Performance of diffusion gradient in thin films to evaluate the toxic fraction of copper to Daphnia magna

This study investigates the relevance of the diffusion gradient in thin films technique (DGT) to measure copper's induced lethality on Daphnia magna in natural water spiked with various organic ligands. Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and glycine were used as artificial organic ligands in controlled solutions of mineral water. With EDTA, DGT measurement makes it possible to predict the toxicity of the mixture because Cu-EDTA complexes are inert, whereas DGT is of no help in the case of NTA, because Cu-NTA complexes are fully labile. The Cu-glycine complexes appear as partly labile and toxic. Humic acids as well as fresh and aged algae extracts also were used as models for natural dissolved organic matters. All three of them form copper complexes that are not toxic to Daphnia magna and appear as partly labile with open-pored DGT. However, the use of restricted gels in DGT greatly reduces the contribution of labile complexes, at least for humic acids and aged algae copper complexes. The DGT with restrictive gels, therefore, appears to be a powerful tool for measuring bioavailable copper in natural water bodies, especially when the dissolved organic matter mostly is of humic origin. The DGTs potential ability to mimic the lability induced by the biological uptake also is discussed.     

Effect of dissolved organic matter of various origins and biodegradabilities on the bioaccumulation of polycyclic aromatic hydrocarbons in Daphnia magna

As a preliminary study of the influence of urban organic matter on the bioavailability of polycyclic aromatic hydrocarbons (PAHs), the effect of different types of dissolved organic matter (DOM) on the bioaccumulation of fluoranthene, pyrene, or benzo[a]pyrene in Daphnia magna was studied. Commercial humic substances, DOM from the aeration basin of a wastewater treatment plant, and highly biodegradable DOM (algae or animal extracts) were tested. The bioaccumulation of benzo[a]pyrene was reduced by each DOM (up to 80% reduction with humic substances). Pyrene bioaccumulation was also decreased by each DOM to a lesser extent. Fluoranthene bioaccumulation was affected by the presence of humic acids only. In each experiment, the solution containing humic DOM led to the lowest bioaccumulation. Supposing that only dissolved PAHs were bioavailable, the reduction of bioaccumulation allowed a biological estimate of the partition coefficients of DOM and PAH, K(DOC). The estimated coefficients were positively related to the aromaticity of DOM and negatively related to its biodegradability.     

Differential bioavailability of copper complexes to bioluminescent Pseudomonas fluorescens reporter strains

Increased levels of Cu in agricultural soils are of concern, because Cu toxicity may adversely affect important soil microorganisms, including pseudomonads. Because total metal concentrations correlate poorly with bioavailability and toxicity, a need exists for more information linking Cu speciation, bioavailability, and toxicity. The objective of the present study was to determine the bioavailability of different Cu complexes to Pseudomonas spp. A Cu-specific bioluminescent Pseudomonas fluorescens reporter strain was used to determine bioavailable Cu, which was operationally defined as those Cu species that induced expression of bioluminescence. Another strain of P. fluorescens, which continuously expressed bioluminescence, was used as a toxicity reporter. Experiments were performed in a defined aqueous medium containing 0.04 microM Cu, which was amended with ethylenediaminetetraacetic acid (EDTA), citrate, or a well-characterized pool of dissolved organic matter (DOM). Bioluminescence emitted by the biosensors was related to data for Cu speciation obtained by geochemical modeling. Changes in Cu bioavailability in the presence of EDTA coincided with modeled changes in Cu2+ activity, indicating that Cu-EDTA complexes were not bioavailable to the Cu-specific reporter. In contrast, changes of Cu bioavailability in the presence of citrate did not correspond to changes in Cu2+, indicating that Cu-citrate complexes were fully bioavailable to the reporter strain. Finally, the response of the Cu-reporter strain to Cu in the presence of DOM indicated that Cu formed bioavailable as well as unavailable complexes with DOM. We conclude that free Cu2+ activity is a poor predictor of Cu bioavailability to Pseudomonas spp. in samples containing organic ligands.     

