts from sugar and carboxylic isomers to specialized proteins and lipid isomers to help S. obliquus cope with stress. These findings highlight the potential use of this green algae as a potential biosorbent and sheds light into the metabolomics of Cd toxicology and insights into microbial metal adaptation.

Biotic ligand model explains the effects of competition but not complexation for Sm biouptake by Chlamydomonas reinhardtii

The applicability of the biotic ligand model (BLM) was tested with respect to the biouptake of the lanthanide Sm by the freshwater green alga, Chlamydomonas reinhardtii. In the absence of organic ligands, Sm uptake was well described by the Michaelis-Menten equation, consistent with the BLM assumption of single transporter, with the maximum influx rate (J_max) of 1.5 × 10^-14 mol cm^-2 s^-1 and a binding constant (K_Sm) of 10^7.0 M^-1. The addition of organic ligands (i.e., malic acid, diglycolic acid and citric acid) decreased Sm influx rates, however, the decreases were much less than that predicted by the BLM, possibly due to the direct contribution of the Sm complexes. Competition effects of two major cations (Ca²⁺ and Mg²⁺) and three lanthanide cations (La³⁺, Ce³⁺ and Eu³⁺) were successfully modeled by the BLM, with binding constants corresponding to K_Ca = 10^4.0 M^-1, K_Mg = 10^2.7 M^-1, K_La = 10^6.8 M^-1, K_Ce = 10^6.9 M^-1 and K_Eu = 10^7.0 M^-1. The binding constants and J_max were very similar among the four investigated lanthanides and varied progressively with atomic number; therefore, the results obtained in the present study can probably be extrapolated to other rare earth metals.

Cadmium accumulation and toxicity in the unicellular alga Pseudokirchneriella subcapitata: Influence of metal-binding exudates and exposure time

Predicting metal availability and toxicity for chronic (several hours or days) metal exposure scenarios, even for unicellular algae, is a major challenge to existing toxicity models. This is because several factors affecting metal uptake and toxicity, such as the release of metal-binding exudates, changes in the kinetics of metal uptake and toxicity over time, and algal physiological acclimation to internalized metals, are still poorly understood. The present study assessed the influence of these factors on Cd uptake and toxicity in laboratory batch cultures of the freshwater alga Pseudokirchneriella subcapitata. To do so, changes in the free Cd(2+) concentrations caused by the release of metal-binding algal exudates were monitored, (109)Cd accumulation in algal cells was measured, and Cd-induced inhibition of algal growth as a function of exposure time (from 12 h to 96 h) was followed. Results indicate that metal-binding exudates may decrease the proportion of the free Cd(2+) ion in solution up to 2-fold, a decrease that affects Cd uptake and toxicity. Pseudokirchneriella subcapitata has the capacity to decrease net Cd uptake rate on short time scales (<24 h), but this reduction in the Cd uptake rate disappeared after 24 h, and Cd toxicity occurred at relatively high Cd concentrations in solution. These data illustrate some of the pitfalls of standard algal toxicity assays, which were designed for acute exposures, and suggest how robust chronic bioassays might be developed.

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.

Modelling metal accumulation using humic acid as a surrogate for plant roots

Metal accumulation in roots was modelled with WHAM VII using humic acid (HA) as a surrogate for root surface. Metal accumulation was simulated as a function of computed metal binding to HA, with a correction term (E(HA)) to account for the differences in binding site density between HA and root surface. The approach was able to model metal accumulation in roots to within one order of magnitude for 95% of the data points. Total concentrations of Mn in roots of Vigna unguiculata, total concentrations of Ni, Zn, Cu and Cd in roots of Pisum sativum, as well as internalized concentrations of Cd, Ni, Pb and Zn in roots of Lolium perenne, were significantly correlated to the computed metal binding to HA. The method was less successful at modelling metal accumulation at low concentrations and in soil experiments. Measured concentrations of Cu internalized in L. perenne roots were not related to Cu binding to HA modelled and deviated from the predictions by over one order of magnitude. The results indicate that metal uptake by roots may under certain conditions be influenced by conditional physiological processes that cannot simulated by geochemical equilibrium. Processes occurring in chronic exposure of plants grown in soil to metals at low concentrations complicate the relationship between computed metal binding to HA and measured metal accumulation in roots.

Joint toxicity of tetracycline with copper(II) and cadmium(II) to Vibrio fischeri: effect of complexation reaction

Co-contamination of antibiotic and heavy metals commonly occurs in the environment. Tetracycline (TC), a common antibiotic, can behave as an efficient organic ligand to complex with cations. In this paper, the joint toxicity of TC with two commonly existing metals, copper(II) and cadmium(II), towards a luminescent bacteria, Vibrio fischeri, are investigated. Results showed that coexistence of TC and Cu(II) showed a significant antagonistic effect, while TC and Cd(II) showed a synergistic effect. The aqueous speciation of TC with two metal cations was calculated using a chemical equilibrium software Visual MINTEQ and results indicated that a strong complexation exist between TC and Cu(II), while much weaker interaction between TC and Cd(II). Traditional joint toxicity prediction model based on independent action failed to predict the combined toxicity of TC with metals. A new method based on speciation calculation was used to evaluate the joint toxicity of ligands and cations. It is assumed that the metal-ligand complexes are non-toxic to V. fischeri and the joint toxicity is determined by the sum of toxic unit of free metal-ions and free organic ligands. It explained the joint toxicity of the mixed systems reasonably well. Meanwhile, citric acid (CA) and fulvic acid (FA) were also introduced in this study to provide a benchmark comparison with TC. Results showed it is also valid for mixed systems of CA and FA with metals except for the Cd-CA mixture.

