Mercury species induce metabolic reprogramming in freshwater diatom Cyclotella meneghiniana

Mercury is a hazardous pollutant of global concern. While advances have been made in identifying the detrimental effects caused by Hg species in phytoplankton, knowledge gaps remain regarding the metabolomic perturbations induced by inorganic mercury (Hg(II)) and monomethylmercury (MeHg) in these organisms. Diatoms represent a major phytoplankton group essential in various global biogeochemical cycles. The current study combined targeted metabolomics, bioaccumulation, and physiological response assays to investigate metabolic perturbations in diatom Cyclotella meneghiniana exposed for 2 h to nanomolar concentrations of Hg(II) and MeHg. Our findings highlight that such exposures induce reprogramming of the metabolism of amino acids, nucleotides, fatty acids, carboxylic acids and antioxidants. These alterations were primarily mercury-species dependent. MeHg exposure induced more pronounced reprogramming of the metabolism of diatoms than Hg(II), which led to less pronounced effects on ROS generation, membrane permeability and chlorophyll concentrations. Hg(II) treatments presented distinct physiological responses, with more robust metabolic perturbations at higher exposures. The present study provides first-time insights into the main metabolic alterations in diatom C. meneghiniana during short-term exposure to Hg species, deepening our understanding of the molecular basis of these perturbations.

Exploring the subcellular distribution of mercury in green alga Chlamydomonas reinhardtii and diatom Cyclotella meneghiniana: A comparative study

Mercury (Hg) is a priority pollutant of global concern because of its toxicity, its ability to bioaccumulate throughout the food web and reach significant concentrations in top predators. Phytoplankton bioconcentrate large amounts of Hg and play a key role in the entry of Hg into the aquatic food web. However, the subcellular distribution of Hg in freshwater phytoplankton, known to affect it toxicity and trophic transfer is understudied. The present study aimed at investigating the accumulation of inorganic Hg (iHg) and its subcellular distribution in freshwater phytoplankton species. To this end green alga Chlamydomonas reinhardtii and diatom Cyclotella meneghiniana were exposed to 10 and 100 nM of iHg for 2 h. The concentrations of Hg in the adsorbed, intracellular and subcellular (granules, debris, organelles, heat-stable peptides (HSP) and heat-denaturable proteins (HDP)) fractions were determined. The results showed that C. meneghiniana accumulated more Hg compared to C. reinhardtii at both iHg exposure concentrations (10 nM: 4.41 ± 0.74 vs. 1.10 ± 0.25 amol cell-1; 100 nM: 79.35 ± 10.78 vs. 38.31 ± 4.15 amol cell-1). The evaluation of the subcellular distribution of Hg, revealed that the majority of Hg was concentrated in the organelles fraction (59.7 % and 74.6 %) in the green algae. In the diatom, Hg was mainly found in the organelles (40.9 % and 33.3%) and in the HSP fractions (26.8 % and 40.1 %). The proportion of Hg in HDP fraction decreased in favor of the organelles fraction in C. reinhardtii when the exposure concentration increased, whereas the proportions in the debris and organelles fractions decreased in favor of HSP fraction in C. meneghiniana. This study provides pioneering information on the subcellular distribution of Hg within in freshwater phytoplankton, a knowledge that is essential to understand the toxicity and trophic transfer of Hg in contaminated aquatic environment.

Cytochrome c - silver nanoparticle interactions: Spectroscopy, thermodynamic and enzymatic activity studies

Cytochrome c, an iron containing metalloprotein in the mitochondria of the cells with an oxide/redox property, plays key role in the cell apoptotic pathway. In this study, the interaction of silver nanoparticles (AgNPs) with cytochrome c (Cyt c) was investigated by using a combination of spectroscopic, imaging and thermodynamic techniques, including dynamic light scattering (DLS), ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), fluorescence spectroscopy, near and far circular dichroism (CD) spectroscopy, and isothermal titration calorimetry (ITC). DLS and UV-vis analysis evidenced the formation of surface complexes of Cyt c on AgNPs. The saturation of surface coverage of AgNPs was observed at 4.36 Cyt c molecules per nm2 of AgNPs. The surface complexation resulted in a promotion of the Ag dissolution overtime. The negative sign of enthalpic (ΔH) contribution suggested that electrostatic forces are indicative forces in the interaction between protein and AgNPs. Moreover, the fluorescence spectra revealed that the conformation of protein was altered around tryptophan (Trp) and tyrosine (Tyr) residues indicating the alteration of the tertiary structure of Cyt c. CD analysis evidenced that the secondary structure of Cyt c was modified under AgNPs-Cyt c interactions and the binding of Cyt c onto AgNPs resulted in remarkable structural perturbation around the active site heme, which in turn alter the protein enzymatic activity. The results of the present study contributed to a deeper insight on the mechanisms of interaction between NPs and biomacromolecules and could help establish the in vivo fate of AgNPs on cellular redox homeostasis.

Oxidative stress induced by sub-lethal exposure to copper as a mediator in development of bacterial resistance to antibiotics

Limited information exists on how bacterial resistance to antibiotics is acquired and altered in response to short-term metal stress, and what the prevailing pathways are. Here the precursor mechanisms of development of bacterial antibiotic resistance mediated by oxidative stress induce under sub-lethal Cu²⁺ exposure were explored. The results showed that the overall level of antibiotic resistance in wild-type Escherichia coli and antibiotic-resistant E. coli was enhanced under 4 and 20 mg/L Cu²⁺ exposure, as demonstrated by the 2- to 8-fold increase in minimum inhibitory concentration (MIC). The MIC correlated with the increase of the cellular ROS generation and the enhancement of the antioxidant enzyme activity (p < 0.05), suggesting that changes in antibiotic resistance under sub-lethal Cu²⁺ exposure could be associated with oxidative stress. Likewise, enhanced cell membrane permeability and an increase in the number of bacteria entering the viable but non culturable (VBNC) state contributed to bacterial resistance to antibiotics. Moreover, the variance partitioning analysis demonstrated that the alterations of the antibiotic resistance phenotype of wild-type E. coli was mainly caused by oxidative stress-mediated increase in cell membrane permeability and entry into the VBNC state. The development of antibiotic resistance in resistant E. coli was primarily attributed to changes in the abundance and horizontal transfer ability of its antibiotic resistance genes, both of which contributed up to 20 %. Taken together the results allowed to propose a conseptual scheme on developing bacterial antibiotic resistance mediated by oxidative stress under sub-lethal Cu²⁺ exposure. This result provided a strong basis for reduction of early bacterial resistance.

Comparative study of the sensitivity of two freshwater gastropods, Lymnaea stagnalis and Planorbarius corneus, to silver nanoparticles: bioaccumulation and toxicity

Metal-based nanoparticles (NPs) are considered detrimental to aquatic organisms due to their potential accumulation. However, little is known about the mechanisms underlying these effects and their species-specificity. Here we used stable silver (Ag) NPs (20 nm, from 10 to 500 μg/L) with a low dissolution rate (≤2.4%) to study the bioaccumulation and biological impacts in two freshwater gastropods: Lymnaea stagnalis and Planorbarius corneus. No mortality was detected during the experiments. Ag bioaccumulation showed a dose-related increase with an enhanced concentration in both species after 7d exposure. L. stagnalis displayed a higher accumulation for AgNPs than P. corneus (e.g., up to 18- and 15-fold in hepatopancreas and hemolymph, respectively) which could be due to the more active L. stagnalis having greater contact with suspended AgNPs. Furthermore, the hepatopancreas and stomach were preferred organs for bioaccumulation compared to the kidney, mantle and foot. Regarding biological responses, the hemolymph rather than hepatopancreas appeared more susceptible to oxidative stress elicited by AgNPs, as shown by significantly increasing lipid peroxidation (i.e., formation of malondialdehyde). Neurotoxicity was detected in L. stagnalis when exposed to high concentrations (500 μg/L). Comparison with impacts elicited by dissolved Ag revealed that the effects observed on AgNPs exposure were mainly attributable to NPs. These results highlighted the relationship between the physiological traits, bioaccumulation, and toxicity responses of these two species to AgNPs and demonstrated the necessity of species-specificity considerations when assessing the toxicity of NPs.