Effect of dissolved organic matter on the growth of algae, Pseudokirchneriella subcapitata, in Korean lakes: the importance of complexation reactions

The objective of this study was to investigate the effect of dissolved organic ligands, such as EDTA, humic acids, hydrophobic and hydrophilic fractions of dissolved organic matter (DOM), on the growth of Pseudokirchneriella subcapitata in cultured media and natural lake waters. The growth of algae was gradually increased on the addition of dissolved organic ligands, but markedly declined at high concentrations, due to reduced bioavailable iron concentrations (e.g., Fe3+ and Fe'--inorganic ferric iron). The results demonstrated that the high growth rates of P. subcapitata were correlated with the hydrophobic, but not hydrophilic, DOM from five lakes, spiked under the nutrients-controlled conditions. This was attributed to the role of DOM in controlling the bioavailable iron due to complexation of Fe(III) with -COOH and -OH functional groups on the hydrophobic DOM. Therefore, the hydrophobic DOM, as chelating agent, is a biologically important component in the lake waters, which affects the algal growth by interacting with bioavailable iron in the lake water and can change the results of bioassay experiments.     

Natural DOM Affects Copper Speciation and Bioavailability to Bacteria and Ciliate

This study aimed to investigate the influence of natural dissolved organic materials (DOM) on copper speciation (total dissolved, particulate, and free Cu²⁺ ions) and bioavailability during a two-level experimental microbial food chain. Bacteria were used as the first trophic level, and Paramecium caudatum (protozoan) as the second. The organisms were obtained from a freshwater reservoir and kept under controlled laboratory conditions. Three experimental treatments were performed: exposure of the organisms to copper in the absence of DOM, exposure to DOM in the absence of copper, and exposure to both copper and DOM. Freshwater medium containing natural DOM and copper at a total dissolved concentration of 1.8 × 10⁻⁶ mol L⁻¹ was furnished to bacteria, which was further used as food to the protozoan. The results showed that after bacterial growth, DOM concentration decreased as quantified by total organic carbon determinations. At the same time, free Cu²⁺ ions concentration increased in the medium. A lower copper concentration was detected in both microorganisms in the presence of DOM. We conclude that natural DOM reduced copper accumulation in the organisms on the first and second trophic levels, thus reducing the entrance of copper into the aquatic microbial food chain.     

Measuring the bioavailability of two hydrophobic organic compounds in the presence of dissolved organic matter

Bioavailability of benzo[a]pyrene (BaP) and 3,3',4,4'-tetrachlorobiphenyl (TCB) was studied in natural lake water containing dissolved organic matter (DOM). Lake water was diluted to give a dissolved organic carbon (DOC) range of 1 to 20 mg/L. Partition coefficients for the model compounds were assessed at different DOM concentrations and over time with three different methods, namely equilibrium dialysis and reverse-phase and liquid-liquid extraction. In addition, biological partition coefficients were estimated from the difference in the bioconcentration of the model compounds in Daphnia magna in the presence and absence of DOM. Results showed that bioavailability of the model compounds was reduced by the presence of DOM. The equilibrium dialysis method gave the best estimates for bioavailability of the model compounds when compared with biologically determined values. Both the reverse-phase and the liquid-liquid extraction overestimated the bioavailable fraction. The more pronounced overestimation of bioavailable fraction of TCB suggested that the sorption of TCB was not only lower but the interaction was also weaker than that of BaP. Increasing DOM concentration produced lower partition coefficients and the effect seemed to be more pronounced when measured by the reverse-phase and the extraction methods.     

Use of anodic stripping voltammetry in predicting toxicity of copper in river water

The labile concentration and toxicity of Cu as influenced by alkalinity and different concentrations of ethylenediaminetetraacetic acid (EDTA) and naturally derived fulvic acid (FA) were determined by bioassays carried out in the culture media for Daphnia magna (D. magna). The labile concentration of Cu was obtained by differential pulse anodic stripping voltammetry with a double-acidification method (DAM-DPASV). Changes in water alkalinity did not affect the labile concentration of Cu, but increase in alkalinity did reduce the mortality of D. magna. In the presence of EDTA and FA, both labile concentration of Cu and mortality were reduced. By excluding Cu-carbonate complexes from the labile concentration, a bioavailable concentration of Cu ([Cu*]) was obtained and was used to predict the acute toxicity of Cu on D. magna. For natural waters, the labile concentration of Cu was measured by DAM-DPASV, and [Cu*] was calculated using MINTEQ A2 software (developed by the U.S. Environmental Protection Agency) based on the anion composition of waters. This procedure was tested for waters and sediment elutriates sampled from the Le An River (Jiangxi Province, China) that were severely polluted by the discharges from a copper mine. The results showed that [Cu*] was a good indicator for Cu toxicity and could be used under field conditions.     