A review of environmental characteristics and effects of low-molecular weight organic acids in the surface ecosystem

Low molecular weight organic acids (LMWOAs) are prevalent on the earth's surface. They are vital intermediate products during metabolic pathways of organic matter and participate in the tricarboxylic acid cycle during life activities. Photochemical reactions are pivotal for LMWOAs' origination and play a large role in determining their diversity and their ultimate fate. Within the long time that organic matter is preserved in sediments, it can be decomposed and converted to release organic and inorganic pollutants as well as C, N, and P nutrients, which are of potential ecological risk in causing secondary pollution to lake water. The sediment pool is a comprehensive and complex compartment closely associated with overlying water by various biochemical processes, during which LMWOAs play critical roles to transport and transform elements. This article elucidates geochemical behaviors of LMWOAs in the surface environment in details, taking natural water, soil, and aerosol as examples, focusing on reviewing research developments on sources and characteristics, migration and mineralization of LMWOAs and relevant environmental effects. Simultaneously, this review article depicts the categories and contents of LMWOAs or their contribution to DOC in environmental media, and evaluates their importance during organic matter early diagenesis. Through concluding and discussing the conversion mechanisms and influencing factors, the next research orientations on LMWOAs in lake ecosystems are determined, mainly concerning relationships with hydrochemical parameters and microorganisms, and interactions with pollutants. This will enrich the knowledge on organic matter degradation and related environmental effects, and help reconstruct a theoretical framework for organic compound succession and influencing factors, providing basic data for lake eutrophication and ecological risk assessment, conducive to better control over water pollution and proper management of water quality.

Availability of zinc and the ligands citrate and histidine to wheat: does uptake of entire complexes play a role?

Organic ligands in soils affect the availability of trace metals such as Zn to plants. This study investigated the effects of two of these ligands, citrate and histidine, on Zn uptake by wheat under hydroponic conditions. Uptake of (65)Zn in the presence of these ligands was compared to uptake in the presence of EDTA at the same free Zn concentration (Zn(2+) ~ 50 nM). In the presence of citrate Zn root uptake was enhanced ~3.5 times and in the presence of histidine, by a factor of ~9, compared to the EDTA treatments. Citrate uptake was slightly reduced in the treatment containing ligands and Zn compared to the treatment containing the same ligand concentration but no Zn. In addition, a higher uptake of Zn than of citrate was observed. This suggests that the enhanced Zn uptake was primarily due to increased supply of Zn(2+) by diffusion and dissociation of Zn-citrate complexes at the root surface. Histidine uptake was much higher than citrate uptake and not influenced by the presence of Zn. As histidine forms stronger complexes with Zn than citrate, the results suggest that the enhancement of Zn uptake in the presence of histidine was in part due to the uptake of undissociated Zn-histidine complexes.

Effect of complexing ligands on the surface adsorption, internalization, and bioresponse of copper and cadmium in a soil bacterium, Pseudomonas putida

Environmental quality criteria for metals toxic to soil and water organisms, using the free ion activity model or the biotic ligand model, are based on the concept that the major form of the metal available to the organism is the free metal ion, yet various metal complexes are bioavailable to a variety of soil and water organisms. We test here whether neutral copper or cadmium sulfates, negatively-charged copper or cadmium citrates and positively-charged copper acetate and cadmium chloride are bioavailable to a soil bacterium, Pseudomonas putida. Adsorption onto the cell surface and uptake into the periplasm and cytoplasm of this Gram-negative root colonizing bacterium was studied by adding a single concentration of Cu or Cd and varying the concentration of the ligands to complex 10-100% of the metal. Metal association from the complexes on and within the cell was defined using selective extraction procedures and compared with free ion controls using the Langmuir isotherm. Cellular responses also were assessed using a P. putida biosensor. Both uptake and bioresponse methodologies showed that P. putida was sensitive to the metal complexes. In particular, the bioresponse to Cu and Cd supplied as a citrate complex occurred with activities of free metal ions two orders of magnitude lower than for the control. We concluded that the tested metal complexes for Cu and Cd are taken up into the cell, where they trigger a bioresponse. We also discuss the implications of these findings on interactions between soil and water organisms and nanoparticles that release metal ions.

