Effect of Various Natural Dissolved Organic Carbon on Copper Lability and Toxicity to the Tropical Freshwater Microalga Chlorella sp

Mechanistic insight into multiple effects of copper ion on the photoreactivity of dissolved organic matter

Sunlight irradiation of dissolved organic matter (DOM) in surface water results in the production of photochemically produced reactive intermediates (PPRIs). This process is inevitably influenced by co-existing metal ions in aquatic environments; However, the underlying mechanism remains unclear. In this study, the effect of co-existing copper ion (Cu2 +) on PPRIs produced by irradiation of DOM was systematically investigated, because Cu2+ is a typical redox transient cation and has strong affinity to DOM. The findings demonstrated that Cu2+, acting as cation bridge, caused DOM to aggregate, and had impacts on the optical properties and conformation of DOM. The electron shuttle and catalyst effect of Cu2+ could accelerate the charge transfer processes for the increasing of quantum yield and steady concentrations of hydroxyl radical (·OH) with the increase of concentrations of e-aq, O2.-, hydrogen peroxide (H2O2) and charge separated states of DOM (DOM·+ or DOM·-); On the other hand, Cu2+, as excited state quencher, decrease of apparent quantum yield of triplet state of DOM (3DOM*) and singlet oxygen (1O2) through static quenching of singlet excited of DOM (1DOM*) and dynamic quenching of 3DOM*, respectively. The results provide a deeper understanding of the effect mechanism of Cu2+ on the DOM photochemistry in real environment and will be useful for assessment the photodegradation of organic contaminants in the presence of both DOM and Cu2+.

Self-aggregation effect of the ternary system "Alga EPS-DOM-HMs" and the characterization of the self-adaptation metabolic response of microalgae

Heavy metals (HMs) present in the natural aquatic environment can form a ternary aggregate of "EPS-DOM-HMs" with the prevalent microalgae extracellular polymers substances (EPS) and macromolecular dissolved organic matters (DOMs), which show special molecular structure and biological interaction. This study reveals the formation of "EPS-TA-HMs" and the mechanism of their physiological and metabolic effects on Raphidocelis subcapitata. Results indicate that TA-Cr(III) can bind to EPS to form ternary aggregates with substances coexisting large and small hydrodynamic diameters and that the interactions are dominated by hydrophobic interactions of the protein binding to the pyrrole ring of the polyphenol and hydrogen bonding interactions formed by OC-(N R O). The protein structure of EPS has the largest proportion of proline, glycine, aspartic acid, and tryptophan. These interactions promoted the secretion of EPS components and reduced the growth inhibition of Raphidocelis subcapitata by 45.9 % compared with Cr(III) exposure. TEM analysis combined with EDS analysis indicated that Cr(III) was taken intracellularly and TA-Cr(III) was not. In addition, metabolomics analyses revealed that microalgae initiate adaptive mechanisms via the activation of a two-component system (i.e., maintenance of high metabolic activity). This study underscored the morphology of HMs in real aquatic environments and the mechanisms of metabolic effects on aquatic organisms.

Natural organic matter (NOM) can increase the uptake fluxes of three critical metals (Ga, La, Pt) in a unicellular green alga

Critical metals such as gallium, lanthanum and platinum are considered essential in a modern economy and for the required energy transition. Their relatively recent and increasing use in new technologies have led to an increase in their environmental mobility. As they reach aquatic systems, these metals can interact with organic ligands and especially Natural Organic Matter (NOM). The formation of organic complexes would be expected to reduce metal bioavailability and uptake by living cells, according to the Biotic Ligand Model (BLM). However, exceptions to this model have been determined for several critical metals in the past. The present work compared internalization kinetics of Ga, La and Pt in the green alga Chlamydomonas reinhardtii in the presence of NOMs from different origins: humic and fulvic acids from Suwannee River as well as NOMs from Ontario (Bannister Lake and Luther Marsh). Complexation was determined using a partial ultrafiltration method allowing for a normalization of data based on speciation to compare all conditions based on the concentration of the metal that was not bound to NOM. While internalization metal fluxes varied greatly from one NOM source to the other, uptake was almost always significantly higher than expected based on metal speciation. Quite often, metal internalization fluxes were even significantly increased in the presence of NOM, for the same total metal exposure concentration. For instance, Pt internalization was twice greater in the presence of Bannister Lake NOM than it was in the absence of NOM. The assumption that such exceptions could be explained by NOM characteristics was contradicted by the variable results from one metal to another. To further explore this phenomenon, internalization mechanisms for these individual metals need to be elucidated. This is a necessary step to accurately estimate the risk posed by the presence of these metals in humic aquatic systems.

Molecular variations to the proteome of zebrafish larvae induced by environmentally relevant copper concentrations

Contaminants are increasingly accumulating in aquatic environments and biota, with potential adverse effects on individual organisms, communities and ecosystems. However, studies that explore the molecular changes in fish caused by environmentally relevant concentrations of metals, such as copper (Cu), are limited. This study uses embryos of the model organism zebrafish (Danio rerio) to investigate effect of Cu on the proteome and amino acid (AA) composition of fish. Wild-type embryos at 24 h post-fertilisation were exposed to Cu (2 µg L-1 to 120 µg L-1) for 96 h and the number of healthy larvae were determined based on larvae that had hatched and did not display loss of equilibrium (LOE). The effect concentrations where Cu caused a 10 % (EC10) or 50 % (EC50) decrease in the number of healthy larvae were calculated as 3.7 µg L-1 and 10.9 µg L-1, respectively. Proteomics analysis of embryos exposed to the EC10 and EC50 concentrations of Cu revealed the proteome to differ more strongly after 48 h than 96 h, suggesting the acclimatisation of some larvae. Exposure to excess Cu caused differentially expressed proteins (DEPs) involved in oxidative stress, mitochondrial respiration, and neural transduction as well as the modulation of the AAs (Proline, Glycine and Alanine). This is the first study to suggest that LOE displayed by Cu-stressed fish may involve the disruption to GABAergic proteins and the calcium-dependent inhibitory neurotransmitter GABA. Moreover, this study highlights that proteomics and AA analysis can be used to identify potential biomarkers for environmental monitoring.