The ability of dissolved organic matter (DOM) to influence benzo[a]pyrene bioavailability increases with DOM biodegradation

The biodegradation of two substrates and the ability of dissolved organic matter (DOM) to influence benzo[a]pyrene bioavailability as DOM biodegradation progressed were monitored in reactors. Substrates were composed of algae extracts and an artificial substrate that mimics raw wastewater, which were considered to be autochthonous and anthropogenic allochthonous models for DOM, respectively. The soluble microbial products formed during biomass activity were also studied. The aromaticity of DOM was investigated with specific ultraviolet absorbance. Partitioning coefficients between DOM and benzo[a]pyrene, K(DOC)(biol), were biologically determined by means of 4-h bioaccumulation experiments on Daphnia magna. Parent and degraded substrates always significantly reduced the bioaccumulation of benzo[a]pyrene at environmental DOM concentrations. Soluble microbial products also significantly affected the benzo[a]pyrene bioaccumulation. K(DOC)(biol) ranged between 2 x 10(4) and 4 x 10(5) L/kg. As the artificial wastewater biodegraded, DOM aromaticity increased, as did K(DOC)(biol). During the biodegradation of algae extract DOM, K(DOC)(biol) increased, whereas their aromaticity slightly decreased.     

Effect of suspended solids on bioavailability of pyrethroid insecticides

Runoff and surface-water effluents commonly contain suspended solids. Adsorption to suspended particles and the associated dissolved organic matter (DOM) may significantly decrease the freely dissolved concentration of a hydrophobic compound and, hence, its availability to aquatic organisms. In the present study, we evaluated phase distribution and bioaccumulation of two synthetic pyrethroids, bifenthrin and permethrin, in water samples containing suspended solids from different source sediments. Uptake of [14C]bifenthrin or [14C] permethrin by Daphnia magna after 24 h consistently decreased with increasing levels of suspended solids in the range of 0 to 200 mg/L. The trend of decrease was closely mimicked by pesticide accumulation on polydimethylsiloxane (PDMS) fibers exposed under the same conditions, and the ratio of body residues in D. magna to the concentration detected in the PDMS fiber was consistently around 2.4. Regression analysis showed that the pesticide adsorbed on particles or DOM was completely unavailable to D. magna for uptake during the 24-h exposure. The relative contribution of particles and DOM to the reduced bioavailability depended on the organic matter content and the texture of the source sediment. The influence from particles was predominant for sandy sediments, but contribution from DOM became comparable to or even greater than particles when the organic matter content of the source sediment was 1% or greater. The inhibitory effects of suspended solids on bioavailability should be considered when monitoring runoff and surface-water effluents for synthetic pyrethroids. The proposed PDMS method is simple and inexpensive, and it may serve as an effective option for obtaining ecotoxicologically relevant concentrations.     

Copper toxicity in relation to surface water-dissolved organic matter: biological effects to Daphnia magna