Combined toxicity of polycyclic aromatic hydrocarbons and heavy metals to biochemical and antioxidant responses of free and immobilized Selenastrum capricornutum

The aquatic environment often contains different groups of contaminants, but their combined toxicity on microalgae has seldom been reported. The present study compared the toxic effects of combined mixed polycyclic aromatic hydrocarbons (PAHs) and heavy metals on growth and antioxidant responses of free and immobilized microalga, Selenastrum capricornutum. Five PAHs-phenanthrene, fluorene, fluoranthene, pyrene, and benzo[a]pyrene-and four heavy metals at different concentrations-0.05 to 0.1 µg Cd(2+) ml(-1) , 0.05 to 1 µg Cu(2+) ml(-1) , 0.05 to 1 µg Zn(2+) ml(-1) , and 0.5 to 2.5 µg Ni(2+) ml(-1) -were examined. Results showed that the chlorophyll a content of free and immobilized S. capricornutum was not affected by PAHs but was significantly inhibited by heavy metals. Conversely, the antioxidant parameters, including the content of reduced glutathione (GSH) and the activities of superoxide dismutase and peroxidase, were significantly induced by both PAHs and metals. For the combined toxic effects of PAHs and heavy metals, cell growth and antioxidant responses varied with exposure time and contaminants and differed between free and immobilized cells. The effects of cocontaminants on the GSH content in free cells were mainly synergistic but changed to antagonistic in immobilized cells. The toxic effects of cocontamination on free cells were also more obvious than those on immobilized cells. These findings suggest that immobilization offers some protection to microalgal cells against toxic contaminants causing differences in the interaction and responses to combined toxicants between free and immobilized cells. Immobilized cells might be more suitable for treating wastewater containing toxic contaminants than free cells.

Weak organic ligands enhance zinc uptake in marine phytoplankton

A recent study of the effect of pH on Zn and Cd bioavailability shows that binding to weak organic ligands can increase the pool of metals available to phytoplankton in the presence of strong chelating agents. We explore the underlying mechanism in laboratory experiments with the model species Emiliania huxleyi and Thalassiosira weissflogii. Additions of L- and D- isomers of cysteine (Cys) result in similar increases in Zn uptake rates in the presence of the strong chelator ethylenediaminetetraacetic acid (EDTA) but decrease it in the absence of EDTA, ruling out uptake by a specific Zn-Cys transporter. The effect of Cys does not result from alleviating diffusion limitation of inorganic Zn. The enhancement of Zn uptake kinetics by weak ligands is consistent with a mechanism involving formation of a transient ternary complex with uptake molecules: (1) the enhancement is most dramatic in Zn limited cells whose high affinity transporters should be most effective at extracting Zn from weak ligands; (2) the enhancement occurs with a variety of weak ligands, demonstrating that the underlying mechanism has little chemical specificity; and (3) no enhancement of uptake is seen when Zn is bound in complexes that would make formation of multiligand complexes with uptake molecules difficult. Weak complexing agents which have received heretofore little attention may play a key role in the bioavailability of metals in natural waters.

Effects of Cr III and Pb on the bioaccumulation and toxicity of Cd in tropical periphyton communities: Implications of pulsed metal exposures

Metal exposure pattern, timing, frequency, duration, recovery period, metal type and interactions, has obscured effects on periphyton communities in lotic systems. The objective of this study was to investigate the effects of intermittent exposures of Cr III and Pb on Cd toxicity and bioaccumulation in tropical periphyton communities. Natural periphyton communities were transferred to artificial stream chambers and exposed to metal mixtures at different pulse timing, duration, frequency and recovery periods. Chlorophyll a, dry mass and metal accumulation kinetics were recorded. Cr and Pb decrease the toxic effects of Cd on periphyton communities. Periphyton has high Cd, Cr and Pb accumulation capacity. Cr and Pb reduced the levels of Cd sequestrated by periphyton communities. The closer the frequency and duration of the pulse is to a continuous exposure, the greater the effects of the contaminant on periphyton growth and metal bioaccumulation. Light increased toxic and accumulative effects of metals on the periphyton community.

Effects of Cd, Cu, Ni, and Zn on brown tide alga Aureococcus anophagefferens growth and metal accumulation

Trace metals play important roles in regulating phytoplankton growth and could influence algal bloom development. Laboratory studies were conducted to evaluate the influence of environmentally relevant concentrations of Cd, Cu, Ni, and Zn on Aureococcus anophagefferens bloom (brown tide) development. Results show that the elevated Ni(2+) concentrations, e.g. those of brown tide waters in the northeastern US, greatly stimulated A. anophagefferens growth (as compared to the control without Ni addition), yet, only low amounts of dissolved Ni were sequestered, thus leaving excessive Ni directly promoting A. anophagefferens blooms. The medium effective concentration EC(50) (Me(2+)) suggests A. anophagefferens has similar Cd sensitivity but much greater Cu tolerance as compared to cyanobacteria, as such, excessive Cu could indirectly promote A. anophagefferens blooms by inhibiting competitors such as Synechococcus sp. The effects of Ni and Cu promoting growth are consistent with the recent genomic study of this alga. In addition, Zn(2+) concentrations lower than those in brown tide waters enhance A. anophagefferens growth, but Zn sequestration in A. anophagefferens would not substantially reduce total dissolved Zn in these waters. Overall, this study, showing that excessive Cu and Ni likely promote brown tides, provides evidence for trace metal linkages in algal bloom development.