Aquatic organisms modulate the bioreactivity of engineered nanoparticles: focus on biomolecular corona

The increased use of nanoparticle (NP)-enabled materials in everyday-life products have raised concerns about their environmental implications and safety. This motivated the extensive research in nanoecotoxicology showing the possibility that NPs could cause harm to the aquatic organisms if present at high concentrations. By contrast, studies dealing with influence that organisms could exert on the fate and thus effects of NPs are still very rare. Drawing on the existing up-to-date knowledge we critically discuss the formation of biomolecular corona as one of the mechanisms by which organisms exerted control on the NPs fate in the aquatic and biotic environments. We focused the formation of corona by exogeneous and endogenous biomolecules and illustrated the discussion with the specific example of phytoplankton and aquatic invertebrate species. We highlighted the necessity to incorporate the concept of biomolecular corona within more general framework considering the feedback of aquatic organisms and the control they exert in shaping the fate and impact of NPs in the aquatic and biological environment. In our view such broader perspective will contribute to get novel insights into the drivers of environmental transformations of NPs and their mechanisms, which are important in environmental risk assessment.

Simple Acid Digestion Procedure for the Determination of Total Mercury in Plankton by Cold Vapor Atomic Fluorescence Spectroscopy

Plankton, at the bottom of the food web, play a central role in the entry of mercury into the aquatic biota. To investigate their role in mercury uptake, reliable analytical procedures for Hg analysis are highly sought. Wet digestion procedures for determining total mercury in different biological matrices have been established since years, however only few studies focused on planktonic samples. In the present work, a simple and cost-effective wet digestion method was developed for the determination of total mercury in samples of small plankton material using a cold vapor atomic fluorescence spectroscopy (CVAFS). The optimization of the digestion method was achieved by using glass vessels with Teflon caps, low amount of acids (3 mL w/w 65% HNO₃ or 3 mL 50% v/v HNO₃), a constant temperature of 85 °C, the presence and absence of pre-ultrasound treatment, and a continuous digestion period (12 h). Certified reference materials IAEA-450 (unicellular alga Scenedesmus obliquus) and BRC-414 (plankton matrix) were used to optimize and validate the digestion method. The recovery efficiency of the proposed method for IAEA-450 and BCR-414 (3.1 mg and 21.5 mg) ranged between 94.1 ± 7.6% and 97.2 ± 4.6%. The method displayed a good recovery efficiency and precision for plankton matrices of low size. Thus, allowing better digestion of planktonic samples for mercury analysis using CVAFS techniques.

Asymmetrical Flow Field-Flow Fractionation Coupled to ICP-MS for Characterization of Trace Metal Species in the Environment from Macromolecular to Nano-Assemblage Forms: Current Challenges for Quantification

Asymmetrical flow field-flow fractionation (AF4) is a powerful technique employed for the separation of macromolecules, nanoparticles, and their assemblages according to their hydrodynamic behavior. It is well known that at this size range, complex interactions can occur between components (e.g. surface adsorption, aggregation) controlling the fate of trace metals (TMs) bound to them. AF4 coupling to inductively coupled plasma mass spectrometry (ICP-MS) allows the quantification of metal-containing species at trace levels present in environmental and biological systems on a size-composition basis. The combination of AF4-ICP-MS with other online detectors provides additional information that allows the assessment of the origin of analytes present in mixtures and complex matrixes with minimal sample preparation, which is crucial for understanding the behavior of trace metal contaminants. Despite the increasing use of AF4-ICP-MS in environmental contexts, we acknowledge that the quantification of inorganic species using such combined techniques requires further development of standardized procedures and need certified reference materials. In this review, we also discuss critical endpoints within the ICP-MS instrument coupled to AF4 that need to be controlled before quantitative measurements can be validated. Then, we illustrate how the combination of different online detectors in addition to ICP-MS offers an integrated picture of natural components states, thus providing key information on the changes in behavior of trace metal species and metallic nanoparticles (MNPs) as observed in both environmental samples and biofluids.

Asymmetrical Flow Field-Flow Fractionation Methods for Quantitative Determination and Size Characterization of Thiols and for Mercury Size Speciation Analysis in Organic Matter-Rich Natural Waters

Asymmetrical flow field-flow fractionation (AF4) efficiently separates various macromolecules and nano-components of natural waters according to their hydrodynamic sizes. The online coupling of AF4 with fluorescence (Fluo) and UV absorbance (UV) detectors (FluoD and UVD, respectively) and inductively coupled plasma–mass spectrometry (ICP-MS) provides multidimensional information. This makes it a powerful tool to characterize and quantify the size distributions of organic and inorganic nano-sized components and their interaction with trace metals. In this study, we developed a method combining thiol labeling by monobromo(trimethylammonio)bimane bromide (qBBr) with AF4–FluoD to determine the size distribution and the quantities of thiols in the macromolecular dissolved organic matter (DOM) present in highly colored DOM-rich water sampled from Shuya River and Lake Onego, Russia. We found that the qBBr-labeled components of DOM (qB-DOM) were of humic type, characterized by a low hydrodynamic size (dh &lt; 2 nm), and have concentrations &lt;0.3 μM. After enrichment with mercury, the complexes formed between the nano-sized components and Hg were analyzed using AF4–ICP-MS. The elution profile of Hg followed the distribution of the UV-absorbing components of DOM, characterized by slightly higher sizes than qB-DOM. Only a small proportion of Hg was associated with the larger-sized components containing Fe and Mn, probably inorganic oxides that were identified in most of the samples from river to lake. The size distribution of the Hg–DOM complexes was enlarged when the concentration of added Hg increased (from 10 to 100 nM). This was explained by the presence of small iron oxides, overlapping the size distribution of Hg–DOM, on which Hg bound to a small proportion. In addition, to provide information on the dispersion of macromolecular thiols in colored DOM-rich natural water, our study also illustrated the potential of AF4–FluoD–UVD–ICP-MS to trace or quantify dynamic changes while Hg binds to the natural nano-colloidal components of surface water.

Dual role of titanium dioxide nanoparticles in the accumulation of inorganic and methyl mercury by crustacean Daphnia magna through waterborne and dietary exposure

Titanium dioxide nanoparticles (nTiO₂) are widely used in numerous products, yet their role in the accumulation and transfer of other contaminants in the aquatic food webs is not well understood. The influence of nTiO₂ on inorganic (IHg) and monomethyl mercury (MeHg) accumulation in invertebrate Daphnia magna through waterborne and dietary exposure was thus thoroughly investigated. The results showed that nTiO₂ led to a substantial decrease of the total mercury body burden (THg) in D. magna in direct waterborne exposure to IHg/MeHg. However, exposure to nTiO₂ pre-treated with IHg/MeHg resulted in an increase of the THg body burden in daphnids. The presence of nTiO₂ led to a substantial decrease of the THg in D. magna when exposed to IHg/MeHg via algal food. These effects were more pronounced for IHg than that for MeHg due to the higher adsorption capabilities of nTiO₂ for IHg. In addition, high concentrations of nTiO₂ favored the trophic transfer of IHg/MeHg through feeding on nTiO₂ pre-treated with Hg, however lessened it when D. magna were fed on alga pre-treated with IHg/MeHg. Comparable assimilation efficiency (AE), determined as Hg retained in daphnids after depuration, was observed in D. magna when exposed to IHg/MeHg via algal food regardless the absence or presence of 20 mgL^-1 nTiO₂. By contrast, an increase of the AE of MeHg through feeding on nTiO₂ and alga was found in the presence of higher concentration of 200 mgL^-1 nTiO₂. The present results will help to better understand the role of nTiO₂ on bioavailability and trophic transfer of global contaminants, such as mercury, known to bioaccumulate and biomagnify in the aquatic environment.

