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

A review of metal contamination in seagrasses with an emphasis on metal kinetics and detoxification

Seagrasses are important foundation species in coastal ecosystems, and they provide food and habitat that supports high biodiversity. However, seagrasses are increasingly subjected to anthropogenic disturbances such as metal pollution, which has been implicated as a significant factor driving seagrass losses. There have been several reviews synthesizing the metal concentrations in seagrasses and evaluating their utility as biomonitors for metal pollution in the coastal environment at the local scale. However, the interpretation of metal data in seagrass biomonitors requires a more mechanistic understanding of the processes governing metal bioaccumulation and detoxification. In this review, the progress and trends in metal studies in seagrasses between 1973 and 2022 were analyzed to identify frontier topics in this field. In addition, we tried to (1) analyze and assess the current status of metal contamination in seagrasses on a global scale by incorporating more metal data from tropical and Indo-Pacific seagrasses, (2) summarize the geochemical and biological factors governing metal uptake and loss in seagrasses, and (3) provide an up-to-date understanding of metals' effects on seagrasses and their physiological responses to metal challenges. This review improves our understanding of the highly variable metal concentrations observed in the field.

Comparisons in molecular weight distributions and size-dependent optical properties among model and reference natural dissolved organic matter

Humic acid (HA) and reference natural organic matter (NOM) have been widely used in environmental assessment, biogeochemistry, and ecotoxicity studies. Nevertheless, similarities and differences among the commonly used model/reference NOMs and bulk dissolved organic matter (DOM) have rarely been systematically evaluated. In this study, HA, SNOM (Suwannee River NOM) and MNOM (Mississippi River NOM), both from International Humic Substances Society, and freshly collected unfractionated NOM (FNOM) were concurrently characterized to evaluate their heterogeneous nature and size-dependent chemical properties. We found that molecular weight distributions, PARAFAC-derived fluorescent components, and size-dependent optical properties are NOM-specific and highly variable with pH. The < 1 kDa DOM abundance followed the order of HA < SNOM < MNOM < FNOM. In addition, FNOM was more hydrophilic and contained more protein-like and autochthonous components with a higher UV-absorbance ratio index (URI) and biological fluorescence index, whereas HA and SNOM contained more allochthonous, humic-like components with a higher aromaticity and lower URI. Significant differences in molecular composition and size spectra between FNOM and model/reference NOMs suggest that environmental role of NOMs should be evaluated at the levels of molecular weight and functionalities under the same experimental conditions and that HA and SNOM may not represent bulk NOM in the environment. This study provides new information about similarities and differences in DOM size-spectra and chemical properties between reference NOMs and in-situ NOM and highlights the need to better understand the heterogenous roles of NOMs in regulating the toxicity/bioavailability and environmental fate of pollutants in aquatic environments.

Modelling the bioconcentration of Zn from commercial sunscreens in the marine bivalve Ruditapes philippinarum

Sunscreens contain ZnO particles used as a UV filter cause adverse effects in the marine environment through the release of this metal into seawater and its bioaccumulation in organisms. A mathematical model using sunscreen colloidal residues, seawater and R. philippinarum clams as differentiated compartments, is proposed in order to interpret both the kinetic pattern and the bioaccumulation of Zn in clams. Two kinetic laboratory experiments were conducted, both with and without clams exposed to sunscreen concentrations from 0 to 200 mg L^-1. Both the lowest value of uptake rate coefficient obtained when 5 mg L^-1 of sunscreen is added (0.00688 L g^-1 d^-1) and the highest obtained at sunscreen addition of 100 mg L^-1 (0.0670 L g^-1 d^-1), predict a lower bioavailability of Zn in a complex medium such as the seawater-sunscreen mixtures, in comparison to those studied in the literature. The efflux rate coefficient from clams to seawater increased from 0 to 0.162 d^-1 with the sunscreen concentrations. The estimated value of the inlet rate coefficient at all studied concentrations indicates that there is a negligible colloidal Zn uptake rate by clams, probably due to the great stability of the organic colloidal residue. An equilibrium shift to higher values of Zn in water is predicted due to the bioconcentration of Zn in clams. The kinetic model proposed with no constant Zn (aq) concentrations may contribute to a more realistic prediction of the bioaccumulation of Zn from sunscreens in clams.

Challenges in quantifying and characterizing dissolved organic carbon: Sampling, isolation, storage, and analysis

Despite the advancements in analytical techniques, there are still great challenges and difficulties in accurately and effectively quantifying and characterizing dissolved organic carbon (DOC) in environmental samples. The objectives of this review paper are (a) to understand the roles and variability of DOC along the water continuum; (b) to identify the constraints, inconsistences, limitations, and artifacts in DOC characterization; and (c) to provide recommendations and remarks to improve the analytical accuracy. For the first objective, we summarize the four ecological and engineering roles of DOC along the water continuum from source water to municipal utility, including nutrients and energy sources, controlling the fates of micropollutants, buffering capacity, and treatability and precursors of disinfection byproducts. We also discuss three major challenges in DOC analysis, including spatial and temporal variations, degradability and stability, and unknown structures and formulas. For the second objective, we review the procedures and steps in DOC analysis, including sampling in diverse environmental matrices, isolation of DOC fraction, storage and preservation techniques, and analyses on bulk chemical characteristics. We list and discuss the available options and evaluate the advantages and disadvantages of each choice. Last, we provide recommendations and remarks for each stage: sampling, isolation, storage, and analysis.

Differences in the spectroscopic characteristics of wetland dissolved organic matter binding with Fe3+, Cu2+, Cd2+, Cr3+ and Zn2

Understanding of the binding characteristics of wetland dissolved organic matter (DOM) and different metals is important for the quantitative assessment of the environmental behavior of metals in wetlands. In this study, different types of spectroscopy including ultraviolet-visible absorption, Fourier transform infrared, and fluorescence spectroscopy was used to investigate the binding characteristics of Fe³⁺, Cu²⁺, Cr³⁺, Cd²⁺, and Zn²⁺ with DOM from wetland water. Differential absorption spectra identified binding sites for these five metals in this wetland DOM at 210 nm, 280 nm, 335 nm, and > 400 nm regions. The low binding capacity of DOM in this wetland with Cd and Zn indicated that the toxicity and environmental effects of these metals in this wetland warrant further study. The calculated △EEM combined with fluorescence regional integration (FRI) analysis clearly revealed that Fe and Cu preferred to bind with humic-like DOM while Cd and Zn preferred to bind with protein-like DOM in this wetland. △EEM successfully demonstrated the characteristics of DOM complexing with different metals and could be a compelling tool in evaluating metal-DOM interactions. In addition, 2D-FTIR-COS identified the binding sites and the dynamic processes of binding at the functional group level. Metals preferentially bind with the CO, CO functional group, and then binds to the OH functional group. This study revealed that different DOM components will facilitate the migration of different metals in the environment and provided new slights into an improved understanding of migration and transformation of metals in aquatic environments.

Mitigative effects of natural and model dissolved organic matter with different functionalities on the toxicity of methylmercury in embryonic zebrafish

Dissolved organic matter (DOM) occurs ubiquitously in aquatic environments and plays an intrinsic role in altering the chemical speciation and toxicity of methylmercury (MeHg). However, interactions between MeHg and natural DOM remain poorly understood, especially at the functional group level. We report here the mitigative effects of three natural organic matter (NOM) and five model-DOM under different concentrations (0, 1, 3, 10, 30 and 100 mg-C/L) on the toxicity of MeHg in embryonic zebrafish (<4 h post-fertilization, hpf). NOM are those from the Mississippi River, Yukon River, and Suwannee River, while model-DOM include those containing thiosalicylic acid, L-glutathione, dextran, alginic acid, and humic acid. We selected a MeHg concentration (100 n-mol/L) that reduces the survival rate of embryos at 24 hpf by 18% and increases malformations at 72 and 96 hpf. In the presence of DOM, however, the malformation rates induced by MeHg can be mitigated to a different extent depending on DOM concentrations, specific functional groups, and/or specific components. Model DOM with aromatic thiols was the most effective at mitigating the effects of MeHg, followed by L-glutathione, carbohydrates, and humic acid. NOM also mitigated the toxicity of MeHg dependent on their composition and/or effective DOM components as characterized by fluorescence excitation-emission matrix techniques. Specifically, humic-like DOM components are more effective in reducing the MeHg toxicity in the embryonic zebrafish compared to protein-like components. Further studies are needed to elucidate the interactions between DOM and MeHg and the mitigative mechanisms at the molecular level.