Cadmium speciation and accumulation in periphyton in a small stream with dynamic concentration variations

Accumulation of cadmium in periphyton was investigated under field conditions while Cd concentration and speciation were dynamically varying in a small stream during rain events. Speciation in water was determined in situ by diffusion gradient in thin-films (DGT) and by modeling of complexation with fulvic acids. During the rain events, dissolved Cd concentrations increased from 0.17 nM to 0.27-0.36 nM, and 70-97% were DGT-labile. Cd content in periphyton closely followed Cd concentrations in water, despite higher concentrations of Zn and Mn, and may be controlled by either free or DGT-labile Cd concentrations. Decrease of Cd content in periphyton after the rain events was slower than the decrease of Cd concentration in water. Concentrations of Zn, Mn, Cu, Pb and Fe in periphyton also followed the dynamic variations of metal concentrations in water. Repeated exposure of periphyton to elevated dissolved Cd may lead to Cd accumulation.

Accumulation of cadmium in periphyton under various freshwater speciation conditions

The relationship between cadmium speciation and accumulation in periphyton was examined at environmentally relevant Cd concentrations under natural freshwater conditions. Periphyton was exposed in artificial recirculating channels containing natural freshwater to two Cd concentrations (20 and 40 nM), for which speciation was modified by the addition of a synthetic organic ligand (nitrilotriacetate, NTA). Labile metal concentrations were measured with the technique of diffusion gradient in thin-films (DGT) and major Cd species were estimated by modeling. Total and intracellular Cd content in periphyton increased within both Cd exposure concentrations with NTA additions and were related to an increase in DGT-labile Cd, which was caused by the competition of NTA with probably colloidal species. Bioaccumulation was thus not controlled by the free Cd concentrations, as predicted by the free ion activity model, but by the diffusion of labile Cd-NTA complexes, which constituted a large fraction of DGT-labile Cd. These findings confirm the importance of labile species for Cd accumulation in periphyton under freshwater conditions, as predicted by models considering diffusion and uptake kinetics.

Uptake of hydrophobic metal complexes by three freshwater algae: unexpected influence of pH

Cadmium forms neutral, lipophilic Cd(L)2(0) complexes with diethyldithiocarbamate (DDC) and with ethylxanthate (XANT). Uptake of these complexes bythree unicellularfreshwater green algae (Chlamydomonas reinhardtii, Chlorella fusca, and Pseudokirchneriella subcapitata) was determined at two pH values (7.0 and 5.5) and compared to uptake of the free, uncomplexed Cd2+ ion. Uptake of the lipophilic complexes over time, characterized by high initial uptake rates but tending toward a plateau after about 30 min, could be modeled successfully as the result of the following processes: first-order uptake from solution, partitioning of the accumulated Cd into two internal pools (labile and nonlabile), and first-order loss of Cd from the labile pool. At pH 7.0 initial uptake rates for both Cd(L)2(0) complexes were much higher than for Cd2+ alone (e.g., up to approximately 90 times higher for comparable dissolved Cd concentrations of approximately 0.4 nM). However, the initial uptake rates for the lipophilic complexes dropped dramatically when the pH was lowered from 7.0 to 5.5 (2- to 60-fold decrease, depending on the algal species and the nature of the neutral complex). Loss rates for the accumulated complexes also decreased atthe lower pH. The lipophilicity of the neutral complexes, as estimated from their octanol-water distribution coefficient (Dow), was not affected by the decrease in pH from 7.0 to 5.5. We thus conclude that the acidification of the external medium, i.e., the interaction of protons with the algal membrane, strongly affects algal membrane permeability.

Influence of N, P additions on the transfer of nickel from phytoplankton to copepods

We examined the influence of macronutrient (nitrate and phosphate) additions on Ni uptake by phytoplankton (Prorocentrum donghaiense and Skeletonema costatum) and its subsequent transfer to marine copepods (Calanus sinicus and Labidocera euchaeta). Ni uptake by phytoplankton after 24h of exposure was markedly dependent on nutrient conditions, with a higher nutrient quota facilitating Ni accumulation in the algae. Trophic transfer was quantified by measurements of the Ni assimilation efficiency in C. sinicus and L. euchaeta, feeding on the algae under different nutrient treatments. Ni assimilation efficiency generally increased with an increase of nutrient concentration in the algae. A significant positive-correlation was found between the Ni assimilation efficiencies of the copepods and the %intracellular Ni in the algal cells. However, ambient nutritional conditions had little effect on the physiological turnover rate constant of Ni by copepods. Thus, nutrient enrichment may lead to an increase in Ni uptake and transfer in marine plankton.