Metabolic alterations in alga Chlamydomonas reinhardtii exposed to nTiO2 materials

Nano-sized titanium dioxide (nTiO₂) is one of the most commonly used materials, however the knowledge about the molecular basis for metabolic and physiological changes in phytoplankton is yet to be explored. In the present study we use a combination of targeted metabolomics, transcriptomics and physiological response studies to decipher the metabolic perturbation in green alga Chlamydomonas reinhardtii exposed for 72 h to increasing concentrations (2, 20, 100 and 200 mg L⁻¹) of nTiO₂ with primary sizes of 5, 15 and 20 nm. Results show that the exposure to all three nTiO₂ materials induced perturbation of the metabolism of amino acids, nucleotides, fatty acids, tricarboxylic acids, antioxidants but not in the photosynthesis. The alterations of the most responsive metabolites were concentration and primary size-dependent despite the significant formation of micrometer-size aggregates and their sedimentation. The metabolic perturbations corroborate the observed physiological responses and transcriptomic results and confirmed the importance of oxidative stress as a major toxicity mechanism for nTiO₂. Transcriptomics revealed also an important influence of nTiO₂ treatments on the transport, adenosine triphosphate binding cassette transporters, and metal transporters, suggesting a perturbation in a global nutrition of the microalgal cell, which was most pronounced for exposure to 5 nm nTiO₂. The present study provides for the first-time evidence for the main metabolic perturbations in green alga C. reinhardtii exposed to nTiO₂ and helps to improve biological understanding of the molecular basis of these perturbations.

Combination of transcriptomics, metabolomics and physiology studies highlighted the nanoparticle size- and concentration-dependent disturbance in algal metabolism induced by nTiO₂.

Light-trapped caddisflies to decipher the role of species traits and habitats in Hg accumulation and transfer

We present a novel meta-community approach to explore the influence of species traits, such as adult body size, larval feeding type and microhabitat, as well as larval macrohabitat (main river channel vs. floodplain water bodies) on the concentration of total Hg accumulated ([THg]) in assemblages of adult caddisflies. We analyzed [THg] in 157 light-trapped adult caddisflies in a floodplain sector of the French upper Rhône River and used a linear mixed effect model to decipher the role of species traits and habitats in Hg accumulation. Variation of [THg] between species was best explained by the larval feeding type, whereas the contributions of adult size and larval micro and macro-habitat were minor. Results showed that [THg] in species associated with floodplain macrohabitats in the larval stage was lower than in those associated with the main river channel. This difference could depend on complexation of Hg by DOM (in the floodplain) and MES (in the main channel). This research provides a first evidence of the potential of an entire caddisfly assemblage for the assessment of contamination in large alluvial rivers. The implications of the results are discussed in view of the possible role of caddisflies as vectors of Hg to riparian predators.

Species-specific isotope tracking of mercury uptake and transformations by pico-nanoplankton in an eutrophic lake

The present study aims to explore the bioaccumulation and biotic transformations of inorganic (iHg) and monomethyl mercury (MMHg) by natural pico-nanoplankton community from eutrophic lake Soppen, Switzerland. Pico-nanoplankton encompass mainly bacterioplankton, mycoplankton and phytoplankton groups with size between 0.2 and 20 μm. Species-specific enriched isotope mixture of ¹⁹⁹iHg and ²⁰¹MMHg was used to explore the accumulation, the subcellular distribution and transformations occurring in natural pico-nanoplankton sampled at 2 different depths (6.6 m and 8.3 m). Cyanobacteria, diatoms, cryptophyta, green algae and heterotrophic microorganisms were identified as the major groups of pico-nanoplankton with diatoms prevailing at deeper samples. Results showed that pico-nanoplankton accumulated both iHg and MMHg preferentially in the cell membrane/organelles, despite observed losses. The ratios between the iHg and MMHg concentrations measured in the membrane/organelles and cytosol were comparable for iHg and MMHg. Pico-nanoplankton demethylate added ²⁰¹MMHg (~4 and 12% per day depending on cellular compartment), although the involved pathways are to further explore. Comparison of the concentrations of ²⁰¹iHg formed from ²⁰¹MMHg demethylation in whole system, medium and whole cells showed that 82% of the demethylation was biologically mediated by pico-nanoplankton. No significant methylation of iHg by pico-nanoplankton was observed. The accumulation of iHg and MMHg and the percentage of demethylated MMHg correlated positively with the relative abundance of diatoms and heterotrophic microorganisms in the pico-nanoplankton, the concentrations of TN, Mg²⁺, NO₃^-, NO₂^-, NH₄⁺ and negatively with the concentrations of DOC, K⁺, Na⁺, Ca²⁺, SO₄^2-. Taken together the results of the present field study confirm the role of pico-nanoplankton in Hg bioaccumulation and demethylation, however further research is needed to better understand the underlying mechanisms and interconnection between heterotrophic and autotrophic microorganisms.

Determination of the Intracellular Complexation of Inorganic and Methylmercury in Cyanobacterium Synechocystis sp. PCC 6803

Understanding of mercury (Hg) complexation with low molecular weight (LMW) bioligands will help elucidate its speciation. In natural waters, the rate of this complexation is governed by physicochemical, geochemical, and biochemical parameters. However, the role of bioligands involved in Hg intracellular handling by aquatic microorganisms is not well documented. Here, we combine the use of isotopically labeled Hg species (inorganic and monomethylmercury, iHg and MeHg) with gas or liquid chromatography coupling to elemental and molecular mass spectrometry to explore the role of intracellular biogenic ligands involved in iHg and MeHg speciation in cyanobacterium Synechocystis sp. PCC 6803, a representative phytoplankton species. This approach allowed to track resulting metabolic and newly found intracellular Hg biocomplexes (e.g., organic thiols) in Synechocystis sp. PCC 6803 finding different intracellular Hg species binding affinities with both high and low molecular weight (HMW and LMW) bioligands in the exponential and stationary phase. Furthermore, the parallel detection with both elemental and molecular ionization sources allowed the sensitive detection and molecular identification of glutathione (GSH) as the main low molecular weight binding ligand to iHg ((GS)₂-Hg) and MeHg (GS-MeHg) in the cytosolic fraction. Such a novel experimental approach expands our knowledge on the role of biogenic ligands involved in iHg and MeHg intracellular handling in cyanobacteria.