Multiple stressors unpredictably affect primary producers and decomposition in a model freshwater ecosystem

Freshwater ecosystems are affected by various stressors, such as contamination and exotic species, making them amongst the most imperilled biological systems on the planet. In Australia and elsewhere, copper is one of the most common metal contaminants in freshwater systems and the European carp (Cyprinus carpio L.) is one of the most pervasive and widespread invasive fish species. Copper (Cu) and carp can both directly affect primary production and decomposition, which are critical and interrelated nutrient cycling processes and ecosystem services. The aim of this study was to explore the direct and indirect effects of Cu and carp individually, and together on periphyton cover, chlorophyll a concentration, growth of the macrophyte Vallisneria spiralis L., and the decomposition of leaf litter and cotton strips in a controlled, factorial experiment in outdoor experimental ponds. In isolation, Cu reduced macrophyte growth and organic matter decomposition, while chlorophyll a concentrations and periphyton cover remained unchanged, possibly due to the Low-Cu concentrations in the overlying water. Carp addition alone had a direct negative effect on the biomass of aquatic plants outside protective cages, but also increased plant biomass inside the cages, periphyton cover and chlorophyll a concentrations. Leaf litter was more decomposed in the carp only ponds compared to controls, while there was no significant effect on cotton strip decomposition. Aquatic plants were absent in the Cu + carp ponds caused by the combined effects of Cu toxicity, carp disturbance and the increase in turbidity due to carp bioturbation. Increases in periphyton cover in Low-Cu + carp, while absence in the High-Cu + carp ponds, and differences in the decomposition of surface and buried cotton strips were not as predicted, which highlights the need for such studies to understand the complex interactions among stressors for environmental risk assessment.

Toxic effects of thallium (Tl+) on prokaryotic alga Microcystis aeruginosa: Short and long-term influences by potassium and humic acid

Thallium (Tl) is a priority pollutant regulated by the US EPA. It is also a critical element commonly used in high technology industries; with an increasing demand for semiconductors nowadays, wastewater discharges from manufacturing plants or metal mining activities may result in elevated levels of thallium in receiving water harming aquatic organisms. Regarding the impact of thallium on freshwater algae, little attention has been paid to prokaryotic physiology through various exposure periods. In this bench-scale study, prokaryotic alga Microcystis aeruginosa PCC 7806 was cultured in modified BG11 medium and exposed to Tl+ (TlNO3) ranging from 250 to 1250 μg/L for 4 and 14 days. Throughout the experiment using flow cytometry assays, algal population, cell membrane integrity, oxidation stress level, and chlorophyll fluorescence were exacerbated following the exposure to 750 μg Tl/L (approximately 4-day effective concentration of Tl+ for reducing 50% of algal population). Potassium and humic acid (HA) (1-5 mg/L) were added to study their influences on the thallium toxicity. With the additions of potassium, thallium toxicities to algal population and physiology were not significantly changed within 4 days, while they were alleviated within 14 days. With the addition of HA at 1 mg/L, cell membrane integrity was significantly attenuated within 4 days; ameliorating effects on algal population and oxidative stress were not observed until day 14. Thallium toxicities on oxidative stress level and photosynthesis activity were exacerbated in the presence of HA at 3-5 mg/L. The study provides useful information for further studies on the mode of toxic action of Tl+ in prokaryotic algae; it also demonstrates the necessity of considering short and long-term exposure durations while incorporating water chemistry into assessment of thallium toxicity to algae.

Applicability of Chronic Multiple Linear Regression Models for Predicting Zinc Toxicity in Australian and New Zealand Freshwaters

Bioavailability models, for example, multiple linear regressions (MLRs) of water quality parameters, are increasingly being used to develop bioavailability-based water quality criteria for metals. However, models developed for the Northern Hemisphere cannot be adopted for Australia and New Zealand without first validating them against local species and local water chemistry characteristics. We investigated the applicability of zinc chronic bioavailability models to predict toxicity in a range of uncontaminated natural waters in Australia and New Zealand. Water chemistry data were compiled to guide a selection of waters with different zinc toxicity-modifying factors. Predicted toxicities using several bioavailability models were compared with observed chronic toxicities for the green alga Raphidocelis subcapitata and the native cladocerans Ceriodaphnia cf. dubia and Daphnia thomsoni. The most sensitive species to zinc in five New Zealand freshwaters was R. subcapitata (72-h growth rate), with toxicity ameliorated by high dissolved organic carbon (DOC) or low pH, and hardness having a minimal influence. Zinc toxicity to D. thomsoni (reproduction) was ameliorated by both high DOC and hardness in these same waters. No single trophic level-specific effect concentration, 10% (EC10) MLR was the best predictor of chronic toxicity to the cladocerans, and MLRs based on EC10 values both over- and under-predicted zinc toxicity. The EC50 MLRs better predicted toxicities to both the Australian and New Zealand cladocerans to within a factor of 2 of the observed toxicities in most waters. These findings suggest that existing MLRs may be useful for normalizing local ecotoxicity data to derive water quality criteria for Australia and New Zealand. The final choice of models will depend on their predictive ability, level of protection, and ease of use. Environ Toxicol Chem 2023;42:2614-2629. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Responses of microalgae under different physiological phases to struvite as a buffering nutrient source for biomass and lipid production