Artificial humification of lignin architecture: Top-down and bottom-up approaches

Humic substances readily identifiable in the environment are involved in several biotic and abiotic reactions affecting carbon turnover, soil fertility, plant nutrition and stimulation, xenobiotic transformation and microbial respiration. Inspired by natural roles of humic substances, several applications of these substances, including crop stimulants, redox mediators, anti-oxidants, human medicines, environmental remediation and fish feeding, have been developed. The annual market for humic substances has grown rapidly for these reasons and due to eco-conscious features, but there is a limited supply of natural coal-related resources such as lignite and leonardite from which humic substances are extracted in bulk. The structural similarity between humic substances and lignin suggests that lignocellulosic refinery resulting in lignin residues as a by-product could be a potential candidate for a bulk source of humic-like substances, but structural differences between the two polymeric materials indicate that additional transformation procedures allowing lignin architecture to fully mimic commercial humic substances are required. In this review, we introduce the emerging concept of artificial humification of lignin-related materials as a promising strategy for lignin valorization. First, the core structural features of humic substances and the relationship between these features and the physicochemical properties, natural functions and versatile applications of the substances are described. In particular, the mechanism by which humic substances stimulate the growth of plants and hence can improve crop productivity is highlighted. Second, top-down and bottom-up transformation pathways for scalable humification of small lignin-derived phenols, technical lignins and lignin-containing plant residues are described in detail. Finally, future directions are suggested for research and development of artificial lignin humification to achieve alternative ways of producing customized analogues of humic substances.

Dissolved organic matter binding with Pb(II) as characterized by differential spectra and 2D UV-FTIR heterospectral correlation analysis

Dissolved organic matter (DOM) in aquatic environment significantly influences the behavior and fate of heavy metals via binding, complexation and thus changes the metal speciation; however detailed interfacial processes and mechanisms are still unclear. Here, differential absorbance and fluorescence spectra and two dimensional UV-FTIR heterospectral correlation analysis were applied to probe into the Pb(II)-DOM interaction at a wide range of pH and ionic strength (IS). The absorbance of DOM molecules under all conditions increased with metal addition, while the different extents of absorbance variations along the wavelength range in the differential zero-order and log-transformed absorbance spectra indicated the site heterogeneity within the DOM pool for metal binding. Spectral parameters, namely differential fluorescent components 1 and 2 (DFC1 and DFC2) and differential slopes of log-transformed absorbance in the range of wavelength 350-400 nm (DSlope_350-400) were found to be highly correlated with the total amounts of DOM-bound Pb(II) predicted by the NICA-Donnan model, while the differential absorbance spectra at 235 nm (DA₂₃₅) was related to the extent of Pb(II) bound by carboxylic groups. Thus, these parameters are an indicator or proxy for the in situ Pb(II)-DOM interaction extent. Aryl C-H gave the fastest response to Pb(II) binding at lower pH and IS (e.g., pH 4.7 and IS = 0.01 M), followed by carboxyl C=O and polysaccharide C-OH and then chromophoric groups at 265 nm (CDOM₂₆₅). However, the CDOM₂₆₅ bound to Pb(II) prior to aryl C-H and polysaccharide C-OH groups at higher pH and IS (6.0 and 0.1 M, respectively), showing that the binding sequences were highly dependent on solution chemistry. Differential spectra combined with two dimensional UV-FTIR heterospectral correlation analysis can be used as a promising approach to elucidate metal-DOM interaction processes, including site heterogeneity, binding sensitivity and sequence at the functional group level.

Trace metals in oysters: molecular and cellular mechanisms and ecotoxicological impacts

Oysters are important benthic bivalves in coastal and estuarine environments. They are widely farmed due to their rapid growth and taste; they are also widely applied in environmental monitoring of coastal pollution due to their accumulation of contaminants. Most importantly, oysters are among the few marine organisms that are considered to be hyper-accumulators of many toxic metals, such as cadmium, copper and zinc. As such, there is a tremendous call to study the interactions between metals and oysters, especially due to the increasing metal pollution in many coastal and estuarine waters. Over the past decades, many studies have focused on metal accumulation in oysters as well as the ecotoxicological effects of metals on oysters. In this review, we summarize the recent progress in our understanding of the molecular and cellular mechanisms of metal accumulation, sequestration and toxicity in oysters. Applications of modern technologies such as omics and nanoscale imaging have added significantly to our knowledge of metal biology in oysters. Variations between different metals also demonstrate the diversity of the interactions between oysters and metals. Despite this recent progress, however, there is a need for further study of the molecular mechanisms of metal uptake and toxicity as well as the joint effects of metal mixtures on oyster populations. Oysters have higher numbers of stress responsive genes than most animals, which may have been induced by gene duplication during the evolution of their intertidal environmental adaptations. The divergent expression of stress responsive genes may explain the different tolerances for metals among different species. These fundamental studies may eventually provide promising solutions for reducing toxic metal concentrations in oysters for safe consumption by humans. To conclude, the complexity of life history and metal chemistry of oysters coupled with emerging pollution and application of modern techniques represents an important and exciting research area in modern ecotoxicology.

Element uptake and physiological responses of Lactuca sativa upon co-exposures to tourmaline and dissolved humic acids

Element migration and physiological response in Lactuca sativa upon co-exposure to tourmaline (T) and dissolved humic acids (DHAs) were investigated. Different fractions of DHA1 and DHA4 and three different doses of T were introduced into Hoagland's solution. The results indicated that T enhanced the contents of elements such as N and C, Si and Al in the roots and shoots. The correlation between TF values of Si and Al (R2 = 0.7387) was higher than that of Si and Mn (R2 = 0.4961) without the presence of DHAs. However, both DHA1 and DHA4 increased the correlation between Si and Mn, but decreased the one between Si and Al. CAT activities in T treatments were positively correlated to the contents of N and Al in the shoots, whose R 2 was 0.9994 and 0.9897, respectively. In the co-exposure of DHAs and tourmaline, DHA4 exhibited more impacts on element uptake, CAT activities, as well as ABA contents in comparison with the presence of DHA1, regardless of the T exposure doses. These results suggested that DHAs have effects on mineral element behaviors and physiological response in Lactuca sativa upon exposure to tourmaline for the first time, which had great use in guiding soil remediation.

Lessons learned from studies with the freshwater mussel Dreissena polymorpha exposed to platinum, palladium and rhodium

The platinum group elements (PGE) platinum, palladium and rhodium gain increasing (eco-)toxicological interest due to their cumulative introduction into ecosystems. So far, most PGE exposure studies investigating biological availability, uptake and bioaccumulation of PGE as well as their effects on different toxicological endpoints were performed under non-standardized conditions which occasionally make an interpretation and comparison of the results difficult. Here we compare the results of different PGE exposure studies with zebra mussels (Dreissena polymorpha) showing influences due to the PGE source, the exposure medium, the exposure concentration and period as well as the test system. Problems associated to the performance and evaluation of these studies were identified and recommendations as well as needs for future studies are given. As nominal exposure concentrations often did not reflect real exposure conditions the reference for exposure concentration has to be chosen with caution, i.e. nominal versus quantified aqueous concentrations. The determination of bioaccumulation factors can be problematic when PGE concentrations in the exposure medium and in the test organism did not reach steady state even after several weeks of exposure. For future studies it would be advantageous to regularly correlating PGE bioaccumulation and biomarker responses to increase the knowledge on potential adverse effects of these metals, preferably using environmentally relevant PGE concentration scenarios. Many aspects discussed in the present study for zebra mussels and PGE can be transferred to other aquatic animals and other metals, respectively.