Microbial community diversity and composition in river sediments contaminated with tetrabromobisphenol A and copper

The microbial community composition in aquatic ecosystems have received increased attention. However, the knowledge gap relative to the responses of bacterial, archaeal and fungal communities in co-contaminated river sediments remain poorly studied. Here, we investigated the changes of tetrabromobisphenol A (TBBPA) and copper (Cu) concentrations and the responses of microbial communities in co-contaminated sediments during long-term incubation. TBBPA concentrations significantly decreased over time, whereas Cu concentrations remained relatively stable over the 60-day incubation. Abundances of the bacterial 16S rRNA, archaeal 16S rRNA and fungal ITS genes ranged from 6.53 × 10⁶ to 1.26 × 10⁹ copies g^-1, 1.12 × 10⁶ to 5.47 × 10⁶ copies g^-1 and 5.33 × 10³ to 7.51 × 10⁴ copies g^-1 in the samples, respectively. A total of 11, 6 and 5 bacterial, archaeal and fungal phyla were identified across all samples. Bacterial, archaeal and fungal communities mainly consisted of members from the phyla Proteobacteria and Acidobacteria, Methanomicrobia and Woesearchaeia as well as Agaricales and Helotiales, respectively. Fungal communities showed a stronger response to pollutant addition after a long incubation compared with bacterial and archaeal communities. The variance analysis results revealed that the bacterial, archaeal and fungal microbial communities of all treatments were distinctly distributed into two separated clusters according to incubation time. However, the three microbial communities did not significantly change in response to pollutant types, which was consistent with variation in relative abundances of the three microbial communities. These findings improve our understanding of the ecotoxicological effects of co-exposure on sediment microbial communities.

Metabolomic Responses of Green Alga Chlamydomonas reinhardtii Exposed to Sublethal Concentrations of Inorganic and Methylmercury

Metabolomics characterizes low-molecular-weight molecules involved in different biochemical reactions and provides an integrated assessment of the physiological state of an organism. By using liquid chromatography-mass spectrometry targeted metabolomics, we examined the response of green alga Chlamydomonas reinhardtii to sublethal concentrations of inorganic mercury (IHg) and monomethylmercury (MeHg). We quantified the changes in the levels of 93 metabolites preselected based on the disturbed metabolic pathways obtained in a previous transcriptomics study. Metabolites are downstream products of the gene transcription; hence, metabolite quantification provided information about the biochemical status of the algal cells exposed to Hg compounds. The results showed that the alga adjusts its metabolism during 2 h exposure to 5 × 10^-9 and 5 × 10^-8 mol L^-1 IHg and MeHg by increasing the level of various metabolites involved in amino acid and nucleotide metabolism, photorespiration, and tricarboxylic acid (TCA) cycle, as well as the metabolism of fatty acids, carbohydrates, and antioxidants. Most of the metabolic perturbations in the alga were common for IHg and MeHg treatments. However, the exposure to IHg resulted in more pronounced perturbations in the fatty acid and TCA metabolism as compared with the exposure to MeHg. The observed metabolic perturbations were generally consistent with our previously published transcriptomics results for C. reinhardtii exposed to the comparable level of IHg and MeHg. The results highlight the potential of metabolomics for toxicity evaluation, especially to detect effects at an early stage of exposure prior to their physiological appearance.

Morphological plasticity in Chlamydomonas reinhardtii and acclimation to micropollutant stress

Phytoplankton are characterized by a great phenotypic plasticity and amazing morphological variability, both playing a primary role in the acclimation to changing environments. However, there is a knowledge gap concerning the role of algal morphological plasticity in stress responses and acclimation to micropollutants. The present study aims at examining palmelloid colony formation of the green alga Chlamydomonas reinhardtii upon micropollutants exposure. Cells were exposed to four micropollutants (MPs, copper, cadmium, PFOS and paraquat) with different modes of action for a duration of 72 h. Effects of MPs on palmelloid formation, growth and physiological traits (chlorophyll fluorescence, membrane integrity and oxidative stress) were monitored by flow cytometry and fluorescence microscopy. Palmelloid formation was observed upon treatment with the four micropollutants. Number of palmelloid colonies and their size were dependent on MP concentration and exposure duration. Cells reverted to their unicellular lifestyle when colonies were harvested and inoculated in fresh medium indicating that palmelloid formation is a plastic response to micropollutants. No physiological effects of these compounds were observed in cells forming palmelloids. Palmelloid colonies accumulated lower Cd concentration than unicellular C. reinhardtii suggesting that colony formation protects the cells from MPs stress. The results show that colony formation in Chlamydomonas reinhardtii is a stress response strategy activated to face sub-lethal micropollutant concentrations.

The interplay of flow processes shapes aquatic invertebrate successions in floodplain channels - A modelling applied to restoration scenarios

The high biotic diversity supported by floodplains is ruled by the interplay of geomorphic and hydrological processes at various time scales, from daily fluctuations to decennial successions. Because understanding such processes is a key question in river restoration, we attempted to model changes in taxonomic richness in an assemblage of 58 macroinvertebrate taxa (21 gastropoda and 37 ephemeroptera, plecoptera and trichoptera, EPT) along two successional sequences typical for former braided channels. Individual models relating the occurrence of taxa to overflow and backflow durations were developed from field measurements in 19 floodplain channels of the Rhône floodplain (France) monitored over 10 years. The models were combined to simulate diversity changes along a progressive alluviation and disconnection sequence after the reconnection with the main river of a previously isolated channel. Two scenarios were considered: (i) an upstream + downstream reconnection creating a lotic channel, (ii) a downstream reconnection creating a semi-lotic channel. Reconnection led to a direct increase in invertebrate richness (on average x2.5). However, taxonomical richness showed a constant decrease as isolation progressed and reached an average of 2 for EPT and 7 for gastropods at the end of the scenarios. With more than 80% of the taxonomic models with an AUC equal or higher than 0.7 and slopes of linear relations between observed and predicted richness of 0.75 (gastropods) and 1 (EPT), the Boosted Regression Trees (BRT) provided a good basis for prediction of species assemblages. These models can be used to quantify a priori the sustainability and ecological efficiency of restoration actions and help floodplain restoration planning and management.

Academic expertise in assisting private companies in the fields of environment and environmental toxicology: the role of individual expertise

The scientific knowledge produced by academic research can be valued in all sectors of human activity, including private sector. The ROVALTAIN Foundation organized a round-table during its scientific day in 2019. It crossed the points of view of academic scientists and industrial partners, addressing five main topics. The first one concerned the validation of a common definition of the academic research/private partners interface. Then, the group discussed the place for academic expertise in the corporate world; the advantages of involving academic researchers in expertise for the private sector; and the limits of this model. To conclude, the need of a third party, like the ROVALTAIN Foundation, as a catalyzer in building the interface between academic research and private partners has been discussed.

Distinguishing the effects of Ce nanoparticles from their dissolution products: identification of transcriptomic biomarkers that are specific for ionic Ce in Chlamydomonas reinhardtii

Cerium (Ce) is a rare earth element that is incorporated in numerous consumer products, either in its cationic form or as engineered nanoparticles (ENPs). Given the propensity of small oxide particles to dissolve, it is unclear whether biological responses induced by ENPs will be due to the nanoparticles themselves or rather due to their dissolution. This study provides the foundation for the development of transcriptomic biomarkers that are specific for ionic Ce in the freshwater alga, Chlamydomonas reinhardtii, exposed either to ionic Ce or to two different types of small Ce ENPs (uncoated, ∼10 nm, or citrate-coated, ∼4 nm). Quantitative reverse transcription PCR was used to analyse mRNA levels of four ionic Ce-specific genes (Cre17g.737300, MMP6, GTR12, and HSP22E) that were previously identified by whole transcriptome analysis in addition to two oxidative stress biomarkers (APX1 and GPX5). Expression was characterized for exposures to 0.03-3 µM Ce, for 60-360 min and for pH 5.0-8.0. Near-linear concentration-response curves were obtained for the ionic Ce and as a function of exposure time. Some variability in the transcriptomic response was observed as a function of pH, which was attributed to the formation of metastable Ce species in solution. Oxidative stress biomarkers analysed at transcriptomic and cellular levels confirmed that different effects were induced for dissolved Ce in comparison to Ce ENPs. The measured expression levels confirmed that changes in Ce speciation and the dissolution of Ce ENPs greatly influence Ce bioavailability.