Microalgae cultivation for biodiesel production is promising, but the high demand for nutrients, such as nitrogen and phosphorus, remains a limiting factor. This study investigated effects of struvite, a low-cost nutrient source, on microalgae production under different physiological phases. Changes in element concentrations were determined to characterize the controllable nutrient release properties of struvite. Results showed that nutrient elements could be effectively supplemented by struvite. However, responses of microalgae under different growth stages to struvite varied obviously, achieving the highest biomass (0.53 g/L) and the lowest (0.32 g/L). Moreover, the microalgal lipid production was obviously increased by adding struvite during the growth phase, providing the first evidence that struvite could serve as an alternative buffering nutrient source to culture microalgae. The integration of microalgae cultivation with struvite as a buffering nutrient source provides a novel strategy for high ammonia nitrogen wastewater treatment with microalgae for biodiesel production.

Toxicity mechanisms and bioavailability of copper to fish based on an adverse outcome pathway analysis

Copper (Cu) exists in a variety of forms in different aquatic environments, and affects their bioavailability. In this study we provide a systematic review on toxicity of Cu which focuses on identifying evidence in the mechanisms of Cu toxicity, and apply an adverse outcome pathway (AOP) analysis to identify multiple potential mechanisms and their interactions of Cu toxicity to fish. This analysis process included the mechanisms of behavior toxicant, oxidative toxicant, ion regulation disruption toxicity, as well as endocrine disruption toxicity. It was found that at low levels of Cu exposure, swimming, avoid predators, locating prey and other sensory functions will be impaired, and the organism will suffer from metabolic alkalosis and respiratory acidosis following the inhibition of the carbonic anhydrase active. The main pathway of acute toxicity of Cu to fish is the inhibition of the Na⁺/K⁺-ATPase enzyme, and lead to reduced intracellular sodium absorption, as well as Cu-induced increased cell permeability, in turn resulting in increased sodium ion loss, leading to cardiovascular collapse and respiratory insufficiency. The endocrine disruption toxicity of Cu to fish caused growth inhibition and reproductive reduction. In addition, there are several key pathways of Cu toxicity that are affected by hardness (e.g., Ca²⁺) and intracellular DOC concentrations, including inhibiting Cu-induction, improving branchial gas exchange, altering membrane transport functions, decreasing Na⁺ loss, and increasing Na⁺ uptake. The results of the AOP analysis will provide a robust framework for future directed research on the mechanisms of Cu toxicity.

Investigation of cytotoxic cadmium in aquatic green algae by synchrotron radiation-based Fourier transform infrared spectroscopy: Role of dissolved organic matter

The heavy metal Cd can cause severe toxicity on aquatic algae, but there are few studies on the cytotoxicity of heavy metal on algae based on synchrotron radiation technology. In this study, synchrotron radiation-based Fourier transform infrared spectromicroscopy (SR-FTIR) was used to characterize in vivo the toxic effects of Cd on Cosmarium sp. cells, emphasizing the influence of dissolved organic matter (DOM) on Cd toxicity. Results showed that, in the absence of DOM, obvious growth inhibition, cell volume reduction, and photosynthesis disruption could be observed with increasing Cd concentrations (0-500 μg/L). Based on the SR-FTIR imaging and functional group quantification, it was shown that the biosynthesis of biomolecules such as proteins, lipids, and carbohydrates was inhibited in algal cells. However, the addition of DOM caused significant heterogeneities in biomacromolecule biosynthesis that an increased biosynthesis of carbohydrates and structural lipids but an inhibited biosynthesis of proteins and storage lipids were observed. Furthermore, the correlation analysis and principal component analysis showed a good correlation between v(C-OH)/Amide II and biochemical parameters, indicating that changes of carbohydrates could be used as the biomarker to indicate the cytotoxicity of heavy metals to algal cells. These findings provide insight into the mechanisms of heavy metal cytotoxicity to aquatic algae and systematic cytotoxicity assessment under various aquatic conditions.

Nutrient removal and lipid production by the co-cultivation of Chlorella vulgaris and Scenedesmus dimorphus in landfill leachate diluted with recycled harvesting water

Applying microalgae for landfill leachate (LL) treatment is promising. However, LL usually needs to be diluted with much fresh water, aggravating water shortage. In this study, mono- and co-culturing microalgae (Chlorella vulgaris and Scenedesmus dimorphus) were used to treat LL diluted with recycled harvesting water, to investigate nutrient removal and lipid production. The results showed that microalgae in co-culture treatment had more biomass and stronger superoxide dismutase activity, which might be related to humic acids contained in recycled harvesting water, according to dissolved organic matters (DOMs) analysis. In addition, the lipid content and yield of co-cultured microalgae reached 27.60 % and 66.87 mg·L^-1, respectively, higher than those of mono-culture, proving the potential of co-culture for the improvement of lipid production. This study provided a freshwater-saving dilution method for LL treatment with recycled harvesting water as well as a strategy for the increase of biomass and lipid accumulation by microalgae co-cultivation.