Composition of dissolved organic matter (DOM) from periodically submerged soils in the Three Gorges Reservoir areas as determined by elemental and optical analysis, infrared spectroscopy, pyrolysis-GC-MS and thermally assisted hydrolysis and methylation

Soil-derived dissolved organic matter (DOM) has a major influence in biogeochemical processes related to contaminant dynamics and greenhouse gas emissions, due to its reactivity and its bridging role between the soil and aquatic systems. Within the Three Gorges Reservoir (TGR, China) area, an extensive water-fluctuation zone periodically submerges the surrounding soils. Here we report a characterization study of soil-derived DOM across the TGR areas, using elemental and optical analysis, infrared spectroscopy (FTIR), pyrolysis-GC-MS (Py-GC-MS) and thermally assisted hydrolysis and methylation (THM-GC-MS). The results showed that the soil DOM from the TGR area is a mixture of "allochthonous" (i.e., plant-derived/terrigenous) and "autochthonous" (i.e., microbial) origins. The terrigenous DOM is composed primarily of phenolic and aliphatic structures from lignin and aliphatic biopolymers (i.e. cutin, suberin), respectively. Multivariate statistics differentiated between two fractions of the microbial DOM, i.e. chitin-derived, perhaps from fungi and arthropods in soil, and protein-derived, partially sourced from algal or aquatic organisms. Molecular proxies of source and degradation state were in good agreement with optical parameters such as SUVA₂₅₄, the fluorescence index (FI) and the humification index (HIX). The combined use of elemental analysis, fluorescence spectroscopy, and Py-GC-MS provides rigorous and detailed DOM characterization, whereas THM-GC-MS is useful for more precise but qualitative identification of the different phenolic (cinnamyl, p-hydroxyphenyl, guaiacyl, syringyl and tannin-derived) and aliphatic materials. With the multi-methodological approach used in this study, FTIR was the least informative, in part, because of the interference of inorganic matter in the soil DOM samples. The soil DOM from the TGR's water fluctuation zone exhibited considerable compositional diversity, mainly related to the balance between DOM source (microbial- or plant-derived), local vegetation and anthropogenic activities (e.g., agriculture). Finally, the relationship between DOM composition and its potential reactivity with substances of environmental concerns in the TGR area are also discussed.

Combined effects of dissolved humic acids and tourmaline on the accumulation of 2, 2', 4, 4', 5, 5'- hexabrominated diphenyl ether (BDE-153) in Lactuca sativa

In order to investigate the effects of dissolved humic acid (DHA) and tourmaline on uptake of 2, 2', 4, 4', 5, 5'- hexabrominated diphenyl ether (BDE-153) by Lactuca sativa, different fractions of DHA, including DHA₁ and DHA₄, as well as different doses of tourmaline were introduced into BDE-153 contaminated solutions for plant growth. The levels of BDE-153 in L. sativa tissues were positively correlated with the Fe levels (R² = 0.9264) in seedings of the treatments with different doses of tourmaline. However, when adding DHA₁ and DHA₄ into the system, the correlation coefficients (R²) decreased to 0.6976 and 0.5451 from 0.9264, respectively. In contrast with the Fe contents, the presence of DHAs didn't affect the R² between the levels of BDE-153 and the lipid contents in plant tissues. Our results indicated that both DHA₁ and DHA₄ could severely alter the BDE-153 uptake by L. sativa through reducing the Fe uptake instead of the lipid contents. Additionally, DHA₄ exhibited much stronger abilities to alter the BDE-153 accumulation than DHA₁. Transmission electron microscopy (TEM) observations indicated that either DHA₁ or tourmaline or co-treatment with DHA and tourmaline had no negative impact on L. sativa at the cellular level. The present study provides important information for the impacts of different fractions of DHA extracted from soil on the BDE-153 migration in plant systems. Moreover, we elucidated the importance of the iron in tourmaline for migration of the polybrominated diphenyl ethers (PBDEs) in plant systems.

Time-dependent uptake and toxicity of nickel to Enchytraeus crypticus in the presence of humic acid and fulvic acid

The present study aimed to investigate the influence of different fractions of dissolved organic carbon (DOC) on the uptake and toxicity of nickel (Ni) in the soil invertebrate Enchytraeus crypticus after different exposure times. The addition of DOC as humic acid or fulvic acid significantly reduced Ni uptake by E. crypticus in the soil-solution test system. Median lethal effect concentrations were calculated based on total dissolved Ni concentrations (LC50[Ni]), free Ni ion activity (LC50{Ni²⁺ }), and Ni body concentrations (LC50_Body-Ni ). The LC50[Ni] values increased with increasing DOC levels and decreased with exposure time (4, 7, and 10 d). Humic acid exerted a greater protective effect on Ni toxicity than fulvic acid, but the protective effects decreased with prolonged exposure time. The LC50{Ni²⁺ } values also decreased with exposure time but were almost constant with variation in DOC levels, indicating that the protective effect of DOC is mainly through complexation with free Ni ions to reduce Ni bioavailability. The LC50_Body-Ni value was independent of DOC concentration and exposure time, with an estimated overall value of 22.1 µg/g dry weight. The present study shows that body concentration could serve as an effective indicator for predicting Ni toxicity with variations in the exposure environment (e.g., DOC) and exposure time. Environ Toxicol Chem 2017;36:3019-3027. © 2017 SETAC.

Salinity, dissolved organic carbon, and interpopulation variability hardly influence the accumulation and effect of copper in Mytilus edulis

To improve the ecological relevance of environmental risk assessment, an improved understanding is needed of 1) the influence of environmental conditions on the toxicity of pollutants, and 2) the effect of these factors in combination with possible interpopulation variability. The influences of salinity and dissolved organic carbon (DOC) on the accumulation and effect of copper (Cu) to settled mussels were investigated with mussels from a North Sea and a Baltic Sea population. We found that both populations were equally Cu-sensitive, even though the Baltic Sea population lives in suboptimal conditions. Baltic Sea mussels, however, accumulated more Cu. This suggests that these populations may have different ways of coping with excess Cu. The influence of salinity on Cu toxicity to settled mussels was limited for both populations. An increase in DOC did not decrease the Cu accumulation or effect in either population. This suggests that DOC-Cu complexes are bioavailable for settled mussels. These findings are in contrast with previous research which indicated that DOC decreased the toxicity and accumulation of Cu in the D-larvae life stage. As a consequence, the mussel larval stage is not the most Cu-sensitive life stage at high DOC concentrations. Furthermore, a DOC correction factor for Cu toxicity cannot be used for settled mussels. This should be accounted for in future marine Cu environmental risk assessment. Environ Toxicol Chem 2017;36:2074-2082. © 2017 SETAC.

Impacts of rapeseed dregs on Cd availability in contaminated acid soil and Cd translocation and accumulation in rice plants

The objective of the present study was to investigate the effects of rapeseed dregs (RSD, a commonly organic fertilizer in rural China) at application rates of 0, 0.75, 1.5, and 3.0 % on Cd availability in soil and its accumulation in rice plants (Oryza sativa L., Xiangwanxian 12^#, and Weiyou 46^#) by means of a pot experiment. The results showed that application of RSD resulted in a sharp decrease in the soil TCLP-extractable Cd content. However, the soil TCLP-extractable Cd content in amended soil gradually increased during the rice growing period. Application of RSD significantly increased Cd transport from root to shoot and the amount of Cd accumulated in the aerial part. RSD was an effective organic additive for increasing rice grain yield, but total Cd content in rice grain was also increased. At an application rate of 1.5-3.0 % RSD, the total Cd content in Weiyou 46^# brown rice was 0.27-0.31 mg kg^-1, which exceeded the standard safe limit (0.2 mg kg^-1) and was also higher than that of Xiangwanxian 12^# (0.04-0.14 mg kg^-1). Therefore, Weiyou 46^# had a higher dietary risk than Xiangwanxian 12^# with RSD application. We do not recommend planting Weiyou 46^# and applying more than 0.75 % RSD in Cd-contaminated paddy fields.