A density gradient centrifugation method for rapid separation of nanoTiO2 and TiO2 aggregates from microalgal cells in complex mixtures with mercury

In natural environment, the microorganisms are exposed to complex mixtures of contaminants, including manufactured nanoparticles and their aggregates. Evaluation of the toxicant accumulation in biota exposed to such cocktails is a challenging task because the microorganisms need to be separated from nanomaterial aggregates often of a comparable size. We propose a method for separation of TiO₂ aggregates from green microalga Chlamydomonas reinhardtii and subsequent determination of cellular Hg concentration in algae exposed to mixture of Hg with nanoTiO₂, known also to adsorb Hg. The method is based on differences in specific weight of algae and TiO₂ aggregates, using medium speed centrifugation on a step gradient of sucrose. The efficiency of the separation method was tested with nanoTiO₂ of three different primary sizes at four concentrations: 2, 20, 100 and 200 mg L⁻¹. The method gives a possibility to separate nanoTiO₂ and their aggregates from the algae with a mean recovery of 83.3% of algal cells, thus allowing a reliable determination of Hg accumulation by microalgae when co-exposed to Hg and nanoTiO₂.

• A rapid and reliable method to separate algal cells and nanoparticle aggregates of comparable size.

• A method to measure the cellular amount of Hg in green alga co-exposed to Hg and nanoTiO₂.

Impact of anthropogenic activities on the occurrence and distribution of toxic metals, extending-spectra β-lactamases and carbapenem resistance in sub-Saharan African urban rivers

The occurrence and dissemination of toxic metals, antibiotic resistant bacteria and their resistance genes (ARGs) in the aquatic ecosystems of sub-Saharan African countries are still understudied, despite their potential to threat human health and aquatic organisms. In this context, the co-contamination and seasonal distribution of toxic metals and ARG in river sediments receiving untreated urban sewages and hospital effluents from Kinshasa, the capital city of the Democratic Republic of the Congo were investigated. ARGs including β-lactam resistance (bla_CTX-M and bla_SHV), carbapenem resistance (bla_VIM, bla_IMP, bla_KPC, bla_OXA-48 and bla_NDM) and total bacterial load were quantified by using quantitative polymerase chain reaction (qPCR) in total DNA extracted from sediment. The amount of toxic metals in sediments was quantified using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The results highlight high abundance of 16S rRNA and ARGs copy numbers in sediment samples. Strong pollution of rivers by toxic metals was found, with max values (mg kg^-1) of 81.85(Cr), 5.09(Co), 33.84(Ni), 203.46 (Cu), 1055.92(Zn), 324.24(Pb) and 2.96(Hg). Results also highlight the high abundance of bacterial markers (8.06 × 10⁹-2.42 × 10¹² 16S rRNA/g^-1 DS) as well as antibiotic resistance genes (up to 4.58 × 10⁸ ARG. g^-1 DS) in the studied rivers. Significant correlations were observed between (i) metals (except Cd and Hg) and organic matter (R > 0.60, p < 0.05); and (ii) ARGs (except bla_NDM) and 16S rRNA (R > 0.57, p < 0.05) suggesting a tight link between (i) metal contamination and anthropogenic pressure and (ii) microbial contamination of river and dissemination of antibiotic resistance. Results demonstrated that multi-diffuse pollution originating from human activity contribute to the spread of toxic metals and ARGs into the aquatic ecosystems.

NanoTiO2 materials mitigate mercury uptake and effects on green alga Chlamydomonas reinhardtii in mixture exposure

The present study examined the effect of titanium dioxide nanoparticles (nanoTiO₂) and mercury (Hg) compounds on the green alga, Chlamydomonas reinhardtii. Mixtures containing nanoTiO₂ of different primary sizes (5 nm, 15 nm and 20 nm), inorganic Hg (IHg) or monomethyl Hg (CH₃Hg⁺, MeHg) were studied and compared with individual treatments. Oxidative stress and membrane damage were examined. Stability of nanoTiO₂ materials in terms of hydrodynamic size and surface charge as well as Hg adsorption on different nanoTiO₂ materials were characterized. The uptake of Hg compounds in the absence and presence of nanoTiO₂ was also quantified. Results show that increasing concentrations of nanoTiO₂ with different primary size diminished oxidative stress and membrane damage induced by high concentrations of IHg or MeHg, due to the adsorption of Hg on the nanoTiO₂ aggregates and consequent decrease of cellular Hg concentrations. The observed alleviation effect of nanoTiO₂ materials on Hg biouptake and toxicity was more pronounced for the materials with smaller primary size. IHg adsorbed onto the nanoTiO₂ materials to a higher extent than MeHg. The present study highlights that the effects of contaminants are modulated by the co-existing engineered nanomaterials; therefore, it is essential to get a better understanding of their combined effect in the environment.

When Environmental Chemistry Meets Ecotoxicology: Bioavailability of Inorganic Nanoparticles to Phytoplankton

The present review critically examines the state-of-the-art of the research concerning the likely environmental implications of engineered nanoparticles (ENPs) with specific emphasis on their interactions with phytoplankton in the aquatic environment. Phytoplankton plays a key role in the global carbon cycle and contributes to the half of the global primary production, thus representing some of the Earth ' s most critical organisms making the life on our planet possible. With examples from our own research and the literature, we illustrate what happens when aquatic organisms are unintentionally exposed to metal-containing ENPs, which are increasingly released into the environment from nano-enabled materials. We highlight the complexity of the ENPs behavior in the aquatic environment and focus on the three key steps of the bioavailability process: exposure availability, uptake availability and toxico-availability. The influence of the phytoplankton on the ENPs fate in the aquatic environment is discussed, too.

Dissolved Organic Matter and Associated Trace Metal Dynamics from River to Lake, Under Ice-Covered and Ice-Free Conditions

The present study investigates the changes in dissolved organic matter (DOM) composition and its influences on trace metal dispersion from the Shuya River (SR) in the Petrozavodsk Bay of Lake Onega during ice-covered and ice-free periods. Humic substances (HS) found in the SR dominated the composition of DOM through the river-bay-lake continuum in both periods. When the bay was ice-covered, both the aromaticity and the size of HS varied in the water column according to a horizontal stratification and decreased in the bay, while under ice-free conditions, they decreased along the river-lake gradient, suggesting in both cases a decrease in the proportion of HS with high aromatic character. These findings were associated with an overall decrease in the proportion of HS components that have the highest molecular masses. The quantification of metal bound to HS revealed that these characteristics were associated with a decrease in the binding capacity of the HS for Fe and Al but not Cu while dispersing in the bay to the lake. Pb was found to bind on HS, but its behavior in the bay could not be related to the HS dispersion nor to the changes in HS properties.