Natural organic matter source, concentration, and pH influences the toxicity of zinc to a freshwater microalga

Zinc is a contaminant of concern in aquatic environments and is a known toxicant to many aquatic organisms. Dissolved organic matter (DOM) is a toxicity modifying factor for zinc and is an important water chemistry parameter. This study investigated the influence of DOM concentration, source, and water pH on the chronic toxicity of zinc to a freshwater microalga, Chlorella sp. The influence of DOM on zinc toxicity was dependent on both concentration and source. In the absence of DOM, the 72-h EC50 was 112 μg Zn.L^-1. In the presence of a DOM high in fulvic-like components, zinc toxicity was either slightly decreased (<4-fold increase in EC10s across 15 mg C.L^-1 range) or unchanged (minimal difference in EC50s). In the presence of a DOM high in humic-like (aromatic and high molecular weight) components, zinc toxicity was slightly decreased at the EC10 level and strongly increased at the EC50 level. The influence of pH on zinc toxicity was dependent on the source of DOM present in the water. In the presence of DOM high in humic-like components pH did not influence toxicity. In the presence of DOM high in fulvic-like components, pH had a significant effect on EC50 values. Labile zinc (measured by diffusive gradients in thin-films) followed linear relationships with dissolved zinc but could not explain the changes in observed toxicity, with similar DGT-labile zinc relationships shown for the two DOMs despite each DOM influencing toxicity differently. This indicates changes in toxicity may be unrelated to changes in zinc lability. The results suggest that increased toxicity of zinc in the presence of DOM may be due to direct uptake of Zn-DOM complexes. This study highlights the importance of considering DOM source and characteristics when incorporating DOM into water quality guidelines through bioavailability models.

How phytoplankton biomass controls metal(loid) bioaccumulation in size-fractionated plankton in anthropogenic-impacted subtropical lakes: A comprehensive study in the Yangtze River Delta, China

Phytoplankton biomass can significantly affect metal(loid) bioaccumulation in plankton, but the underlying mechanisms are still controversial. We investigated the bioaccumulation of eight metal(loid)s (As, Co, Cu, Hg, Mn, Pb, Se, and Zn) in three size categories of planktonic organisms - seston (0.7-64 μm), mesozooplankton (200-500 μm), and macrozooplankton (>500 μm) - sampled from six freshwater lakes in two seasons in the Yangtze River Delta, China. Our results highlight phytoplankton biomass is the major driver on metal(loid) bioaccumulation in the studied anthropogenic-impacted subtropical lakes, mainly via affecting site-specific water physiochemical characteristics and plankton communities. However, such impact is highly dependent on chlorophyll a (Chl-a) concentration. The bioaccumulation of metal(loid)s in size-fractionated plankton declined significantly with increasing phytoplankton biomass when Chl-a was below ∼50 μg L^-1, mainly owing to the reduced metal(loid) bioavailability and subsequent bioaccumulation at more productive sites (with elevated pH and dissolved organic carbon), rather than algal bloom dilution. To a lesser extent, phytoplankton growth dilution and the smaller body-size of zooplankton at more productive sites also contributed to the lower metal(loid) bioaccumulation. The bioaccumulation of metal(loid)s was enhanced under severe algal bloom conditions (when Chl-a concentration was higher than ∼50 μg L^-1). Although the underlying mechanisms still require further investigations, the potential risks of metal(loid) bioaccumulation under severe algal bloom conditions deserve special attention.

Quantifying the bioaccumulation of Pb to Chlorella vulgaris in the presence of dissolved organic matters with different molecular weights

Dissolved organic matter (DOM) is ubiquitous in natural waters which exhibits obvious effects on the toxicity of heavy metals. However, information on the toxicity of heavy metals in the presence of DOMs with different molecular weights (MWs) was still unclear. In this study, Suwannee river humic acid (SRHA) and algae-derived organic matter (ADOM) were selected as typical terrestrial and microbial DOMs, with the bulk DOMs fractionating into high MW (HMW-, 1 kDa ~ 0.45 μm) and low MW (LMW-, < 1 kDa) fractions to explore the MW-dependent heterogeneities in the bioaccumulation of Pb to Chlorella vulgaris. Results showed that, regardless of DOM types, the LMW fraction exhibited more acidic groups and humic-like substances than the HMW counterparts. Presence of bulk DOM can decrease the bioaccumulation of Pb, while the specific effects were MW- and type-dependent. The LMW-SRHA enhanced the bioaccumulation of Pb while the HMW counterpart alleviated the effects. However, both the HMW- and LMW-ADOM can reduce the bioaccumulation of Pb to C. vulgaris. Moreover, the correlation analysis showed a significant positive correlation between the content of phenolic-OH and the adsorbed/internalized amounts of Pb, demonstrating that the phenolic-OH played a critical role in altering the bioaccumulation of Pb. The results obtained in this study suggest that distribution of MWs, number of acidic functional groups, and metal complexation capacity within DOM pool should be considered for the eco-environmental risk assessment of heavy metals in aquatic environments.

The influence of hardness at varying pH on zinc toxicity and lability to a freshwater microalga, Chlorella sp

Zinc is an essential element for aquatic organisms, however, activities such as mining and refining, as well as zinc's ubiquitous role in modern society can contribute to elevated environmental concentrations of zinc. Water hardness is widely accepted as an important toxicity modifying factor for metals in aquatic systems, though other factors such as pH are also important. This study investigated the influence of increasing water hardness, at three different pH values (6.7, 7.6 and 8.3), on the chronic toxicity of zinc to the growth rate of a microalgae, Chlorella sp. Zinc toxicity decreased with increasing hardness from 5 to 93 mg CaCO₃ L^-1 at all three pH values tested. The 72 h growth rate inhibition EC50 values ranged from 6.2 μg Zn L^-1 (at 5 mg CaCO₃ L^-1, pH 8.3) to 184 μg Zn L^-1 (at 92 mg CaCO₃ L^-1, pH 6.7). Increases in hardness from 93 to 402 mg CaCO₃ L^-1 generally resulted in no significant (p > 0.05) reduction in zinc toxicity. DGT-labile zinc measurements did not correspond with the observed changes in zinc toxicity as hardness was varied within a pH treatment. This suggests that cationic competition from increased hardness is decreasing zinc toxicity, rather than changes in metal lability. This study highlighted that current hardness algorithms used in water quality guidelines may not be sufficiently protective of sensitive species, such as Chlorella sp., in high hardness waters.