Dynamic modeling of copper bioaccumulation by Mytilus edulis in the presence of humic acid aggregates

Copper (Cu) complexation by humic acids (HA) is expected to decrease Cu bioavailability for aquatic organisms as predicted by metal bioavailability models, such as the biotic ligand model (BLM). This has been confirmed for non-feeding organisms such as marine invertebrate embryos or microalgae, but for filter-feeding organisms such as the mussel Mytilus edulis, Cu bioaccumulation was higher in the presence of HA, suggesting that part of the Cu-HA complexes were available for uptake. This study shows the dynamic modeling of Cu accumulation kinetics in the gills and rest of the soft-body of M. edulis in the absence and presence of HA. Assuming that truly dissolved Cu is taken in the body via the gills following BLM premises, and including uptake of Cu-HA aggregates via the gut into the rest compartment, this two-compartmental model could successfully explain the observed bioaccumulation data. This modeling approach gives strong evidence to the hypothesis that Cu-HA aggregates can be ingested by mussels leading to Cu absorption in the digestive system.

Reduced cadmium accumulation and toxicity in Daphnia magna under carbon nanotube exposure

With increasing application and commercial production, carbon nanotubes (CNTs) will inevitably be released into aquatic environments and affect the transport and toxicity of toxic metals in ecosystems. The present study examined how CNTs affected the biokinetics and toxicity of a toxic metal, cadmium (Cd), in the freshwater zooplankton Daphnia magna. The authors quantified the dissolved uptake and the 50% lethal concentration (LC50, 48 h and 72 h) of Cd in daphnids in the presence of functionalized multiwalled nanotubes (F-CNTs) with different lengths (10-30 µm vs 0.5-2 µm) and concentrations (4 mg/L and 8 mg/L). Compared with the control treatment without CNTs, both CNTs slowed down the accumulation rate of Cd in D. magna over 8 h of exposure and further reduced the accumulation thereafter. Mechanisms for the reduced Cd uptake were mainly related to the influences of CNTs on the physiological activity of daphnids. The LC50 of D. magna in the presence of Cd and shorter CNTs was almost the same as that of the control group without CNTs. However, the LC50 of the groups with normal CNTs was significantly higher than that of the control group (i.e., F-CNTs decreased Cd toxicity significantly). Meanwhile, CNTs also decreased the tolerance of D. magna to Cd. The present study suggests that different physical properties of CNTs, such as length, need to be considered in the environmental risk assessment of CNTs.

Relating metal exposure and chemical speciation to trace metal accumulation in aquatic insects under natural field conditions

The present study investigated to what extent measured dissolved metal concentrations, WHAM-predicted free metal ion activity and modulating water chemistry factors can predict Ni, Cu, Zn, Cd and Pb accumulation in various aquatic insects under natural field conditions. Total dissolved concentrations and accumulated metal levels in four taxa (Leuctra sp., Simuliidae, Rhithrogena sp. and Perlodidae) were determined and free metal ion activities were calculated in 36 headwater streams located in the north-west part of England. Observed invertebrate body burdens were strongly related to free metal ion activities and competition among cations for uptake in the biota. Taking into account competitive effects generally provided better fits than considering uptake as a function of total dissolved metal levels or the free ion alone. Due to the critical importance and large range in pH (4.09 to 8.33), the H(+) ion activity was the most dominant factor influencing metal accumulation. Adding the influence of Na(+) on Cu(2+) accumulation improved the model goodness of fit for both Rhithrogena sp. and Perlodidae. Effects of hardness ions on metal accumulation were limited, indicating the minor influence of Ca(2+) and Mg(2+) on metal accumulation in soft-water streams (0.01 to 0.94 mM Ca; 0.02 to 0.39 mM Mg). DOC levels (ranging from 0.6 to 8.9 mg L(-1)) significantly affected Cu body burdens, however not the accumulation of the other metals. Our results suggest that 1) uptake and accumulation of free metal ions are most dominantly influenced by competition of free H(+) ions in low-hardness headwaters and 2) invertebrate body burdens in natural waters can be predicted based on the free metal ion activity using speciation modelling and effects of H(+) competition.

Influence of eutrophication on metal bioaccumulation and oral bioavailability in oysters, Crassostrea angulata

Oysters (Crassostrea angulata) are often exposed to eutrophication. However, how these exposures influence metal bioaccumulation and oral bioavailability (OBA) in oysters is unknown. After a four month field experimental cultivation, bioaccumulation factors (BAF) of metals (Fe, Cu, As, Cd, and Pb) from seawater to oysters and metal oral bioavailability in oysters by bionic gastrointestinal tract were determined. A positive effect of macronutrient (nitrate N and total P) concentration in seawater on BAF of Cd in oysters was observed, but such an effect was not significant for Fe, Cu, Pb, and As. Only OBA of As was significantly positively correlated to N and P contents. For Fe, OBA was negatively correlated with N. The regular variation of the OBA of Fe and As may be due to the effect of eutrophication on the synthesis of metal granules and heat-stable protein in oysters, respectively.

Benthic and pelagic pathways of methylmercury bioaccumulation in estuarine food webs of the northeast United States

Methylmercury (MeHg) is a contaminant of global concern that bioaccumulates and bioamagnifies in marine food webs. Lower trophic level fauna are important conduits of MeHg from sediment and water to estuarine and coastal fish harvested for human consumption. However, the sources and pathways of MeHg to these coastal fisheries are poorly known particularly the potential for transfer of MeHg from the sediment to biotic compartments. Across a broad gradient of human land impacts, we analyzed MeHg concentrations in food webs at ten estuarine sites in the Northeast US (from the Hackensack Meadowlands, NJ to the Gulf of Maine). MeHg concentrations in water column particulate material, but not in sediments, were predictive of MeHg concentrations in fish (killifish and Atlantic silversides). Moreover, MeHg concentrations were higher in pelagic fauna than in benthic-feeding fauna suggesting that MeHg delivery to the water column from methylation sites from within or outside of the estuary may be an important driver of MeHg bioaccumulation in estuarine pelagic food webs. In contrast, bulk sediment MeHg concentrations were only predictive of concentrations of MeHg in the infaunal worms. Our results across a broad gradient of sites demonstrate that the pathways of MeHg to lower trophic level estuarine organisms are distinctly different between benthic deposit feeders and forage fish. Thus, even in systems with contaminated sediments, transfer of MeHg into estuarine food webs maybe driven more by the efficiency of processes that determine MeHg input and bioavailability in the water column.

Modeling metal bioaccumulation and tissue distribution in killifish (Fundulus heteroclitus) in three contaminated estuaries

The present study experimentally assessed the uptake, loss, and resulting tissue distribution of As(V), Cd, Cr(III), Hg(II), and methylmercury (MeHg) in killifish (Fundulus heteroclitus) following aqueous exposure in water collected from 3 contaminated field sites-Baltimore Harbor and Elizabeth River (Chesapeake Bay), and Mare Island (San Francisco Bay)-using a radiotracer technique. Uptake rate constants (L g-1 d-1) were highest for MeHg (0.370-0.781) and lowest for As (0.00028-0.00065). Loss rate constants (d-1) were highest for As (0.046-0.096) and lowest for MeHg (0.006-0.009). Tissue distribution data showed that MeHg was redistributed around the body throughout the 9-d depuration period, and drinking may be an uptake mechanism for Cd from the aqueous phase in higher-salinity water. The kinetic parameters calculated in the present study were entered into a bioaccumulation model to calculate the predicted body burden of each metal at steady state and the percentage body burden attributable to dietary exposure on a site-specific basis. Calculated body burdens varied between field sites for all metals except Cr. The predicted values for Cd, Hg(II), and MeHg matched independent field data from contaminated estuaries, indicating that the model can account for the major processes governing metal concentration in killifish. The diet accounted for >97% of the body burden of Cd and MeHg and was the predominant exposure route for As and Cr.