Insect Life Traits Are Key Factors in Mercury Accumulation and Transfer within the Terrestrial Food Web

As plants and associated insects are at the bottom of some terrestrial food webs, they are the primary contributors to mercury (Hg) fluxes in ecosystems. In addition to the trophic position of these organisms, factors related to their life traits have been hypothesized to influence their exposure to Hg. This study investigates the transfer of Hg in a soil-nettle-insect system and the insect-related factors affecting their Hg concentrations in a revegetated chlor-alkali landfill. Twenty-three insect species were identified and classified according to their life traits, their relationship with nettle, and their morphological characteristics. We observed low total mercury (THg) concentrations in nettles, with only 1% methylmercury (MeHg) being detected, while concentrations ranged from 5 to 3700 μg/kg dry wt. in insects with a MeHg percentage of up to 75%. The nettle-related insects were primarily exposed to Hg through the food web with significant biomagnification, particularly at the level of secondary predators. Within the nettle-unrelated group, the insect habitat was the most explanatory factor, with the highest enrichment being for the insects that spent part of their cycle in direct contact with Hg sources. Therefore, these insects require special attention because they are an essential vector of Hg transfer for terrestrial top predators.

Influence of nanoplastic surface charge on eco-corona formation, aggregation and toxicity to freshwater zooplankton

Concerns about possible environmental implications of nano- and micro-plastics are continuously raising. Hence, comprehensive understanding of their behaviour, bioaccumulation and toxicity potential is required. Nevertheless, systematic studies on their fate and possible effects in freshwaters, as well as the influence of particle-specific and environmental factors on their behaviour and impacts are still missing. The aims of the present study are thus two-fold: (i) to examine the role of the surface charge on nanoplastic stability and acute effects to freshwater zooplankton; (ii) to decipher the influence of the refractory natural organic matter (NOM) on the nanoplastic fate and effects. Amidine and carboxyl-stabilized polystyrene (PS) spheres of 200 nm diameter characterized by opposite primary surface charges and neutral buoyancy were selected as model nanoplastics. The results demonstrated that the surface functionalization of the polystyrene nanoplastics controls their aggregation behaviour. Alginate or Suwannee River humic acid (SRHA) modified significantly the surface charge of positively-charged amidine PS nanoplastic and the aggregation state, while had no significant influence on the negatively-charged carboxyl PS nanoplastic. Both amidine and carboxyl PS nanoplastics were ingested by the zooplankton and concentrated mainly in the gut of water flea Daphnia magna and larvae Thamnocephalus platyurus, and the stomach of rotifer Brachionus calyciflorus. Amidine PS nanoplastic was more toxic than carboxyl one. The toxicity decreased in the order D. magna (48 h -immobilization) > B. calyciflorus (24 h - lethality) > T. platyurus (24 h - lethality). Alginate or SRHA reduced significantly the toxicity of both amidine and carboxyl PS nanoplastics to the studied zooplankton representatives. The implications of this laboratory study findings to natural environment were discussed.

Towards early-warning gene signature of Chlamydomonas reinhardtii exposed to Hg-containing complex media

The potential of using gene expression signature as a biomarker of toxicants exposure was explored in the microalga Chlamydomonas reinhardtii exposed 2 h to mercury (Hg) as inorganic mercury (IHg) and methyl mercury (MeHg) in presence of copper (Cu) and Suwannee River Humic Acid (SRHA). Total cellular Hg (THg = IHg + MeHg) decreased in presence of SRHA for 0.7 nM IHg and 0.4 nM MeHg, but increased for 70 nM IHg exposure. In mixtures of IHg + MeHg and (IHg or MeHg) + Cu, SRHA decreased THg uptake, except for 0.7 nM IHg + 0.4 nM MeHg which was unchanged (p-value>0.05). In the absence of SRHA, 0.5 μM Cu strongly decreased intracellular THg concentration for 70 nM IHg, while it had no effect for 0.7 nM IHg and 0.4 nM MeHg. The expression of single transcripts was not correlated with measured THg uptake, but a subset of 60 transcripts showed signatures specific to the exposed metal(s) and was congruent with exposure concentration. Notably, the range of fold change values of this subset correlated with THg bioaccumulation with a two-slope pattern in line with [THg]_intra/[THg]_med ratios. Gene expression signature seems a promising approach to complement chemical analyses to assess bioavailability of toxicants in presence of other metals and organic matter.

Colloidal Size and Redox State of Uranium Species in the Porewater of a Pristine Mountain Wetland

Uranium (U) speciation was investigated in anoxically preserved porewater samples of a natural mountain wetland in Gola di Lago, Ticino, Switzerland. U porewater concentrations ranged from less than 1 μg/L to tens of μg/L, challenging the available analytical approaches for U speciation in natural samples. Asymmetrical flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry allowed the characterization of colloid populations and the determination of the size distribution of U species in the porewater. Most of the U was associated with three fractions: <0.3 kDa, likely including dissolved U and very small U colloids; a 1-3 kDa fraction containing humic-like organic compounds, dispersed Fe, and, to a small extent, Fe nanoparticles; and a third fraction (5-50 nm), containing a higher amount of Fe and a lower amount of organic matter and U relative to the 1-3 kDa fraction. The proportion of U associated with the 1-3 kDa colloids varied spatially and seasonally. Using anion exchange resins, we also found that a significant proportion of U occurs in its reduced form, U(IV). Tetravalent U was interpreted as occurring within the colloidal pool of U. This study suggests that U(IV) can occur as small (1-3 kDa), organic-rich, and thus potentially mobile colloidal species in naturally reducing wetland environments.

Comparative study of Cu uptake and early transcriptome responses in the green microalga Chlamydomonas reinhardtii and the macrophyte Elodea nuttallii

Microalgae are widely used as representative primary producers in ecotoxicology, while macrophytes are much less studied. Here we compared the bioavailability and cellular toxicity pathways of 2 h-exposure to 10^-6 mol L^-1 Cu in the macrophyte Elodea nuttallii and the green microalga Chlamydomonas reinhardtii. Uptake rate was similar but faster in the algae than in the macrophyte, while RNA-Sequencing revealed a similar number of regulated genes. Early-regulated genes were congruent with expected adverse outcome pathways for Cu with Gene Ontology terms including gene regulation, energy metabolism, transport, cell processes, stress, antioxidant metabolism and development. However, the gene regulation level was higher in E. nuttallii than in C. reinhardtii and several categories were more represented in the macrophyte than in the microalga. Moreover, several categories including oxidative pentose phosphate pathway (OPP), nitrate metabolism and metal handling were only found for E. nuttallii, whereas categories such as cell motility, polyamine metabolism, mitochondrial electron transport and tricarboxylic acid cycle (TCA) were unique to C. reinhardtii. These differences were attributed to morphological and metabolic differences and highlighted dissimilarities between a sessile and a mobile species. Our results highlight the efficiency of transcriptomics to assess early molecular responses in biota, and the importance of studying more aquatic plants for a better understanding on the impact and fate of environmental contaminants.

Species-species interactions modulate copper toxicity under different visible light conditions

Combination of biotic and abiotic factors influences the effects of naturally occurring or anthropogenic chemicals on photosynthetic microorganisms in the aquatic environment. Nonetheless, the combined effects of physical stressors and species-species interaction on chemicals' toxicity are still poorly understood. The present study examines the responses of the green alga Chlamydomonas reinhardtii and the cyanobacterium Synechocystis sp. alone and in mixtures to copper exposure under increasing visible light intensities. Cell growth, chlorophyll bleaching, oxidative stress and membrane permeability were determined by flow cytometry in both mono- and multi-species tests. The results revealed that species-species interactions influenced copper toxicity under different light regimes at 4 h and 48 h - exposure. For a given light condition, monocultures of Synechocystis sp. were more sensitive to copper than those of C. reinhardtii. In long-term incubation C. reinhardtii sensitivity to copper diminished in presence of Synechocystis sp. under low-intensity light, however it was enhanced under high-intensity light. The present results revealed the complex interplay between visible light intensity variations, species-species interaction and copper effects to phytoplankton in long- term exposure.