Changes to the amino acid profile and proteome of the tropical freshwater microalga Chlorella sp. in response to copper stress

Contamination of freshwaters is increasing globally, with microalgae considered one of the most sensitive taxa to metal pollution. Here, we used 72 h bioassays to explore the biochemical effects of copper (Cu) on the amino acid (AA) profile and proteome of Chlorella sp. and advance our understanding of the molecular changes that occur in algal cells during exposure to environmentally realistic Cu concentrations. The Cu concentrations required to inhibit algal growth rate by 10% (EC₁₀) and 50% (EC₅₀) were 1.0 (0.7-1.2) µg L^-1 and 2.0 (1.9-2.4) µg L^-1, respectively. The AA profile of Chlorella sp. showed increases in glycine and decreases in isoleucine, leucine, valine, and arginine, with increasing Cu. Proteomic analysis revealed the modulation of several proteins involved in energy production pathways, including: photosynthesis, carbon fixation, glycolysis, and oxidative phosphorylation, which likely assists in meeting increased energy demands under Cu-stressed conditions. Copper exposure also caused up-regulation of cellular processes and signalling proteins, and the down-regulation of proteins related to ribosomal structure and protein translation. These changes in biomolecular pathways have direct effects on the AA profile and total protein content and provide an explanation for the observed changes in amino acid profile, cell growth and morphology. This study shows the complex mode of action of Cu on Chlorella under environmentally realistic Cu concentrations and highlights several potential biomarkers for future investigations.

Probabilistic risk assessment of mine-derived copper in the Ok Tedi/Fly River, Papua New Guinea

The Ok Tedi mine discharges waste rock and tailings into the Ok Tedi River in Papua New Guinea. This has resulted in elevated copper concentrations throughout the Ok Tedi/Fly River system, which can potentially impact aquatic biota. Ten years of measured copper and toxicity monitoring data were used to assess the risk of chronic effects from the mine-derived copper. Cumulative probability plots of dissolved and labile copper were compared to a species sensitivity distribution (SSD) of published copper toxicity data for four regions of the river. The Cu-SSD was used to estimate the risk of chronic effects to aquatic organisms in the Ok Tedi/Fly River at a range of potential copper exposure scenarios. The risk to species at the median labile copper concentration for each region showed a gradient effect with distance downstream from the mine and only the most sensitive (0.2-11%) species were at risk. There were copper exceedances of the region-specific guideline values (GV) and default guideline value (DGV) 88% and 74% of the time, respectively, in the Ok Tedi region (closest to the mine) and this is considered a high risk of chronic effects. Measured copper concentrations in the middle Fly River, lower Fly River (farthest downstream of the mine) and the river at Kiunga (reference site) exceeded the region-specific GVs and DGVs less frequently to rarely and present a lower risk of chronic effects from copper. The risk was supported using toxicity tests with the local microalgal species Chlorella sp. Comparison of recent (2010-2020) and historical (1996-2004) copper monitoring data from the Ok Tedi/Fly River indicates a decrease in the labile copper concentrations (30-76%) at key sites from impacted regions and a subsequent decrease in risk. This coincides with improved mining practices aimed at reducing the copper load into the Ok Tedi/Fly River.

Effective concentration signature of Zn in a natural water derived from various speciation techniques

The uptake of nutrients or toxicants by different organisms in aquatic systems is known to correlate with different fractions of the nutrient's or toxicant's total concentration. These fractions can be provided by different analytical techniques, from which the better correlation is expected to be found for those with a characteristic length comparable to that in the considered organism uptake. An effective concentration signature can be built up with the concentration values associated to the availability (i.e. fluxes in dynamic techniques) of the nutrient or toxicant measured by various analytical techniques with different characteristic lengths. Here, this new representation was obtained for the pool of Zn complexes in the Mediterranean stream Riera d'Osor (Girona, Catalonia, Spain) with a suite of four analytical techniques. Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) and Polymer Inclusion Membrane (PIM) devices provided the free Zn concentration. Linear Anodic Stripping Voltammetry provided a labile fraction (defined here as c_LASV, higher than the free concentration), related to the diffusion layer scale. Diffusion Gradients in Thin-films provided higher labile fractions (known as DGT concentrations, c_DGT) connected to the different characteristic lengths of different configurations (e.g. one or two resin discs) longer, in any case, than that corresponding to LASV. The combination of the information retrieved by the techniques allowed to quantify lability degrees of the pool of Zn complexes and to build up the effective concentration signature for this water.

Intracellular Biotransformation of Cu(II)/Cu(I) Explained High Cu Toxicity to Phytoplankton Chlamydomonas reinhardtii

The toxicity of Cu is related to its redox species, but the differential toxicity of Cu(II) and Cu(I) remains unknown. In the present study, we developed a novel protocol to simultaneously detect the biologically produced extracellular Cu(I) and internalized Cu(II) in a freshwater phytoplankton Chlamydomonas reinhardtii. The intracellular Cu(I) was further imaged using a fluorometric probe. Combining these pieces of evidence, we demonstrated that Cu(I) dominated the Cu toxicity in algal cells under Fe-deficient conditions. Our results showed that the labile Cu(I) content increased significantly in the low Fe quota cells. Intracellular biotransformation from Cu(II) to Cu(I) rather than the direct uptake of Cu(I) was responsible for the high Cu toxicity. The abnormal biotransformation from Cu(II) to Cu(I) under Fe deficiency was not resulted from the increase of overall Cu bioaccumulation but was likely due to the change of Cu(II) metabolism. High contents of Cu(II) were accumulated in the normal cells and the low Zn quota cells upon Cu exposure but did not induce cell death, further suggesting that Cu(I) dominated the Cu toxicity to the algae. This is the first study to simultaneously consider the effect of Cu(I) and Cu(II) during Cu exposure in phytoplankton. The results uncovered the underlying mechanisms of high Cu toxicity under Fe deficiency and highlighted the critical role of modulation of Cu metabolism in phytoplankton.