Mercury contaminated sediment sites-an evaluation of remedial options

Mercury (Hg) is a naturally-occurring element that is ubiquitous in the aquatic environment. Though efforts have been made in recent years to decrease Hg emissions, historically-emitted Hg can be retained in the sediments of aquatic bodies where they may be slowly converted to methylmercury (MeHg). Consequently, Hg in historically-contaminated sediments can result in high levels of significant exposure for aquatic species, wildlife and human populations consuming fish. Even if source control of contaminated wastewater is achievable, it may take a very long time, perhaps decades, for Hg-contaminated aquatic systems to reach relatively safe Hg levels in both water and surface sediment naturally. It may take even longer if Hg is present at higher concentration levels in deep sediment. Hg contaminated sediment results from previous releases or ongoing contributions from sources that are difficult to identify. Due to human activities or physical, chemical, or biological processes (e.g. hydrodynamic flows, bioturbation, molecular diffusion, and chemical transformation), the buried Hg can be remobilized into the overlying water. Hg speciation in the water column and sediments critically affect the reactivity (i.e. conversion of inorganic Hg(II) to MeHg), transport, and its exposure to living organisms. Also, geochemical conditions affect the activity of methylating bacteria and its availability for methylation. This review paper discusses remedial considerations (e.g. key chemical factors in fate and transport of Hg, source characterization and control, environmental management procedures, remediation options, modeling tools) and includes practical case studies for cleaning up Hg-contaminated sediment sites.

Influences of ambient carbon nanotubes on toxic metals accumulation in Daphnia magna

With the rapid increase of carbon nanotube (CNT) applications, there are considerable concerns of their inevitable releases into the aquatic environments. CNTs may interact with and further influence the fate and transport of other pollutants such as toxic metals. In the present study, non-covalent and nontoxic dispersant polyvinyl pyrrolidone (PVP) was used to provide a relatively stable test solution for CNTs. The dissolved uptake rate constant (ku) and the dietary assimilation efficiency (AE) of cadmium (Cd) and zinc (Zn) were then quantified in a freshwater zooplankton Daphnia magna in the presence of different CNTs (without functionalized - single-walled nanotubes-SWNTs, multi-walled nanotubes-MWNTs, and with functionalized - F-SWNTs, F-MWNTs, containing oxygen functional groups at the defect sites of CNTs) concentrations. We demonstrated that different CNTs exposures led to distinctive metal accumulation patterns. Non-functionalized CNTs significantly decreased the metal uptake rate from the dissolved phase, possibly because of their effects on the physiological activity of animals. In contrast, the F-CNTs (F-SWNTs and F-MWNTs) adsorbed the metals and increased the metal accumulation in daphnids in a concentration-dependent manner, due to the ingestion of F-CNTs associated metals. The AEs of metals in D. magna were elevated by CNTs physical blocking of the animal guts. Our present study showed that CNTs could serve as a new pathway for metal accumulation. This raised a new environmental problem of CNTs since they may induce the accumulation of toxic metals from the dietary exposure.

Temporal and spatial variation in Hg accumulation in zebra mussels (Dreissena polymorpha): possible influences of DOC and diet

Zebra mussels (Dreissena polymorpha) are filter feeders located near the base of the foodweb and these animals are able to utilize a variety of carbon sources that may also vary seasonally. We conducted both a spatial and a temporal study in order to test the hypotheses: (1) dissolved organic carbon (DOC) concentrations influence Hg accumulation in zebra mussels sampled from a series of lakes and (2) seasonal variations in diet influence Hg accumulation. In the spatial study, we found a significant negative relationship between Hg concentrations and DOC concentrations, suggesting an influence of DOC on Hg bioaccumulation. In the temporal study, we used stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C) as ecological tools to provide a temporally integrated description of the feeding ecology of zebra mussels. Both δ(15)N and δ(13)C varied seasonally in a similar manner: more depleted values occurred in the summer and more enriched values occurred in the fall. Mercury concentrations also varied significantly over the year, with highest concentrations occurring in the summer, followed by a progressive decrease in concentrations into the fall. The C/N ratio of zebra mussels also varied significantly over the year with the lowest values occurring mid-summer and then values increased in the fall and winter, suggesting that there was significant variation in lipid stores. These results indicate that in addition to any effect of seasonal dietary changes, seasonal variation in energy stores also appeared to be related to Hg levels in the zebra mussels. Collectively results from this study suggest that DOC concentrations, seasonal variation in diet and seasonal depletion of energy stores are all important variables to consider when understanding Hg accumulation in zebra mussels.

Influence of humic acid on the uptake of aqueous metals by the killifish Fundulus heteroclitus

The role of humic acids, over a concentration range of 0 to 20 mg L(-1) , was investigated in the uptake of three metals (Cd, Cr, and Hg-as both inorganic Hg [Hg(II)] and methylmercury [MeHg]) and a metalloid (As) from the aqueous phase by the killifish (Fundulus heteroclitus). Cadmium uptake showed no relationship with humic acid concentration, whereas Cr, Hg(II), and MeHg uptake showed an inverse relationship, and As uptake increased with increasing humic acid concentration. Concentration factors were >1 for Cd, Hg(II), and MeHg at all humic acid concentrations, indicating killifish were more enriched in the metal than the experimental media, whereas As and Cr generally had concentration factors <1 at the end of a 72-h exposure. The uptake of As and Cr reached steady state within the 72-h exposure, whereas uptake of Cd, Hg(II), and MeHg did not. Uptake rate constants (k(u) s; ml g(-1)  d(-1) ) were highest for MeHg (91-3,936), followed by Hg(II), Cd, and Cr, and lowest for As (0.17-0.29). Dissection data revealed that the gills generally had the highest concentration of all metals under all humic acid treatments. The present study concludes that changes in humic acid concentration can influence the accumulation of aqueous metals in killifish and should be considered when modeling metal bioaccumulation.

Interactions of Pb and Cd mixtures in the presence or absence of natural organic matter with the fish gill

Metal gill binding and toxicity can be modeled using the concentration addition model, in which the toxic unit (TU) concept is used to determine if constituent metals are acting in a strictly additive, less than, or greater than additive fashion. To test this hypothesis, rainbow trout (Oncorhynchus mykiss) were exposed to a matrix of Pb plus Cd mixtures (nominal concentrations=0.75, 1.5, 2.25, 3.0 μmol L(-1)), in the presence or absence of mainly terrigenous (allochthonous; 10 mg CL(-1)) natural organic matter (NOM), and metal-gill binding, and toxicity was quantified. Based on its greater affinity for metal-gill binding sites, Cd-gill binding was expected to exceed Pb-gill binding during metal mixture exposure, but this only occurred at the lowest metal concentrations (0.75 μmol L(-1)); at higher concentrations Pb-gill binding was greater than Cd-gill binding. These unexpected observations were because Pb and Cd likely bind to different populations of high affinity, low capacity binding sites on the gill, which was borne out in subsequent attempts to mathematically model metal-gill interactions during metal-mixture exposure. The presence of an additional low affinity, high capacity population of Pb-gill binding sites also contributed to higher Pb-gill accumulation. Metal-gill interactions were complicated by NOM, which exacerbated toxicity during Cd-only exposure despite lowering Cd-gill accumulation. NOM also promoted Cd-gill binding in the presence of low-moderate concentrations of Pb (0.75 and 1.50 μmol L(-1)). We suggest that direct interactions of Cd-NOM complexes with the gill, and increases in Cd bioavailability due to Pb outcompeting Cd for NOM-metal binding sites due to its greater affinity for such ligands, accounted for greater Cd-gill binding and toxicity. We conclude that interactions of Pb and Cd with the gill cannot be predicted using the concentration addition model, and that NOM is not universally protective against metal-gill binding and toxicity when fish are exposed to metal mixtures.