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.

Cadmium--copper antagonism in seaweeds inhabiting coastal areas affected by copper mine waste disposals

Cadmium and copper accumulation by macroalgae was studied in a coastal area exposed to upwelling events and high levels of Cu, the latter resulting from mine disposals. Eight species were studied, and all had very high concentrations of Cd outside of the Cu-contaminated area. Cu in algal tissues was much higher in contaminated than in reference sites. High Cu appeared to suppress Cd bioaccumulation; Cd in algal tissues was much lower in the Cu-contaminated area than in the reference sites. Transplant experiments with Lessonia nigrescens revealed a depuration of Cd in individuals transplanted to areas with high Cu. However, Cd depuration occurs more slowly than Cu uptake. These differences suggest that while Cd and Cu are linked mechanistically, itis nota simple substitution. Overall, the work confirms that macroalgae are useful indicators of metal contamination and may be used as in situ biomonitors for labile forms of metals, like free Cu2+. However, antagonistic relationships between metals must be clearly understood in order to properly interpret their concentrations in macroalgae.

Modeling and separation-detection methods to evaluate the speciation of metals for toxicity assessment

There is an increasing appreciation for the importance of speciation in the assessment of metal toxicity. In this review, two approaches to speciation are discussed, with an emphasis on their application to biological samples. One approach is the direct separation and detection of metal species of toxicological interest. Various "hyphenated" techniques, consisting of a chromatographic system coupled to inductively coupled plasma-mass spectrometry (ICP-MS), are discussed. The chromatographic strategies employed for separation emphasize liquid chromatography (LC), but the increasing use of gas chromatography (GC) and capillary electrophoresis (CE) in speciation analysis is discussed. The second approach to speciation is the use of computer models to calculate the speciation of a metal ion within a complex mixture of ligands. This approach is applicable to systems in which the metal cation exchanges ligands rapidly, so that the sample represents an equilibrium mixture of metal complexes. These computational models are based on the equilibrium constants for the metal complexes and a series of mass balance equations and give the distribution of metal complexes in the original sample. This approach is illustrated using the speciation of Al(III) in serum as an example.

Copper and cadmium complexation by high molecular weight materials of dominant microalgae and of water from a eutrophic reservoir

High molecular weight materials (HMWM, >12000-14000 Da) excreted by the two cyanophyte species (Microcystis aeruginosa and Anabaena spiroides) and a diatom (Aulacoseira granulata) which are dominant phytoplankton species in a eutrophic reservoir, Barra Bonita, Brazil were investigated as copper (Cu) and cadmium (Cd) complexation agents and their monosaccharide and elemental analysis of C, H, N and S determined. Also, HMWM obtained from the reservoir water as well as from a mixture of the three algae materials were studied. The HMWM of the cyanophytes and the mixture of the three algae materials complexed Cu and Cd, whereas the HMWM of the diatom and that from the reservoir water complexed only Cu. Two classes of ligands of intermediate to weak binding strength were obtained after Scatchard plot analysis of the titration data. The cyanophytes and the mixture HMWM presented higher conditional stability constants for Cu class-1 ligands (logK1' = 9.2-9.5) than the HMWM derived from the diatom and the reservoir water (logK1' = 8.6-8.8). Higher proportions of acidic monosaccharides corresponded to higher K1' of Cu and Cd complexation, yet no relation was observed among complexation parameters and elemental analysis. This study points out Cu ligands of intermediate to weak binding strength in the excreted HMWM of dominant microalgae and in the HMWM of the reservoir water, while Cd was solely complexed by ligands isolated from the cyanophyte HMWM.

Comparison of a rhizosphere-based method with other one-step extraction methods for assessing the bioavailability of soil metals to wheat

There is no method recognized as a universal approach for evaluation of bioavailability of heavy metals in soil. Based on the simulation of the rhizosphere soil conditions and integration of the combined effects of root-soil interactions as a whole, a rhizosphere-based method has been proposed. Wet fresh rhizosphere soil was extracted by low-molecular-weight organic acids (LMWOAs) to fractionate metal fractions of soil pools, which were then correlated with the metal contents of wheat roots and shoots. The rhizosphere-based method was compared with other one-step extraction methods using DTPA, EDTA, CaCl2, and NaNO3 as extractants and the first step of the Community Bureau of Reference (BCR) method. Simple correlation and stepwise multiple regression analysis were used for the comparison. Simple correlation indicated that the extractable Cu, Zn, Cr, and Cd of soils by the rhizosphere-based method were significantly correlated with the metal contents of wheat roots. For DTPA, BCR1 and EDTA methods there was a relatively poor correlation between the extractable Cu, Zn and Cd of soil and metal contents of wheat roots. Stepwise multiple regression analysis revealed that the equation of the rhizosphere-based method was the simplest one, and no soil properties variables needed to be added. In contrast, the equations of other one-step extraction methods were more complicated, and soil properties variables needed to be entered. The most distinct feature of the rhizosphere-based method was that the recommended method was suitable for acidic, neutral and near alkaline soils. However, the DTPA and EDTA extraction methods were suitable for calcareous soils only-or-only for acidic soils. The CaCl2, and NaNO3 extraction methods were only suitable for exchangeable metals. In short, the rhizosphere-based method was the most robust approach for evaluation of bioavailability of heavy metals in soils to wheat.