Metal lability and environmental risk in anthropogenically disturbed Antarctic melt streams

Antarctic melt streams are important ecosystems that increasingly face contaminant pressures from anthropogenic sources. Metal contaminants are often reported in the limno-terrestrial environment but their speciation is not well characterised, making environmental risk assessments difficult. This paper characterises labile metal concentrations in five melt streams and three shallow lakes around the Casey and Wilkes research stations in East Antarctica using chemical extracts and field deployments of diffusive gradients in thin-film (DGT) samplers. An acute toxicity test with field-collected Ceratadon purpeus and taxonomic identification of diatoms in melt streams were used to infer environmental risk. Copper and zinc were the most labile metals in the melt streams. DGT-labile copper concentrations were up to 3 μg Cu L^-1 in melt-stream waters but not labile below the sediment-water interface. DGT-labile zinc concentrations were consistent above and below the sediment-water interface at concentrations up to 14 μg Zn L^-1 in four streams, but one stream showed evidence of zinc mineralisation in the sediment with a flux to overlying and pore waters attributed to the reductive dissolution of iron and manganese oxides. Other metals, such as chromium, nickel, and lead were acid-extractable from the sediments, but not labile in pore waters or overlying waters. All streams had unique compositions of freshwater diatoms, but one had particularly reduced diversity and richness, which correlated to metal contamination and sediment physico-chemical properties such as a finer particle size. In laboratory bioassays with field-collected samples of the Antarctic moss C. purpeus, there was no change in photosynthetic efficiency following 28-d exposure to 700, 900, 1060, or 530 μg L^-1 of cadmium, copper, nickel, and zinc, respectively. This study shows that microorganisms such as diatoms may be at greater risk from contaminants than mosses, and highlights the importance of geochemical factors controlling metal lability.

The Influence of pH on Zinc Lability and Toxicity to a Tropical Freshwater Microalga

Increased focus on the development and application of bioavailability-based metal water quality guideline values requires increased understanding of the influence of water chemistry on metal bioavailability and toxicity. Development of empirical models, such as multiple linear regression models, requires the assessment of the influence of individual water quality parameters as toxicity-modifying factors. The present study investigated the effect of pH on the lability and toxicity of zinc (Zn) to a tropical green microalga (Chlorella sp.). Zinc speciation and lability were explored using the Windermere Humic Aqueous Model (WHAM7), ultrafiltration, and diffusive gradients in thin films (DGT). Zinc toxicity increased significantly with increasing pH from 6.7 to 8.3, with 50% growth inhibition effect concentrations decreasing from 185 to 53 µg l^-1 across the pH range. Linear relationships between DGT-labile Zn and dissolved Zn did not vary across the tested pH range, nor did the linear relationship between dissolved (<0.45 µm) and ultrafiltered (<3 kDa) Zn. Our findings show that Zn toxicity to this freshwater alga is altered as a function of pH across environmentally realistic pH ranges and that these toxicity changes could not be explained by Zn speciation and lability as measured by DGT and WHAM7. Environ Toxicol Chem 2021;40:2836-2845. © 2021 SETAC.

Field application of a novel active-passive sampling technique for the simultaneous measurement of a wide range of contaminants in water

A first test of the field capabilities of a novel in situ sampling technique combining active and passive sampling (APS) was conducted in the sea. The proof-of-concept device uses a pump to draw water into a diffusion cell where dissolved target substances are accumulated onto sorbents which are selective for different classes of contaminants (i.e., metal cations, polar and non-polar organic compounds), simultaneously. A controlled laminar flow established in the diffusion cell enables measurements of contaminant concentrations that are fully independent from the hydrodynamic conditions in the bulk solution. APS measurements were consistent with those obtained using conventional passive sampling techniques such as organic diffusive gradients in thin films (o-DGT) and silicone rubber (SR) samplers (generally < 40% difference), taking into account the prevailing hydrodynamic conditions. The use of performance reference compounds (PRC) for hydrophobic contaminants provided additional information. Field measurements of metal ions in seawater showed large variability due to issues related to the device configuration. An improved field set-up deployed in supplementary freshwater mesocosm experiments provided metal speciation data that was consistent with passive sampling measurements (DGT), taking into account the hydrodynamic conditions. Overall, the results indicate that the APS technique provides a promising approach for the determination of a wide range of contaminants simultaneously, and independently from the hydrodynamic conditions in the bulk solution.

Effect of Dissolved Organic Matter Concentration and Source on the Chronic Toxicity of Copper and Nickel Mixtures to Chlorella sp

There have been limited studies on the effects of toxicity-modifying factors, such as dissolved organic matter (DOM), on the toxicity of metal mixtures to aquatic biota. The present study investigated the effects of DOM concentration (low, 2.8 ± 0.1 mg C/L; high, 11 ± 1.0 mg C/L) and DOM source (predominantly terrestrial or microbial) on the chronic toxicity of copper (Cu) and nickel (Ni) binary mixtures to the green freshwater microalga Chlorella sp. This was assessed by using a full factorial design of 72-h growth inhibition bioassays. Measured algal growth rate was compared with growth predicted by the concentration addition and independent action reference models. Model predictions were based on concentrations of dissolved metals, labile metals (measured by diffusive gradients in thin films [DGT]), and calculated free metal ions (determined by the Windermere Humic Aqueous Model). Copper/Ni mixture toxicity was synergistic to Chlorella sp. in the absence of added DOM, with evidence of metal concentration-dependent toxicity at low effect concentrations. As DOM concentration increased, the mixture interaction changed from synergism to noninteraction or antagonism depending on the metal speciation method used. The DOM source had no significant effect on mixture interaction when based on dissolved and free metal ion concentrations but was significantly different when based on DGT-labile metal concentrations. Ratio-dependent mixture interaction was observed in all treatments, with increased deviation from the reference model predictions as the mixture changed from Ni- to Cu-dominated. The present study demonstrated that both DOM concentration and source can significantly change metal mixture toxicity interactions and that these interactions can be interpreted differently depending on the metal speciation method used. Environ Toxicol Chem 2021;40:1908-1918. © 2021 SETAC.