Optimisation and application of the voltammetric technique for speciation of chromium in the Patos Lagoon Estuary--Brazil

The chemical speciation analysis of chromium in one of the most important South American Estuary was performed for the first time. Samples were collected in Patos Lagoon Estuary (Brazil) and were analysed by adsorptive cathodic stripping voltammetry, with the following analytical figures of merit: limit of detection, 0.1 nmol L(-1); precision RSD = 3%, n = 7; linearity, from limit of quantitation up to 20 nmol L(-1); and accuracy of 99.8%, expressed as recovery. No labile chromium forms were identified in samples, beside industries and a city were near the study area. It is pointed out a reverse correlation between total and non-active chromium and salinity, which could be explained by biogeochemical processes.

Copper uptake by the marine mussel Mytilus edulis in the presence of fulvic acids

Copper uptake and accumulation by the marine mussel Mytilus edulis were studied at different Cu concentrations in chemically defined artificial seawater in the presence and absence of fulvic acids. Both short-term uptake of Cu by excised mussel gills and Cu accumulation in whole mussels after 24 h of exposure decreased in the presence of fulvic acids compared with their absence at similar dissolved Cu concentrations. Calculations of Cu speciation based on previous measurements of labile Cu by anodic stripping voltammetry demonstrated that Cu uptake and accumulation depended on the concentration of labile Cu, in agreement with the free ion activity model. No evidence of a significant uptake of Cu-fulvic acid complexes was observed.

Combined effects of titanium dioxide and humic acid on the bioaccumulation of cadmium in Zebrafish

The combined effects of titanium dioxide (TiO2) nanoparticles and humic acid (HA) on the bioaccumulation of cadmium (Cd) in Zebrafish were investigated. Experimental data on the equilibrium Cd bioaccumulation suggest that only the dissolved Cd effectively contributed to Cd bioaccumulation in HA solutions whereas both the dissolved and TiO2 associated Cd were accumulated in TiO2 or the mixture of HA and TiO2 solutions, due likely to the additional intestine uptake of the TiO2-bound Cd. The equilibrium Cd bioaccumulation in the mixed system was comparable to that in the corresponding HA solutions, and significantly lower than that in the corresponding TiO2 solutions (n=3, p<0.05). The presence of either HA or TiO2 (5-20 mg L(-1)) in water slightly increased the uptake rate constants of Cd bioaccumulation whereas combining HA and TiO2 reduced the uptake rate constants.

Pb uptake by the marine mussel Mytilus sp. Interactions with dissolved organic matter

It is well known that dissolved organic matter binds metal ions and buffers them in natural waters. Although it is believed that a decrease in metal ion concentration should lead to a decrease in metal bioavailability, previous work has shown that Pb uptake by Mytilus edulis gills is greatly enhanced in the presence of humic acids. In the present work, the effect of more soluble organic matter (fulvic acids and DOM extracted from river) on Pb uptake by mussels and their gills is studied. Pb complexation by these organic substances was measured by anodic stripping voltammetry (ASV) and it is proven that Pb uptake by mussel gills in the presence of fulvic acids can be successfully predicted according to ASV-labile Pb concentrations. However, Pb uptake by whole mussels in the presence of river DOM is slightly higher than predicted on the basis of ASV measurements. The possible reasons leading to different effects of DOM on Pb uptake by mussels are discussed according to physicochemical properties of DOM.

Utility of tissue residues for predicting effects of metals on aquatic organisms

As part of a SETAC Pellston Workshop, we evaluated the potential use of metal tissue residues for predicting effects in aquatic organisms. This evaluation included consideration of different conceptual models and then development of several case studies on how tissue residues might be applied for metals, assessing the strengths and weaknesses of these different approaches. We further developed a new conceptual model in which metal tissue concentrations from metal-accumulating organisms (principally invertebrates) that are relatively insensitive to metal toxicity could be used as predictors of effects in metal-sensitive taxa that typically do not accumulate metals to a significant degree. Overall, we conclude that the use of tissue residue assessment for metals other than organometals has not led to the development of a generalized approach as in the case of organic substances. Species-specific and site-specific approaches have been developed for one or more metals (e.g., Ni). The use of gill tissue residues within the biotic ligand model is another successful application. Aquatic organisms contain a diverse array of homeostatic mechanisms that are both metal- and species-specific. As a result, use of whole-body measurements (and often specific organs) for metals does not lead to a defensible position regarding risk to the organism. Rather, we suggest that in the short term, with sufficient validation, species- and site-specific approaches for metals can be developed. In the longer term it may be possible to use metal-accumulating species to predict toxicity to metal-sensitive species with appropriate field validation.

Minerals as additives for decreasing the toxicity of Mediterranean contaminated dredged sediments

The management of dredged sediments is a priority issue in the Mediterranean sea where sediments are historically polluted. The aims of this study were to evaluate the toxicity of port sediment samples and the effect of three mineral additives (hematite, zerovalent iron (ZVI) and natural zeolite (NZ)) on sediment elutriate toxicity. Four sediments (A, B, C and D) were provided by port authorities after composting procedure; particle size, particulate organic carbon, metals and organic pollutants (TBT, PAHs, PCBs) were determined in whole sediments. Elutriates from these composted sediments were analyzed by determining toxicity level using oyster (Crassostrea gigas) larvae bioassay, metal and dissolved organic carbon concentrations. Toxicity, measured on undiluted elutriates (250 g/L), decreased as follows: A≥B>C∼D. The treatment of sediments with mineral additives (5%) revealed that hematite tends to decrease the elutriate toxicity in all samples, particularly in samples B and C. This effect may be related to metal concentration decrease in elutriates, in particular Cu and Zn, that have a significant toxic effect on oyster larvae. ZVI and NZ have a variable influence on elutriate toxicity. Results suggest that hematite may be a possible candidate for decreasing chemical concentration and improving the quality of elutriates. Hematite could be used for sediment stabilization prior to the deposit in a specific site or landfill.

Chemical contamination assessment of Gulf of Mexico oysters in response to hurricanes Katrina and Rita

Hurricane Katrina made landfall on August 29, 2005 and caused widespread devastation along the central Gulf Coast states. Less than a month later Hurricane Rita followed a similar track slightly west of Katrina's. A coordinated multi-agency response followed to collect water, sediment and tissue samples for a variety of chemical, biological and toxicological indicators. The National Oceanic and Atmospheric Administration's National Status and Trends Program (NS&T) participated in this effort by measuring chemical contamination in sediment and oyster tissue as part of the Mussel Watch Program, a long-term monitoring program to assess spatial and temporal trends in a wide range of coastal pollutants. This paper describes results for contaminants measured in oyster tissue collected between September 29 and October 10, 2005 and discusses the results in the context of Mussel Watch and its 20-year record of chemical contamination in the region and the nation. In general, levels of metals in oyster tissue were higher then pre- hurricane levels while organic contaminants were at or near record lows. No contaminant reported here exceeded the FDA action level for food safety.