Water quality standards for copper are usually stated in total element concentrations. It is known, however, that a major part of the copper can be bound in complexes that are biologically not available. Natural organic matter, such as humic and fulvic acids, are strong complexing agents that may affect the bioavailable copper (Cu2+) concentration. The aim of this study was to quantify the relation between the concentration of dissolved natural organic matter and free Cu2+ in surface waters, and the biological effect, as measured in a standardized ecotoxicological test (48 h-median effective concentration [EC50] Daphnia magna, mobility). Six typical Dutch surface waters and an artificial water, ranging from 0.1 to 22 mg/L dissolved organic carbon (DOC), were collected and analyzed quarterly. Chemical speciation modeling was used as supporting evidence to assess bioavailability. The results show clear evidence of a linear relation between the concentration of dissolved organic carbon (in milligrams DOC/L) and the ecotoxicological effect (as effect concentration, EC50, expressed as micrograms Cu/L): 48-h EC50 (Daphnia, mobility) = 17.2 x DOC + 30.2 (r2 = 0.80, n = 22). Except for a brook with atypical water quality characteristics, no differences were observed among water type or season. When ultraviolet (UV)-absorption (380 nm) was used to characterize the dissolved organic carbon, a linear correlation was found as well. The importance of the free copper concentration was demonstrated by speciation calculations: In humic-rich waters the free Cu2+ concentration was estimated at approximately 10(-11) M, whereas in medium to low dissolved organic carbon waters the [Cu2+] was approximately 10(-10) M. Speciation calculations performed for copper concentrations at the effective concentration level (where the biological effect is considered the same) resulted in very similar free copper concentrations (approximately 10(-8) M Cu) in these surface waters with different characteristics. These observations consistently show that the presence of organic matter decreases the bioavailability, uptake, and ecotoxicity of copper in the aquatic environment. It demonstrates that the DOC content must be included in site-specific environmental risk assessment for trace metals (at least for copper). It is the quantification of the effects described that allows policy makers to review the criteria for copper in surface waters.     

Dissolved fraction of standard laboratory cladoceran food alters toxicity of waterborne silver to Ceriodaphnia dubia

The biotic ligand model (BLM) for the acute toxicity of cationic metals to aquatic organisms incorporates the toxicity-modifying effects of dissolved organic matter (DOM), but the default parameterization (i.e., assuming 10% of DOM is humic acid) does not differentiate DOM from different sources. We exposed a cladoceran (Ceriodaphnia dubia) to Ag in the presence of DOM from filtered YCT (standard yeast-Cerophyll(R)-trout chow food recommended by the U.S. Environmental Protection Agency [EPA] for cladocerans), from the Suwannee River (GA, USA; relatively little anthropogenic input), and from the Desjardins Canal in Hamilton (ON, Canada; receives treated municipal wastewater effluent). In all three treatments, the dissolved organic carbon (DOC) concentration was 2 mg/L (the concentration following addition of YCT slurry at the U.S. EPA-recommended volume ratio). The average 48-h median effects concentration (EC50) ratios for dissolved Ag in the presence and absence of DOM [i.e., (EC50 with DOM)/(EC50 without DOM)] were as follows: Suwannee River, 1.6; Desjardins Canal, 2.2; and YCT filtrate, 26.8. Therefore, YCT filtrate provided much more protection against Ag toxicity than that provided by DOM from the surface waters. The major spectral characteristic that differentiated YCT filtrate from the other two types of DOM was a strong tryptophan peak in the excitation- emission matrix for YCT. These results have important implications for interpreting Ag toxicity tests in which organisms are fed YCT, and they suggest BLM-calculated toxicity predictions might be improved by incorporating specific chemical constituents or surrogate indices of DOM. Another component of the protective effect against Ag toxicity, however, might be that the dissolved fraction of YCT served as an energy and/or nutrient source for C. dubia.     

Phase distribution of synthetic pyrethroids in runoff and stream water

Synthetic pyrethroids (SPs) are a group of hydrophobic compounds with significant aquatic toxicity. Their strong affinity to suspended solids and humic materials suggests that SPs in natural surface water are distributed in solid-adsorbed, dissolved organic matter (DOM)-adsorbed, and freely dissolved phases. The freely dissolved phase is of particular importance because of its mobility and bioavailability. In the present study, we used solid-phase microextraction to detect the freely dissolved phase, and we evaluated the phase distribution of bifenthrin and permethrin in stream and runoff waters. In stream water, most SPs were associated with the suspended solids and, to a lesser extent, with DOM. The freely dissolved phase contributed only 0.4% to 1.0%. In runoff effluents, the freely dissolved concentration was 10% to 27% of the overall concentration. The predominant partitioning into the adsorbed phases implies that the toxicity of SPs in surface water is reduced because of decreased bioavailability. This also suggests that monitoring protocols that do not selectively define the freely dissolved phase can lead to significant overestimation of toxicity or water-quality impacts by SPs.     