Influence of total organic carbon and UV-B radiation on zinc toxicity and bioaccumulation in aquatic communities

The effects of total organic carbon (TOC) and UV-B radiation on Zn toxicity and bioaccumulation in a Rocky Mountain stream community were assessed in a 10-d microcosm experiment. We predicted that TOC would mitigate Zn toxicity and that the combined effects of Zn and UV-B would be greater than Zn alone. However, TOC did not mitigate Zn toxicity in this study. In fact, treatments with TOC plus Zn had significantly lower community respiration as compared with the controls and Zn concentrations associated with the periphyton increased in the presence of TOC. UV-B had no additive effect on periphyton Zn accumulation or community respiration. Heptageniid mayflies (Ephemeroptera) were particularly sensitive to Zn, and reduced abundances were observed in all Zn treatments. UV-B did not additionally impact Heptageniid abundances; however UV-B did have a greater effect on macroinvertebrate drift than Zn alone. Ephemeroptera, Plecoptera, and Trichoptera (groups typically classified as sensitive to disturbance) were found in highest numbers in the drift of UV-B + Zn treatments. Measures of Zn accumulation in the caddisfly Arctopsyche grandis, periphyton biomass, and total macroinvertebrate abundance were not sufficiently sensitive to differentiate effects of TOC, UV-B, and Zn. These results indicate that UV-B and TOC affect Zn bioavailability and toxicity by impacting species abundance, behavior, and ecosystem processes.

Biokinetics of cadmium, selenium, and zinc in freshwater alga Scenedesmus obliquus under different phosphorus and nitrogen conditions and metal transfer to Daphnia magna

The uptake of Cd, Se(IV) and Zn by the freshwater alga Scenedesmus obliquus and the subsequent transfer and release budget in Daphnia magna were investigated under different nutrient additions and cell incubation conditions. An increase in ambient phosphate concentrations from 0.5 micromol l(-1) to 50 micromol l(-1) significantly increased the intracellular accumulation of Cd (by 18x) and Zn (by 5x), but decreased the accumulation of Se (by 126x) in the alga. The percentage of these metals distributing in the intracellular pool of algae also increased substantially with increasing ambient P concentrations. Nitrate addition from 5.0 to 200 micromol l(-1) did not influence the uptake of any of the three metals, although a significant decrease in the intracellular Se distribution was observed. Radiolabeled algae under different nutrient manipulations (semi-continuous culture, starvation, and P-pulse treatments) were used to measure trophic transfer assimilation efficiency (AE) in Daphnia. When the algal cells were grown in a semi-continuous culture, starved for N and P, or were treated with P-pulse, the AEs of Cd and Zn were generally independent of the nutritional conditions, but the Se AE was significantly affected by different P levels. The efflux rate constants, determined during 10 d depuration following 7 days of dietary uptake, decreased significantly for Cd and Zn, but were relatively constant for Se with increasing P concentration. N-addition caused no effect on the metal efflux rate constants. P- or N-additions did not influence the release budget (including molting, neonates, excretion and feces) for all three elements in Daphnia. Our study indicated that phosphate enrichment may substantially increase metal uptake in green alga S. obliquus. Responses of trophic transfer in Daphnia to nutrient enrichment were metal specific. P-enrichment can possibly lead to considerable decrease on Se transfer from algae to zooplankton.

Influence of metal speciation in natural freshwater on bioaccumulation of copper and zinc in periphyton: a microcosm study

The free ion activity model (FIAM) has already been confirmed under laboratory conditions for many trace metals but has still to be validated under natural conditions where the presence of natural organic ligands influences metal speciation and bioavailability. The goal of this study was to test if the FIAM is followed under natural conditions by measuring copper and zinc speciation as well as metal accumulation in periphyton. Periphyton was exposed in microcosms to natural river water with different added concentrations of copper (25-258 nM) or zinc (18-501 nM) and additions of a synthetic ligand (NTA). Free Cu2+ was in the range of 10(-16.5)-10(-14.5) M and Zn(2+) was in the range of 0.7-8.7 nM, as measured by competitive ligand exchange coupled with cathodic/anodic stripping voltammetry. Other metal complexes were either measured or computed. Bioaccumulation of zinc in periphyton appeared to be controlled by the free zinc ion concentration, confirming the FIAM. In contrast, bioaccumulation of copper was controlled by weakly complexed copper (including Cu2+ plus inorganic and weak organic complexes), which is in disagreement with the FIAM, and appears to be caused by limitation of copper diffusion due to very low free Cu2+ occurring in natural environments.