Speciation of nickel and its toxicity to Chlorella sp. in the presence of three distinct dissolved organic matter (DOM)

Nickel is often a metal of interest in regulatory settings given its increasing prevalence in disturbed freshwaters and as a known toxicant to fish and algae. Dissolved organic matter (DOM) is a toxicity modifying factor for nickel and a ubiquitous water physicochemical parameter. This study investigated the effect of DOM concentration and source on the chronic toxicity of nickel to Chlorella sp. using three DOM at two concentrations (3.1 ± 1.8 and 12 ± 1.3 mg C/L). Nickel toxicity to Chlorella sp. was not strongly influenced by DOM concentration. In the absence of DOM, the 72-h EC₅₀ for Chlorella sp. was 120 μg Ni/L. In the low DOM treatment, nickel toxicity was either unchanged or slightly increased (87-140 μg Ni/L) and unchanged or slightly decreased in the high DOM treatment (130-240 μg Ni/L). DOM source also had little effect on nickel toxicity, the largest differences in nickel toxicity occurring in the high DOM treatment. Labile nickel (measured by diffusive gradients in thin-films, DGT) followed strong linear relationships with dissolved nickel (R² > 0.97). DOM concentration and source had limited effect on DGT-labile nickel. DGT-labile nickel decreased with increasing DOM concentration for only one of the three DOM. Modelled labile nickel concentrations (expressed as maximum dynamic concentrations, c^dyn_max) largely agreed with DGT-labile nickel and suggested that toxicity is explained by free Ni²⁺ concentrations. This study confirms that nickel toxicity is largely unaffected by DOM concentration or source and that both measured (DGT) and modelled (c^dyn_max and free Ni²⁺) nickel concentrations can explain nickel toxicity.

Speciation of Inorganic Compounds in Aquatic Systems Using Diffusive Gradients in Thin-Films: A Review

The speciation of trace metals in an aquatic system involves the determination of free ions, complexes (labile and non-labile), colloids, and the total dissolved concentration. In this paper, we review the integrated assessment of free ions and labile metal complexes using Diffusive Gradients in Thin-films (DGT), a dynamic speciation technique. The device consists of a diffusive hydrogel layer made of polyacrylamide, backed by a layer of resin (usually Chelex-100) for all trace metals except for Hg. The best results for Hg speciation are obtained with agarose as hydrogel and a thiol-based resin. The diffusive domain controls the diffusion flux of the metal ions and complexes to the resin, which strongly binds all free ions. By using DGT devices with different thicknesses of the diffusive or resin gels and exploiting expressions derived from kinetic models, one can determine the labile concentrations, mobilities, and labilities of different species of an element in an aquatic system. This procedure has been applied to the determination of the organic pool of trace metals in freshwaters or to the characterization of organic and inorganic complexes in sea waters. The concentrations that are obtained represent time-weighted averages (TWA) over the deployment period.

Bioavailability and phytotoxicity of rare earth metals to Triticum aestivum under various exposure scenarios

It is a daunting challenge to predict toxicity and accumulation of rare earth metals (REMs) in different exposure scenarios (e.g., varying water chemistry and metal combinations). Herein, we investigated the toxicity and uptake of La and Ce in the presence of various levels of Ca, Mg, Na, K, and at different pH values, as well as the combined effects of La and Ce in wheat Triticum aestivum. Major cations (Ca²⁺ and Mg²⁺) significantly mitigated the toxicity and accumulation of La³⁺/Ce³⁺. Toxicity and uptake of La, Ce, and La-Ce mixtures could be well quantified by the multi-metal biotic ligand model (BLM) and by the Langmuir-type uptake model with the consideration of the competitive effects of Ca²⁺ and Mg²⁺, with more than 85.1% of variations explained. The derived binding constants of Ca, Mg, La, and Ce to wheat root were respectively 3.87, 3.59, 6.97, and 6.48 on the basis of toxicity data, and 3.23, 2.84, 6.07, and 5.27 on the basis of uptake data. The use of the alternative WHAM-F_tox approach, requiring fewer model parameters than the BLM but with similar Akaike information criterion (AIC) values, successfully predicted the toxicity and accumulation of La/Ce as well as toxicity of La-Ce mixtures, with at least 76.4% of variations explained. However, caution should be taken when using this approach to explain the uptake of La-Ce mixtures. Our results provided promising tools for delineating REMs toxicity/uptake in the presence of other toxicity-modifying factors or in mixture scenarios.