Molecular Effects, Speciation, and Competition of Inorganic and Methyl Mercury in the Aquatic Plant Elodea nuttallii

Mercury (Hg) remains hazardous in aquatic environments because of its biomagnification in food webs. Nonetheless, Hg uptake and impact in primary producers is still poorly understood. Here, we compared the cellular toxicity of inorganic and methyl Hg (IHg and MeHg, respectively) in the aquatic plant Elodea nuttallii. IHg and MeHg regulated contigs involved in similar categories (e.g., energy metabolism, development, transport, secondary metabolism), but MeHg regulated more contigs, supporting a higher molecular impact than IHg. At the organism level, MeHg induced antioxidants, while IHg decreased chlorophyll content. The uptake of Hg and expression of a subset of contigs was subsequently studied in complex media. Measured uptake pointed to a contrasted impact of cell walls and copper (Cu) on IHg and MeHg. Using a speciation modeling, differences in uptake were attributed to the differences in affinities of IHg and MeHg to organic matter in relation to Cu speciation. We also identified a distinct gene expression signature for IHg, MeHg, and Cu, further supporting different molecular toxicity of these trace elements. Our data provided fundamental knowledge on IHg and MeHg uptake in a key aquatic primary producer and confirmed the potential of transcriptomics to assess Hg exposure in environmentally realistic systems.

Biological effects of four iron-containing nanoremediation materials on the green alga Chlamydomonas sp

As nanoremediation strategies for in-situ groundwater treatment extend beyond nanoiron-based applications to adsorption and oxidation, ecotoxicological evaluations of newly developed materials are required. The biological effects of four new materials with different iron (Fe) speciations ([i] FerMEG12 - pristine flake-like milled Fe(0) nanoparticles (nZVI), [ii] Carbo-Iron^® - Fe(0)-nanoclusters containing activated carbon (AC) composite, [iii] Trap-Ox® Fe-BEA35 (Fe-zeolite) - Fe-doped zeolite, and [iv] Nano-Goethite - 'pure' FeOOH) were studied using the unicellular green alga Chlamydomonas sp. as a model test system. Algal growth rate, chlorophyll fluorescence, efficiency of photosystem II, membrane integrity and reactive oxygen species (ROS) generation were assessed following exposure to 10, 50 and 500 mg L^-1 of the particles for 2 h and 24 h. The particles had a concentration-, material- and time-dependent effect on Chlamydomonas sp., with increased algal growth rate after 24 h. Conversely, significant intracellular ROS levels were detected after 2 h, with much lower levels after 24 h. All Fe-nanomaterials displayed similar Z-average sizes and zeta-potentials at 2 h and 24 h. Effects on Chlamydomonas sp. decreased in the order FerMEG12 > Carbo-Iron® > Fe-zeolite > Nano-Goethite. Ecotoxicological studies were challenged due to some particle properties, i.e. dark colour, effect of constituents and a tendency to agglomerate, especially at high concentrations. All particles exhibited potential to induce significant toxicity at high concentrations (500 mg L^-1), though such concentrations would rapidly decrease to mg or µg L^-1 in aquatic environments, levels harmless to Chlamydomonas sp. The presented findings contribute to the practical usage of particle-based nanoremediation in environmental restoration.

Green Synthesis of Metal and Metal Oxide Nanoparticles and Their Effect on the Unicellular Alga Chlamydomonas reinhardtii

Recently, the green synthesis of metal nanoparticles has attracted wide attention due to its feasibility and very low environmental impact. This approach was applied in this study to synthesise nanoscale gold (Au), platinum (Pt), palladium (Pd), silver (Ag) and copper oxide (CuO) materials in simple aqueous media using the natural polymer gum karaya as a reducing and stabilising agent. The nanoparticles' (NPs) zeta-potential, stability and size were characterised by Zetasizer Nano, UV-Vis spectroscopy and by electron microscopy. Moreover, the biological effect of the NPs (concentration range 1.0-20.0 mg/L) on a unicellular green alga (Chlamydomonas reinhardtii) was investigated by assessing algal growth, membrane integrity, oxidative stress, chlorophyll (Chl) fluorescence and photosystem II photosynthetic efficiency. The resulting NPs had a mean size of 42 (Au), 12 (Pt), 1.5 (Pd), 5 (Ag) and 180 (CuO) nm and showed high stability over 6 months. At concentrations of 5 mg/L, Au and Pt NPs only slightly reduced algal growth, while Pd, Ag and CuO NPs completely inhibited growth. Ag, Pd and CuO NPs showed strong biocidal properties and can be used for algae prevention in swimming pools (CuO) or in other antimicrobial applications (Pd, Ag), whereas Au and Pt lack these properties and can be ranked as harmless to green alga.

Modeling whole body trace metal concentrations in aquatic invertebrate communities: A trait-based approach

Trace metal risk assessment and environmental quality standard definition require realistic models that quantify metal exposure and accumulation by biota. In the present study we propose a novel trait-based approach to predict whole body concentrations of metals in aquatic invertebrates from concentrations measured in different environmental compartments. Field data from a large riverine floodplain was used to calibrate and test the model. The prediction performance of the trait-based model was unbiased and uncertainty was below the twofold of measured concentrations for the four studied metals (Ni, Cu, Cd, Pb). The relative contribution of feeding, respiration and locomotion patterns as well as metal concentrations in three environmental compartments provided insights into the importance of different uptake pathways. The relation with the sediment (i.e., to what degree taxa live in or directly on the sediment) was shown to be the most important trait to predict metal accumulation. Overall, this study demonstrated the potential use of bioecological traits for the modeling of whole body metal concentrations of entire aquatic invertebrate communities.

Effects of two-hour exposure to environmental and high concentrations of methylmercury on the transcriptome of the macrophyte Elodea nuttallii

The effects of two methylmercury (CH₃Hg⁺, MeHg) concentrations, representative of environmental level and extreme contamination, were investigated on the macrophyte Elodea nuttallii during a 2h-exposure combining transcriptomic (RNA-Seq), physiological endpoints (pigment contents, activity of anti-oxidative stress enzymes) and bioaccumulation. Exposure to MeHg induced the up- and down-regulation of numerous genes (4389 and 16853 for 10ngL^-1 and 10μgL^-1 MeHg exposure, respectively) involved in sugar, amino acid and secondary metabolism (e.g. cinnamic acid, flavonoids) at both concentrations. Genes coding for photosynthesis, membrane integrity, metal homeostasis, water transport and anti-oxidative enzymes were additionally up- and down-regulated at the higher concentration. At the physiological level, exposure to both MeHg concentrations resulted in a strong increase of anthocyanin content in shoots. Chlorophyll content and antioxidant enzyme activities were unchanged. The data suggest that the macrophyte was able to efficiently cope with the stress resulting from MeHg exposure, possibly by using anthocyanin as anti-oxidant and S-rich amino acids (such as cysteine and methionine) as chelators. Transcriptomics analysis enabled gaining novel insights on molecular effects of MeHg in primary producers, which are one of the main entry pathway of hazardous MeHg in aquatic food webs.