Partition of six phthalic acid esters in soluble and solid residual fractions of wastewater sludges

Distributions of six priority controlled Phthalic acid esters (PAEs), including di-methyl phthalate (DMP), di-ethyl phthalate (DEP), di-butyl phthalate (DBP), benzyl butyl phthalate (BBP), di-(2-ethylhexyl) phthalate (DEHP), di-octyl phthalate (DOP) were studied based on soluble and insoluble fractions of sludge samples collected at four wastewater treatment plants in Shanghai, China. Three sludge samples comprised hydrophilic colloidal particles of high protein contents and low aromaticity. Meanwhile, these sludges contained DBP of 4.2 to approximately 5.7 mg kg(-1) dried solids (ds) and DEHP of 21.1 to approximately 55.6 mg kg(-1) ds, respectively. Another sludge sample comprised mainly hydrophobic colloidal particles of humic substances and high aromaticity. It contained DBP of 1.18 mg kg(-1) ds and DEHP of 2.89 mg kg(-1) ds, respectively. The most abundant components noted amongst the six studied PAEs were DBP and DEHP, which mostly associated with the insoluble fraction of sludge. Specifically, the DBP and DEHP in insoluble fraction (the solid residual phase) accounted for 89.8 to approximately 98.2% and 88.6 to approximately 99.6% of those in the whole sludge. The partition coefficients of DBP and DEHP for the soluble and insoluble fractions of sludge correlated with the suspension SUVA254, suggesting that interaction between pi-electrons of DBP or DEHP and those of organic particulates in suspension contributes most of the sorption processes.

Fluorescence characterization of cross flow ultrafiltration derived freshwater colloidal and dissolved organic matter

3-D fluorescence excitation-emission matrix (EEM) spectrophotometry was applied to investigate the fluorescence characterization of colloidal organic matter (COM) and truly dissolved organic matter (DOM) from an urban lake and a rural river fractionated by the cross flow ultrafiltration (CFUF) process with a 1kDa membrane. Relatively high tryptophan-like fluorescence intensity is found in the urban water, although the fluorescence of both water samples is mainly dominated by humic/fulvic-like fluorophores. During CFUF processing, the fluorescence intensities of humic/fulvic-like materials in the retentate increased rapidly, but a slight increase is also observed in the permeate fluorescence intensity. Very different ultrafiltration behaviour occurred with respect to the tryptophan-like fluorophore, where both permeate and retentate fluorescence intensities increase substantially at the beginning of the CFUF process, then tend to remain constant at high concentration factor (cf) values. Comparison with tryptophan standards demonstrates that freshwater tryptophan-like fluorescence is not dissolved and 'free', but is, in part, colloidal and related to the ultrafiltration behaviour of fulvic/humic-like matter. A good linear relationship between the retentate humic/fulvic-like fluorescence intensity and organic carbon concentration further reveals that fluorescent humic/fulvic-like substances are the dominant contributors to colloidal organic carbon, mainly in the colloidal fraction.

Impact of humic substances on the aqueous solubility, uptake and bioaccumulation of platinum, palladium and rhodium in exposure studies with Dreissena polymorpha

Zebra mussels (Dreissena polymorpha) were exposed to different types of water containing PGE salts (PtCl4, PdSO4, RhCl3) to investigate the influence of humic substances on the aqueous solubility, uptake and bioaccumulation of noble metals. The results showed a time dependent decrease of the aqueous PGE concentrations in tank water for all groups. This could mainly be related to non-biological processes. The aqueous solubility of Pd and Rh was higher in humic water compared with non-chlorinated tap water, whereas Pt showed opposing results. Highest metal uptake rates and highest bioaccumulation plateaus were found for Pd, followed by Pt and Rh. Pd uptake and bioaccumulation was significantly hampered by humic substances, whose presence appear to increase Pt uptake and bioaccumulation. No clear trend emerged for Rh. Differences in effects of humic matter among the PGE may be explained by formation of metal complexes with different fractions of humic substances.

Exposure of caged mussels to metals in a primary-treated municipal wastewater plume

The biological availability of metals in municipal wastewater effluents is strongly influenced by the physical and chemical conditions of both the effluent and the receiving water. Aquatic organisms are exposed to both dissolved and particulate (food ingestion) forms of these metals. In the present study, the distribution of metals in specific tissues was used to distinguish between exposure routes (i.e. dissolved vs. particulate phase) and to examine metal bioavailability in mussels exposed to municipal effluents. Caged Elliptio complanata mussels were deployed at sites located between 1.5 km upstream and 12 km downstream of a major effluent outfall in the St. Lawrence River. Metals in surface water samples were fractionated by filtration techniques to determine their dissolved, truly-dissolved (<10 kDa), total-particulate and acid-reactive-particulate forms. At the end of the exposure period (90 days), pooled mussel soft tissues (digestive gland, gills, gonad, foot and mantle) were analyzed for several metals. The results showed that gills and digestive gland were generally the most important target tissues for metal bioaccumulation, while gill/digestive gland metal ratios suggest that both exposure routes should be considered for mussels exposed to municipal effluents. We also found that Ag and Cd in the dispersion plume nearest the outfall, in contrast to other metals such as Cu and Zn, are more closely associated with colloids and were generally less bioavailable than at the reference site in the St. Lawrence River.

Copper uptake by Mytilus edulis in the presence of humic acids

The effects of humic acids (HA) on Cu uptake by the blue mussel (Mytilus edulis) were studied in chemically defined seawater. Short-term uptake by excised gills was studied and compared with whole-mussel Cu accumulation. Copper uptake in gills is not a saturable process within the time frame and concentration range tested, being a linear function of time (0-120 min) and Cu concentration (0-150 microg/L). For the whole mussel, Cu uptake is not linear with time (0-72 h) and Cu concentration (0-130 microg/L). The presence of HA (0-10 mg/L) clearly reduced Cu uptake by gills, but it did not have such an effect on whole-animal uptake. A simple complexation model obtained from voltammetric measurements was used to determine the noncomplexed (labile) fraction of Cu in the presence of HA to test the effect of speciation on Cu uptake. In most cases, Cu uptake by gills was better explained by labile Cu concentrations than by total Cu exposure concentrations, which is in agreement with the free-ion activity model. In whole-animal experiments, Cu uptake was not related to labile Cu concentrations in the presence of HA, indicating that Cu-HA complexes are at least partially available for uptake by the mussels.

Influences of dissolved and colloidal organic carbon on the uptake of Ag, Cd, and Cr by the marine mussel Perna viridis

The cross-flow ultrafiltration and radiotracer techniques were used to study the influences of natural dissolved organic carbon (DOC) and colloidal organic carbon (COC) on the bioavailability of Ag, Cd, and Cr to the green mussel Perna viridis. We examined the uptake of these metals by the mussels at different concentrations of DOC and COC from different origins (estuarine, coastal, and diatom decomposed). Using the DOC originating from the decomposed diatom (Thalassiosira pseudonana), we demonstrated that Cd and Cr uptake, quantified by the concentration factor (DCF), increased linearly with increasing DOC concentration. There was, however, no consistent influence of natural DOC concentration on the metal uptake when the DOC was obtained from different sources of seawater (coastal and estuarine). The influences of COC on metal bioavailability were metal-specific and dependent on the geochemical properties of colloids and colloid-metal complexation. Cd uptake rate was not influenced by the COC concentrations. Uptake of diatom-decomposed colloidal Cr was enhanced by 3.4x, whereas the uptake of diatom-decomposed colloidal Ag was decreased by 8.2x compared with the uptake of low molecular weight Cr and Ag (<1 kDa). The uptake of diatom-decomposed colloidal Cr and Ag was generally lower than the uptake of metals bound with the same type of colloids for 2 days. Further aging of the colloid-metal binding reduced metal bioavailability to the mussels. In the presence of different sizes of colloidal particles where there was no major binding of colloids with the metals, metal uptake by the mussels was not influenced by different COC concentrations. Overall, our study suggests that although metal dissociation from colloids may be an important step for the uptake of colloidal metals, other mechanisms such as pinocytosis and co-transport may also be involved in the uptake of these metals, especially in aquatic environments with high DOC and COC concentrations.