Speciation and Bioavailability of Trace Metals in Water: Progress Since 1982

The advances in studies of trace metal speciation and bioavailability since Mark Florence’s 1982 review of the topic, published in Talanta, have been comprehensively reviewed. While the relative merits of kinetic and equilibrium approaches are still being determined, advances in the applications of stripping voltammetry, including the application of microelectrodes and an appreciation of detection windows in both CSV and ASV, have been matched by the introduction of new dynamic techniques including diffusive gradients in thin films (DGTs), permeation liquid membranes (PLMs), and improved applications of chelating resins. There have also been improvements in equilibrium techniques such as ion-selective electrodes and Donnan dialysis. The ability of geochemical speciation models to predict metal complexation by natural organic matter has greatly improved, yet the models still require validation against field measurements. More reliable and relevant bioassays have been developed using sensitive species such as algae and bacteria, and improved protocols are eliminating uncertainties particularly due to problems with high cell densities, and are allowing more useful comparisons with chemically estimated bioavailability. While the free-ion activity model has provided an improved interpretation of the relative toxicities observed with different metal species, its extension to the biotic ligand model is leading to better predictions of acute effects at least on higher organisms. The extension of such approaches to studies of chronic effects at ‘natural’ concentrations using unicellular organisms remains a challenge, as does the evaluation of whether such approaches are simplified limiting cases of a more dynamic natural situation where chemical reactivity and rates of metal transport could be important.

Uptake of Neutral Metal Complexes by a Green Alga: Influence of pH and Humic Substances

We have examined the influence of pH and a natural humic acid on the short-term uptake (<40 min) of a neutral, lipophilic metal complex by a unicellular freshwater alga, Pseudokirchneriella subcapitata. Cadmium diethyldithiocarbamate ([Cd(DDC)2]0) was used as a model lipophilic metal complex and Suwannee River Humic Acid (SRHA) was chosen as a representative aquatic humic acid (6.5 mg C L−1). Under the experimental conditions virtually all the Cd was expected to be present as the lipophilic complex ([Cd]T = 0.38 nM; [DDC] 1 μM; [Cd2+] <10−15 M; pH 7.0, 6.0, or 5.5). Uptake of [Cd(DDC)2]0 proved to be sensitive to pH changes. It was lower at pH 6.0 and 5.5 than at pH 7.0. To our knowledge, this is the first demonstration of reduced uptake of a lipophilic metal complex at low pH. The presence of SRHA also affected uptake, either by binding the lipophilic complex in solution and reducing its bioavailability (pH 7.0) or by increasing the permeability of the algal membrane (pH 5.5).

Accumulation of copper and zinc in periphyton in response to dynamic variations of metal speciation in freshwater

Although the free ion activity model (FIAM) has been well-established in laboratory studies, there remains the need for field data in order to validate the applicability of this model in natural systems. The objective of this study was to investigate the response of copper and zinc accumulation in periphyton to short-term variations in metal concentration and speciation in freshwater. During heavy rain events, dissolved Cu in the Furtbach stream increased from 40 to 118 nM, while dissolved Zn increased from 45 to 147 nM due to the release of metals from contaminated sediments. Increases in free copper and free zinc ions in the water (from 10(-14) to 10(-11.5) M for Cu2+; from 1 to 15 nM for Zn2+) were observed during the onset of heavy rain events. Periphytic algae collected from artificial substrates had an intracellular copper content (0.2-2.8 micromol/g dry weight (dw)) that varied as a function of the exchangeable copper in the water (labile form) rather than the free Cu2+. Intracellular zinc content (1.5-8.0 micromol/g dw) was found to follow the same trend as the free zinc ion concentration. Adsorbed Cu and Zn on periphyton showed a very dynamic response to variations in dissolved metal concentration. Different concentrations of dissolved manganese during the two time periods may affect the accumulation of zinc and copper by competition for metal uptake.

Metal bioavailability to phytoplankton--applicability of the biotic ligand model

To elicit a biological response from a target organism and/or to accumulate within this organism, a metal must first interact with a cell membrane. For hydrophilic metal species, this interaction with the cell surface can be represented in terms of the formation of M-X-cell surface complexes, e.g. M(z+)+(-)X-cell<-->M-X-cell, where -X-cell is a cellular ligand present at the cell surface. According to the free-ion model, or its derivative the biotic ligand model (BLM), the biological response elicited by the metal will be proportional to [M-X-cell]. In this paper, using freshwater algae as our test species, we examine some of the key assumptions that underlie the BLM, namely that metal internalization is slow relative to the other steps involved in metal uptake (i.e. the M-X-cell complex is in equilibrium with metal species in solution), that internalization occurs via cation transport, and that internalization must occur for toxicity to appear. Recent experiments with freshwater algae are described, demonstrating anomalously high metal accumulation and/or toxicity in the presence of a common low molecular weight metabolite (alanine), or in the presence of an assimilable inorganic anion (thiosulfate). The possible implications of these findings for the application of the BLM to higher organisms are discussed.