Nickel speciation in the presence of different sources and fractions of dissolved organic matter

This study evaluated nickel (Ni) speciation in the presence of different fractions (humic acid (HA), fulvic acid (FA)) and sources (natural sediment, Suwannee River, peat moss) of dissolved organic matter (DOM) at Ni concentrations toxicologically relevant to the freshwater amphipod, Hyalella azteca. The free Ni ion, Ni(2+), was measured in reconstituted water (with or without DOM) using a miniaturized ion-exchange technique (IET). Ni speciation from these experiments was compared to calculated results obtained from equilibrium modelling (WHAM, Model VI). While it is known that Ni will complex with DOM, it was found that under acutely toxic Ni exposure concentrations ([Ni(Total)]=5mg/L, or 85.1 microM) representative surface-water DOC concentrations ( approximately 10mg/L) played little or no role in Ni speciation. Conversely, at sublethal Ni exposure concentrations ([Ni(Total)]=0.2 and 0.5 microg/L, or 3.4 and 8.51 microM, respectively) DOM significantly affected Ni speciation with [Ni(2+)] decreasing with increasing concentration of DOM. It was found that for similar concentrations of DOC (same fraction, different sources), the measured Ni(2+) concentrations were reduced (relative to the control), but similar to one another. Conversely, at similar DOC concentrations, the HA fraction reduced Ni(2+) levels to a greater extent than the associated FA fraction. Overall, this study provides proof of principle that Suwannee River and peat humic substances are suitable analogues for natural sediment pore-water DOM when evaluating Ni bioavailability in freshwater.     

Effect of dissolved organic matter source on acute copper toxicity to Daphnia magna

The protective effect of dissolved organic matter (DOM) on metal toxicity to aquatic organisms has been reported by numerous authors. Bioavailability models such as the biotic ligand model (BLM) thus account for this factor to predict metal toxicity to aquatic organisms. Until now, however, few attempts have been made to assess the effect of the DOM source on metal speciation and toxicity and, accordingly, on BLM predictions. The aims of this study were to investigate to what extent DOMs differ in their ability to decrease acute copper toxicity to the cladoceran Daphnia magna and to evaluate if ultraviolet (UV) absorbance measurements may be a simple and effective method to incorporate DOM variability into the acute Cu-BLM for D. magna. Acute toxicity tests were carried out in artificial test water enriched with DOMs isolated from six locations in Europe and North America and in seven natural European surface waters. The acute Cu-BLM for D. magna was then used to estimate the copper complexing capacity of each DOM (expressed as % active fulvic acid, %AFA). A factor of 6 difference was observed between the lowest and the highest copper complexing capacity. A significant linear relationship was observed between the UV-absorbance coefficient at 350 nm (epsilon350) and the %AFA. Linking this relationship to the acute Cu-BLM resulted in a significant improvement of the predictive capacity of this BLM. Without accounting for this relationship, 90% of the predicted 48-h 50% effective concentrations (EC50) were within a factor of 2 of the observed EC50s; taking this relationship into account, 90% of the EC50s were predicted with an error of less than factor 1.3. The present study and other studies seem to indicate that UV absorbance may be a good measure of biologically and toxicologically relevant differences in copper binding behavior of DOM.     

Speciation, behavior, and bioavailability of copper downstream of a mine-impacted lake