Increasing UV-B radiation at the earth's surface and potential effects on aqueous mercury cycling and toxicity

In the past two decades, a great deal of attention has been paid to the environmental fate of mercury (Hg), and this is exemplified by the growing number of international conferences devoted uniquely to Hg cycling and its impacts on ecosystem functions and life. This interest in the biogeochemistry of Hg has resulted in a significant improvement of our understanding of its impact on the environment and human health. However, both past and current research, have been primarily oriented toward the study of direct impact of anthropogenic activities on Hg cycling. Besides a few indirect effects such as the increase in Hg methylation observed in acid-rain impacted aquatic systems or the reported enhanced Hg bioaccumulation in newly flooded water reservoirs; changes in Hg transformations/fluxes that may be related to global change have received little attention. A case in point is the depletion of stratospheric ozone and the resulting increase in solar UV-radiation reaching the Earth. This review and critical discussion suggest that increasing UV-B radiation at earth's surface could have a significant and complex impact on Hg cycling including effects on Hg volatilization (photo-reduction), solubilization (photo-oxidation), methyl-Hg demethylation, and Hg methylation. Therefore, this paper is written to provoke discussions, and more importantly, to stimulate research on potential impacts of incoming solar UV-radiation on global Hg fluxes and any toxicity aspects of Hg that may become exacerbated by UV-radiation.

Effect of sub-lethal concentrations of aluminium on the filtration activity of the freshwater mussel Anodonta cygnea L. at neutral pH

Significant amounts of aluminium (Al) are commonly present in rivers and lakes, largely in particulate form in neutral waters. Freshwater bivalves, as filter feeders are therefore exposed to both particulate and dissolved metal and are potentially vulnerable to Al. The effect of Al on filtering behaviour of the freshwater mussel Anodonta cygnea L. was investigated during short (1 hour) and long-term (15 days) exposure to environmentally relevant concentrations (250 and 500 microg l(-1)) at neutral pH. Water flow through the outflow siphon was monitored as an indicator of pumping capacity. Short-term (1 hour) exposure to 500 microg l(-1) added Al produced an irreversible decrease in the duration of filtering periods, presumably as an avoidance response to the toxicant. One-hour exposure 250 microg l(-1) Al had no detectable effect. When mussels were exposed to 250 or 500 microg l(-1) added Al for 15 days, siphon activity measured in days 11-15 of exposure was inhibited by 50% and 65%, respectively, compared to pre-exposure levels. Recovery occurred following transfer of mussels to uncontaminated water. Interaction between Al and freshwater bivalves at neutral pH may affect both the performance of the mussels and the chemical speciation of the metal in the natural environment.

Heterogeneity of natural organic matter from the Chena River, Alaska

Water samples were collected in July 2001 from the Chena River in central Alaska. The natural organic matter (NOM) was size fractionated into particulate (POM,>0.45 microm), colloidal (COM,1kDa-0.45 microm) and dissolved (DOM,<1k Da) organic matter fractions, using filtration and ultrafiltration. The size-fractionated organic matter was then analyzed for organic carbon (OC) and nitrogen (N), isotopic (delta13C and delta15N) and molecular composition, using continuous flow isotope ratio mass spectrometry and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Results of phase partitioning showed that, on average, about 6% of OC and 16% of N occurred in the form of POM while 66% of OC and 57% of N occurred in the form of COM, and 28% of the OC and 27% of the N were in the DOM form. Organic matter in the river water was found to be highly heterogeneous in terms of chemical composition and isotopic signatures. The C/N ratio was as low as 16+/-1 in the POM (n=2) to as high as 48+/-1 in the COM (n=3) and 38+/-4 in the DOM (n=3), suggesting a diagenetically younger POM. Values of delta13C increased with decreasing size, varying from -29.59+/-0.45% in the POM to -27.47+/-0.06% in the COM to -16.93+/-0.08% in the DOM. In contrast, values of delta15N decreased with decreasing size, from 2.64% in POM to 1.64% in COM to 1.33% in DOM. These results, together with radiocarbon measurements, suggest a preferential decomposition of lighter C isotope (12C) and heavier N isotopic (15N) from POM to COM to DOM. Results of py-GC/MS showed that the percentage of polysaccharides decreased with decreasing size, further supporting a degradation pathway of NOM from POM to COM and DOM in Chena River waters. More studies are needed to examine the seasonal and spatial variations of size-fractionated organic matter.

Silver speciation during chronic toxicity tests with the mysid, Americamysis bahia

A 28-day chronic toxicity test and two 7-day chronic estimation toxicity tests were conducted with silver nitrate (AgNO(3)) and the marine invertebrate, Americamysis bahia, in 20 per thousand (parts-per thousand) salinity seawater. One 7-day test was initiated with 7-day-old mysids and the second was initiated with <24-h-old mysids. There was very good agreement between the three toxicity tests. The no-observed-effect concentration (NOEC) values from the 28-day test, the 7-day test initiated with 7-day-old mysids, and the 7-day test initiated with <24-h-old mysids were 34, 65 and 38 microg/l silver, respectively. The 96-h LC50 values from the 28-day toxicity test and the 7-day toxicity test initiated with 7-day old mysids were 260 microg/l, and the 96-h LC50 value from the 7-day toxicity test initiated with <24-h-old mysids was 280 microg/l. Free ionic silver, Ag(+), concentrations measured with a silver electrode were in good agreement with concentrations calculated using total dissolved silver and chloride concentrations. Mean measured concentrations of Ag(+) in the test solutions ranged from 0.99 to 25 ng/l for the dissolved silver concentrations that ranged from 34 to 410 microg/l silver, indicating that free ionic silver varied from 0.003 to 0.006% of the silver dissolved in the 20 per thousand salinity seawater. Understanding the relationship of salinity and silver speciation, and the effect of this relationship on chronic invertebrate toxicity, will be useful for development of a marine biotic ligand model (BLM) and a water quality criterion for silver. This model could provide an important tool for improving the relationship of laboratory toxicity test results and predicted effects in natural environments, where variations in salinity may act to modify the toxicity of silver and other metals.

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.

Metal partitioning between colloidal and dissolved phases and its relation with bioavailability to American oysters

Kinetics and the extent of metal partitioning between colloidal and dissolved phases and coagulation of metals associated with colloids were examined to determine their effects on the bioavailability of selected metals (Cd, Co, Hg, Ag, Fe, and Zn) to American oysters (Crassostrea virginica) using radiotracer and short term exposure experiments. After dispersion of radiolabeled colloids into low molecular weight (LMW, < 1 kDa) seawater, metal partitioning between dissolved (<1 kDa) and colloidal (1 kDa-0.2 microm) phases resulted in a consistent pattern, with a relatively constant percentage in the colloidal phase for each metal. On average, about 90% of Hg and Fe, approximately 60% of Ag and approximately 40% of Zn, Co, and Cd were measured in the colloidal fraction during a short term exposure experiment, consistent with their partitioning in natural waters. Controlled laboratory experiments carried out in parallel using radioactively tagged colloids showed that coagulation of colloidal species, quantified as the fraction retained by a 0.2 microm filter, was insignificant for most metals under the conditions and time periods of the uptake experiments. The bioavailability of colloidally complexed metals, measured in terms of dry weight concentration factor (DCF, ml g(-1)) and uptake rate constant (ml g(-1) h(-1)), was somewhat depressed compared with their counterpart in the LMW treatment, but could be well predicted from the results of the LMW treatment and metal partitioning. Both DCF values and uptake rate constants were higher in the LMW treatment than in the colloidal treatment. In addition, B-type metals, such as Ag, Hg, and Zn, all had higher values of DCF and uptake rate constants, regardless of treatments, except for Cd which had a lower DCF and uptake rate constant. In contrast, Co and Fe had significantly lower DCF values and uptake rate constants. Most of Hg and Ag (60-80%) were measured in the soft tissue of oysters in both LMW and colloidal treatments. In contrast, 80% of Fe, 75% of Co, and approximately 60% of Cd were observed on the shell, while Zn was found evenly distributed between shell and soft tissue of oysters. These results agree well with the variation pattern of both DCF value and uptake rate constant for these two groups of metals.