The biotic ligand model and a cellular approach to class B metal aquatic toxicity

The biotic ligand model (BLM) and a cellular molecular mechanism approach represent two approaches to the correlation of metal speciation with observed toxicity to aquatic organisms. The two approaches are examined in some detail with particular reference to class B, or soft metals. Kinetic arguments are presented to suggest situations that can arise where the BLM criterion of equilibrium between all metal species in the bulk solution and the biotic ligand may not be satisfied and what might the consequences be to BLM predictive capability. Molecular mechanisms of toxicity are discussed in terms of how a class B metal might enter a cell, how it is distributed in a cell, and how the cell might respond to the unwanted metal. Specific examples are given for copper as an organism trace essential metal, which is toxic in excess, and for silver, a non-essential metal. As class B metals all bind strongly to sulfur, regulation of these metals requires that all S(II-) species be accounted for in aquatic systems, even under oxic conditions.

Bioavailability of waterborne strontium to the common carp, Cyprinus carpio, in complexing environments

The uptake of strontium (Sr) and calcium (Ca) in the common carp, Cyprinus carpio, was studied in chemically defined freshwater in the presence of the complexing ligands, ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA). The uptake rates were measured in the whole body, gills, and blood of the fish after an exposure period of 3 h. The uptake rates were determined by using the radiotracers (85)Sr and (45)Ca, and analyzed as a function of the free-ion activity of Sr and Ca in water. Although Sr(2+) activity decreased, the uptake of Sr showed an increase at relatively low concentrations of EDTA and NTA, and a decrease at relatively high concentrations. This can be explained by the decreased competition between Sr(2+) and Ca(2+) at the gill uptake sites due to approximately 30-140-fold higher affinity of EDTA and NTA for Ca(2+) than Sr(2+). With decreasing Ca(2+) activity, Ca uptake rates decreased in the presence of EDTA and NTA, but the effect of NTA was less pronounced. A Michaelis-Menten type competitive inhibition model was derived that could predict the whole-body Sr and Ca uptake rates, taking into account the ambient Sr(2+) and Ca(2+) activities in the presence of EDTA. In case of NTA, the uptake rates were found to be 1.5-3.2 times higher than what was predicted by the model. When the fish were exposed to complexing environments in the complete absence of Ca, an increased uptake of Sr was still observed in case of NTA, but not EDTA. The increased uptake in the presence of NTA is attributed to the direct uptake of SrNTA(-) and CaNTA(-) complexes from water. The results reveal that the uptake of Sr and Ca in carp is not merely a function of the free metal-ion activity but that certain complex species may contribute significantly to overall uptake.

Characterization of an inducible citrate uptake system in Penicillium simplicissimum

When citrate was used as a sole source of carbon, citrate uptake by Penicillium simplicissimum increased 267-fold (if glucose-grown mycelium was adapted to citrate) or 1400-fold (if the fungus was grown on citrate) compared to glucose-grown mycelium. Inhibition of macromolecular synthesis prevented this stimulation of citrate uptake. Citrate uptake by glucose-grown mycelium was low (0.0015 nmol min(-1) (mg DW)(-1)) and most probably due to diffusion of undissociated citric acid. Citrate-adapted mycelium had a K(M) of 65 micromol l(-1) and a V(max) of 0.34 nmol min(-1) (mg DW)(-1). In citrate-grown mycelium K(M) was 318 micromol l(-1) and V(max) was 8.5 nmol min(-1) (mg DW)(-1). Citrate uptake was inhibited by sodium azide and uncouplers (TCS, 3,3',4',5-tetrachlorosalicylanilide; FCCP, carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone). Because of this we postulate that the induced citrate uptake must be an active transport process. The pH optimum of citrate uptake was between pH 6 and 7. EDTA and Mg2+, Mn2+, Cu2+, Zn2+, Fe2+, Ca2+ only weakly influenced the induced citrate uptake. The properties of citrate uptake by Aspergillus niger and P. simplicissimum are compared.

Thiosulfate enhances silver uptake by a green alga: role of anion transporters in metal uptake

Short-term (< 1 h) silver uptake by the green alga Chlamydomonas reinhardtii was measured in the laboratory in defined inorganic media in the presence or absence of ligands (chloride and thiosulfate). In contradiction to the free-ion model of metal uptake, silver accumulation by the alga proved to be sensitive to the choice of ligand used to buffer the free silver concentration. For a low fixed free Ag+ concentration of 10 nM, silver uptake in the presence of thiosulfate (0.11 microM) was 2x greater than in the presence of chloride (4 mM). When sulfate was removed from the exposure medium (i.e., 81 microM-->0 microM), silver uptake in the presence of thiosulfate was even more markedly enhanced (more than 4x greater than in the presence of chloride). Varying the sulfate concentration in the exposure medium only affected silver uptake if thiosulfate was present. We conclude that silver-thiosulfate complexes are transported across the plasma membrane via sulfate/thiosulfate transport systems and that sulfate acts as a competitive inhibitor of this uptake mechanism.