A combination of Cu speciation analysis and toxicity testwork was conducted to assess the behavior, speciation, and bioavailability of Cu in a stream system rich in dissolved organic carbon (DOC) downstream of a mine-impacted lake (East Lake, ON, Canada). Elevated levels (approximately 50 microg/L) of Cu exist in the lake due to the release of dissolved Cu to the water column from underlying sediments. Most of the Cu present in East Lake and downstream is present as filterable species that represent 74 to 100% of the total. Measurements of labile Cu as measured by diffusive gradients in thin films (DGT) suggest that most of the Cu is unavailable to aquatic biota. The DGT results indicate that 9 to 24% of Cu within the receiving environment is biologically available. Decreases in the labile Cu fraction with distance downstream of East Lake correlate well with increases in the concentration of DOC (r(2) = 0.79-0.95), presumably due to the progressive importance of Cu-organic complexes. The relationship between filterable Cu and SO(4)(2-) downstream of East Lake was linear (r(2) = 0.99) for all sampling periods, suggesting that decreases in filterable Cu concentration downstream of East Lake could be attributed solely to dilution (i.e., conservative behavior). Variations in the filterable Cu concentration resulting in 50% mortality (LC50 = 96-203 microg/L) and the concentration resulting in an inhibition of reproduction by 25% (IC25 = 75-156 microg/L) with respect to Ceriodaphnia dubia (7-d incubation) in Cu-spiked solutions could be explained by differences in labile-Cu concentrations as determined by DGT. The considerable complexation capacity afforded by lake and stream waters can be attributed to complexation of Cu with abundant DOC (7-17 mg/L). The relevance of the toxicity data to water-effect ratio testwork, and the associated development of site-specific water quality objectives, are discussed.     

Agricultural soils spiked with copper mine wastes and copper concentrate: implications for copper bioavailability and bioaccumulation

A better understanding of exposure to and effects of copper-rich pollutants in soils is required for accurate environmental risk assessment of copper. A greenhouse experiment was conducted to study copper bioavailability and bioaccumulation in agricultural soils spiked with different types of copper-rich mine solid wastes (copper ore, tailing sand, smelter dust, and smelter slag) and copper concentrate. A copper salt (copper sulfate, CuSO4) that frequently is used to assess soil copper bioavailability and phytotoxicity also was included for comparison. Results showed that smelter dust, tailing sand, and CuSO4 are more likely to be bioavailable and, thus, toxic to plants compared with smelter slag, concentrate, and ore at equivalent total copper concentrations. Differences may be explained by intrinsic differences in copper solubilization from the source materials, but also by their capability to decrease soil pH (confounding effect). The copper toxicity and bioaccumulation in plants also varied according to soil physicochemical characteristics (e.g., pH and total organic carbon) and the available levels of plant nutrients, such as nitrogen, phosphorus, and potassium. Chemistry/mineralogy of mine materials, soil/pore-water chemistry, and plant physiological status thus should be integrated for building adequate models to predict phytotoxicity and environmental risk of copper.     

Development and field validation of a predictive copper toxicity model for the green alga Pseudokirchneriella subcapitata

In this study, the combined effects of pH, water hardness, and dissolved organic carbon (DOC) concentration and type on the chronic (72-h) effect of copper on growth inhibition of the green alga Pseudokirchneriella subcapitata were investigated. Natural dissolved organic matter (DOM) was collected at three sites in Belgium and The Netherlands using reverse osmosis. A full central composite test design was used for one DOM and a subset of the full design for the two other DOMs. For a total number of 35 toxicity tests performed, 72-h effect concentration resulting in 10% growth inhibition (EbC10s) ranged from 14.2 to 175.9 micrograms Cu/L (factor 12) and 72-h EbC50s from 26.9 to 506.8 micrograms Cu/L (factor 20). Statistical analysis demonstrated that DOC concentration, DOM type, and pH had a significant effect on copper toxicity; hardness did not affect toxicity at the levels tested. In general, an increase in pH resulted in increased toxicity, whereas an increase of the DOC concentration resulted in decreased copper toxicity. When expressed as dissolved copper, significant differences of toxicity reduction capacity were noted across the three DOM types tested (up to factor 2.5). When expressed as Cu2+ activity, effect levels were only significantly affected by pH; linear relationships were observed between pH and the logarithm of the effect concentrations expressed as free copper ion activity, that is, log(EbC50Cu2+) and log(EbC10Cu2+): (1) log(EbC50Cu2+)= - 1.431 pH + 2.050 (r2 = 0.95), and (2) log(EbC10cu2+) = -1.140 pH -0.812 (r2 = 0.91). A copper toxicity model was developed by linking these equations to the WHAM V geochemical speciation model. This model predicted 97% of the EbC50dissolved and EbC10dissolved values within a factor of two of the observed values. Further validation using toxicity test results that were obtained previously with copper-spiked European surface waters demonstrated that for 81% of tested waters, effect concentrations were predicted within a factor of two of the observed. The developed model is considered to be an important step forward in accounting for copper bioavailability in natural systems.     