Amelioration of copper toxicity to a tropical freshwater microalga: Effect of natural DOM source and season

Australian tropical freshwaters can experience extreme seasonal variability in rainfall and run off, particularly due to pulse events such as storms and cyclones. This study investigated how seasonal variability in dissolved organic matter (DOM) quality impacted the chronic toxicity of copper to a tropical green alga (Chlorella sp.) in the presence of two concentrations of DOM (low: ∼2 mg C/L; high: ∼10 mg C/L) collected from three tropical waters. Copper speciation and lability were explored using diffusive gradients in thin-films (DGT) and modelled maximum dynamic concentrations (c^dyn_max) using data derived from the Windermere Humic Aqueous Model (WHAM VII). Relationships between copper lability and copper toxicity were assessed as potential tools for predicting toxicity. Copper toxicity varied significantly with DOM concentration, source and season. Copper toxicity decreased with increasing concentrations of DOM, with 50% growth inhibition effect concentrations (EC₅₀) increasing from 1.9 μg Cu/L in synthetic test waters with no added DOM (0.34 mg C/L) up to 63 μg Cu/L at DOM concentrations of 9.9 mg C/L. Copper toxicity varied by up to 2-fold between the three DOM sources and EC₅₀ values were generally lower in the presence of wet season DOM compared to dry season DOM. Linear relationships between DGT-labile copper and dissolved copper were significantly different between DOM source, but not concentration or season. Modelled c^dyn_max consistently under-predicted labile copper in high DOM treatments compared to DGT measurements but performed better in low DOM treatments, indicating that this method is DOM-concentration dependent. Neither speciation method was a good surrogate for copper toxicity in the presence of different sources of natural DOM. Our findings show that DOM source and season, not just DOM concentration, affect copper toxicity to freshwater biota. Therefore, DOM quality should be considered as a toxicity-modifying factor for future derivation of bioavailability-based site-specific water quality guideline values.

The nature of dissolved organic matter determines the biosorption capacity of Cu by algae

The role of dissolved organic matter (DOM) on the biochemical behavior and toxicity of heavy metals in water is very important but complex and unclear. The present work extracted DOM from a natural water and separated it into three fractions, namely humic acid (HA), fulvic acid (FA) and transphilic acid (TPA). Optical detection showed that HA had most aromatic ring skeletons, FA had more aromatic ring hydrophilic groups, and TPA had the largest number of hydroxyl or carboxyl groups. Their effects on the toxicity of Cu by Chlorella pyrenoidosa depended on types and concentration of DOM. In the case of algal exposure to 0.003 mM initial Cu concentration, the final algal optical density increased from 0.317 of the control group to 0.345, 0.645 and 0.435 in the presence of 20, 10 mg L^-1 HA, and 10 mg L^-1 TPA, respectively, but were suppressed to 0.246, 0.117 and 0.234 in the presence of 10, 20 mg L^-1 FA and 20 mg L^-1 TPA. Most adsorption isotherms lost the linearity in the presence of HA, FA and TPA. The adsorbed Cu increased from 0.242 to 0.477 mmol g^-1, following the order of increased concentration of HA, FA, and TPA. The formation of ternary complex and the multi-layer adsorption were proposed to explain the significant enhancement adsorption of Cu in the presence of FA and TPA. This study showed that the type and the density of effective functional groups in DOM determined its effects on Cu toxicity and bioavailability to algae.

Comparison of copper binding properties of DOM derived from fresh and pyrolyzed biomaterials: Insights from multi-spectroscopic investigation

The binding of dissolved organic matter (DOM) with metals affects the latter's biogeochemical processing in the environment. This study used multi-spectroscopic analyses to compare the heterogeneities of the Cu(II) binding properties of DOM derived from fresh and pyrolyzed biomaterials. The results showed that the DOM derived from fresh macrophyte (MDOM) and their corresponding biochar (BDOM) consisted mostly of protein-like and humic-like substances, respectively. The stability constant (log K_M) of protein-like matter in the MDOM was 5.27, and the values of humic-like components in the BDOM were 4.32-5.15. Compared with the MDOM, the BDOM exhibited lower affinities and active binding sites for Cu(II). In addition, the BDOM contents decreased after pyrolysis. Therefore, the pyrolysis of fresh biomaterials into biochar is a promising method for reducing the potential migration risk posed by Cu(II) due to the MDOM being a positive carrier for Cu(II) contamination. Polysaccharide was the only functional group that participated in the binding of Cu(II) in both MDOM and BDOM. Aliphatic groups and amides associated with protein-like matter were responsible for the Cu(II) binding to MDOM, whereas phenolic and aromatic groups mainly participated in the complexation of BDOM-Cu(II). The CO group of amide I in the MDOM, and polysaccharide in the BDOM, showed the fastest response to Cu(II). This study was helpful for elucidating the effects of fresh and pyrolyzed biomaterials (biochars) on the environmental behavior of Cu(II) at the molecular level.

Synergistic toxicity of microcystin-LR and Cu to zebrafish (Danio rerio)

Toxic cyanobacterial blooms often coincide with metal pollution in a freshwater environment because of surface run-off enriched with nutrients and metals. However, the joint toxic effects of cyanobacterial toxins and metals on aquatic animals remain unknown. In this study, single and joint toxic effects and mechanisms of microcystin-LR (MCLR) and copper (Cu) were investigated in the early development of zebrafish (Danio rerio). The LC50_72-h values were 2.79 mg/L for MCLR and 3.23 mg/L for Cu. The sublethal concentrations of MCLR (≤600 μg/L) did not affect the normal development of zebrafish but increased its hatchability. Strong synergistic toxic effects were observed after co-exposure to MCLR and Cu at environmental concentrations (≤60 μg/L). The synergistic toxic effects of these two compounds could be attributed to the increased bioaccumulation of MCLR and Cu, which was mediated by MCLR transporters (e.g., oatp1d1 and oatp2b1) and Cu transporters (e.g., ctr1 and atp7a), in zebrafish. Such bioaccumulation caused oxidative stress, as suggested by the disrupted gene expression of anti-oxidative enzymes (e.g., Cu/Zn-SOD, Mn-SOD, and CAT). Our results revealed for the first time the synergistic toxic effects and potential toxic mechanism of MCLR-Cu in aquatic animals. These synergistic effects should be considered when assessing the ecological risk of toxic cyanobacterial blooms.