The bioavailability of chemicals in soil for earthworms

The bioavailability of chemicals to earthworms can be modified dramatically by soil physical/chemical characteristics, yet expressing exposure as total chemical concentrations does not address this problem. In order to understand the effects of modifying factors on bioavailability, one must measure and express chemical bioavailability to earthworms in a consistent, logical manner. This can be accomplished by direct biological measures of bioavailability (e.g., bioaccumulation, critical body residues), indirect biological measures of bioavailability (e.g., biomarkers, reproduction), or indirect chemical measures of bioavailability (e.g., chemical or solid-phase extracts of soil). If indirect chemical measures of bioavailability are to be used, they must be correlated with some biological response. Bioavailability can be incorporated into ecological risk assessment during risk analysis, primarily in the estimation of exposure. However, in order to be used in the site-specific ecological risk assessment of chemicals, effects concentrations must be developed from laboratory toxicity tests based on exposure estimates utilizing techniques that measure the bioavailable fraction of chemicals in soil, not total chemical concentrations.     

Speciation of aqueous methylmercury influences uptake by a freshwater alga (Selenastrum capricornutum)

Uptake of methylmercury (MeHg) by the alga Selenastrum capricornutum was measured in freshwater batch culture bioassays. The concentration of MeHg in the alga increased rapidly (within 15 min), reached a maximum by 6 h, and then declined because of growth dilution. The alga's rapid growth rate (doubling time, approximately 10 h) contributed to the importance of growth dilution. Conditional first-order rate constants were calculated for uptake (k1 = 6.95 x 10(-9) L/cell/h) and growth (kG = 0.07/h). A competitive synthetic ligand, disodium ethylenediaminetetra-acetate, formed strong complexes with MeHg and reduced MeHg uptake, consistent with the biotic ligand model. A conditional equilibrium formation constant (K) for the MeHg-algae complex was estimated to be approximately 10(16) and was used to model the influence of natural ligands on MeHg bioavailability. Model results suggested MeHg would be most bioavailable at concentrations of dissolved organic matter (DOM) less than 10 mg/L and increasingly unavailable at higher DOM concentrations for the specific humic acid modeled. Similarly, at molar concentrations of sulfide (and, possibly, metal-sulfide clusters) equal to approximately half the MeHg concentration, MeHg was predicted to be unavailable to algae because of the formation of strong 2:1 MeHg-sulfide complexes.     

Influence of dissolved organic matter on the complexation of mercury under sulfidic conditions

The complexation of Hg under sulfidic conditions influences its bioavailability for microbial methylation. Neutral dissolved Hg-sulfide complexes are readily available to Hg-methylating bacteria in culture, and thermodynamic models predict that inorganic Hg-sulfide complexes dominate dissolved Hg speciation under natural sulfidic conditions. However, these models have not been validated in the field. To examine the complexation of Hg in natural sulfidic waters, octanol/water partitioning methods were modified for use under environmentally relevant conditions, and a centrifuge ultrafiltration technique was developed. These techniques demonstrated much lower concentrations of dissolved Hg-sulfide complexes than predicted. Furthermore, the study revealed an interaction between Hg, dissolved organic matter (DOM), and sulfide that is not captured by current thermodynamic models. Whereas Hg forms strong complexes with DOM under oxic conditions, these complexes had not been expected to form in the presence of sulfide because of the stronger affinity of Hg for sulfide relative to its affinity for DOM. The observed interaction between Hg and DOM in the presence of sulfide likely involves the formation of a DOM-Hg-sulfide complex or results from the hydrophobic partitioning of neutral Hg-sulfide complexes into the higher-molecular-weight DOM. An understanding of the mechanism of this interaction and determination of complexation coefficients for the Hg-sulfide-DOM complex are needed to adequately assess how our new finding affects Hg bioavailability, sorption, and flux.