High contamination in the areas surrounding abandoned mines and mining activities: An impact assessment of the Dilala, Luilu and Mpingiri Rivers, Democratic Republic of the Congo

Abandoned mines and mining activities constitute important sources of toxic metals and Rare Earth Elements (REEs) affecting surrounding environmental compartments and biota. This study investigates the contamination degree and distribution of toxic metals and REEs in contrasting sediment, soil and plant samples surrounding rivers in the African copperbelt area characterized by the presence of numerous abandoned mines, artisanal and industrial mining activities. ICP-MS results highlighted the highest concentration of Cu, Co and Pb in sediments reaching values of 146,801, 18,434 and 899 mg kg^-1, respectively. In soil, the values of 175,859, 21,134 and 1164 mg kg^-1 were found for Cu, Co and Pb, respectively. These values are much higher than the sediment guidelines for the protection of aquatic life and international soil clean-up standards. Enrichment factor and geoaccumulation index results indicated important contribution of mining activities to the study sites pollution in addition to natural background. Highest metal accumulation in leaves of Phalaris arundinacea L., was observed, reaching values of 34,061, 5050 and 230 mg kg^-1 for Cu, Co, and Pb, respectively. The ∑REE concentration reached values of 2306, 733, 2796 mg kg^-1 in sediment, soil and plant samples, respectively. The above results were combined with geographical information including satellite imagery, hydrography and mining concessions. Maps were produced to present the results in a comprehensive and compelling visual format. The results will be disseminated through an innovative mapping online platform to simplify access to data and to facilitate dialogue between stakeholders.

Mercury bioavailability, transformations, and effects on freshwater biofilms

Mercury (Hg) compounds represent an important risk to aquatic ecosystems because of their persistence, bioaccumulation, and biomagnification potential. In the present review, we critically examine state-of-the-art studies on the interactions of Hg compounds with freshwater biofilms, with an emphasis on Hg accumulation, transformations, and effects. Freshwater biofilms contain both primary producers (e.g., algae) and decomposers (e.g., bacteria and fungi), which contribute to both aquatic food webs and the microbial loop. Hence they play a central role in shallow water and streams, and also contribute to Hg trophic transfer through their consumption. Both inorganic and methylated mercury compounds accumulate in biofilms, which could transform them mainly by methylation, demethylation, and reduction. Accumulated Hg compounds could induce diverse metabolic and physiological perturbations in the microorganisms embedded in the biofilm matrix and affect their community composition. The bioavailability of Hg compounds, their transformations, and their effects depend on their concentrations and speciation, ambient water characteristics, biofilm matrix composition, and microorganism-specific characteristics. The basic processes governing the interactions of Hg compounds with biofilm constituents are understudied. The development of novel conceptual and methodological approaches allowing an understanding of the chemo- and biodynamic aspects is necessary to improve the knowledge on Hg cycling in shallow water as well as to enable improved use of freshwater biofilms as potential indicators of water quality and to support better informed risk assessment. Environ Toxicol Chem 2017;36:3194-3205. © 2017 SETAC.

Transcriptomic approach for assessment of the impact on microalga and macrophyte of in-situ exposure in river sites contaminated by chlor-alkali plant effluents

Water quality degradation is a worldwide problem, but risk evaluation of chronic pollution in-situ is still a challenge. The present study aimed to evaluate the potential of transcriptomic analyses in representative aquatic primary producers to assess the impact of environmental pollution in-situ: the microalga Chlamydomonas reinhardtii and the macrophyte Elodea nuttallii were exposed 2 h in the Babeni Reservoir of the Olt River impacted by chlor-alkali plant effluent release resulting in increased concentrations of Hg and NaCl in receiving water. The response at the transcriptomic level was strong, resulting in up to 5485, and 8700 dysregulated genes (DG) for the microalga and for the macrophyte exposed in the most contaminated site, respectively. Transcriptomic response was congruent with the concentrations of Hg and NaCl in the water of the impacted reservoir. Genes involved in development, energy metabolism, lipid metabolism, nutrition, and RedOx homeostasis were dysregulated during in-situ exposure of both organisms. In addition, genes involved in the cell motility of C. reinhardtii and development of the cell wall of E. nuttallii were affected. DG were in line with adverse outcome pathways and transcriptomic studies reported after exposure to high concentrations of Hg and NaCl under controlled conditions in the laboratory. Transcriptomic response provided a sensitive measurement of the exposure as well as hints on the tolerance mechanisms of environmental pollution, and is thus promising as an early-warning tool to assess water quality degradation.

Toward Quantitative Understanding of the Bioavailability of Dissolved Organic Matter in Freshwater Lake during Cyanobacteria Blooming

Occurrence of cyanobacterial harmful algal blooms (CyanoHAB) can induce considerable patchiness in the concentration and bioavailability of dissolved organic matter (DOM), which could influence biogeochemical processes and fuel microbial metabolism. In the present study, a laboratory 4-stage plug-flow bioreactor was used to successfully separate the CyanoHAB-derived DOM isolated from the eutrophic Lake Taihu (China) into continuum classes of bioavailable compounds. A combination of new state-of-the-art tools borrowed from analytical chemistry and microbial ecology were used to characterize quantitatively the temporary evolution of DOM and to get deeper insights into its bioavailability. The results showed a total 79% dissolved organic carbon loss over time accompanied by depletion of protein-like fluorescent components, especially the relatively hydrophilic ones. However, hydrophilic humic-like fluorescent components exhibited bioresistant behavior. Consistently, ultrahigh resolution mass spectrometry (FTICR-MS) revealed that smaller, less aromatic, more oxygenated, and nitrogen-rich molecules were preferentially consumed by microorganisms with the production of lipid-like species, whereas recalcitrant molecules were primarily composed of carboxylic-rich alicyclic compounds. Moreover, the bioavailability of DOM was negatively correlated with microbial community diversity in the bioreactor. Results from this study provide deeper insights into the fate of DOM and relevant biogeochemical processes in eutrophic lakes.

Lateral and longitudinal patterns of water physico-chemistry and trace metal distribution and partitioning in a large river floodplain

Floodplain water bodies provide habitat to diversified ecological communities. Floodplains are also among the most impacted aquatic ecosystems. While the link between the lateral connectivity of floodplain sites to the main channel and their plant, fish and invertebrate communities has been well established, detailed information on chemical characteristics and particularly on trace metal spatial distribution and partitioning is scarce. The aim of this study was to examine the link between the lateral connectivity and physico-chemical variables, their trace metal concentrations and partitioning as well as the upstream-downstream gradient of these parameters. In connected and disconnected water bodies of the Rhône River upstream and downstream of the city of Lyon, we measured major ions, dissolved organic carbon, trace metal concentrations (Al, Cr, Ni, Cu, Zn, Cd, Pb and U) in water, suspended particulate matter (SPM) and sediment. The results revealed a clear difference between connected and disconnected water bodies. pH, SPM, Na⁺, and NO₃^- concentrations were lower in disconnected sites while conductivity, DOC, Ca²⁺ and Mg²⁺ were higher. Conductivity and a part of the major ion concentrations increased in the downstream sections. Trace metal concentrations and partitioning varied between connected and disconnected sites. In the dissolved fraction, trace metal concentrations were higher in connected sites. In the surface sediment, concentrations were higher in disconnected sites for the majority of metals. The upstream-downstream gradient was less important than the connected-disconnected gradient. Only three metals in the dissolved fraction (Cu, Cd and Pb) showed a clear increase in downstream sections. Overall, the study shows that the functioning of floodplains produces strong spatial patterns concerning the concentrations and partitioning of trace metals. These findings improve our understanding of trace metal biogeochemistry in floodplains and have important applications for floodplain restoration projects.

Biofilm composition in the Olt River (Romania) reservoirs impacted by a chlor-alkali production plant

Freshwater biofilms can be useful indicators of water quality and offer the possibility to assess contaminant effects at the community level. The present field study examines the effects of chlor-alkali plant effluents on the community composition of biofilms grown in the Olt River (Romania) reservoirs. The relationship between ambient water quality variables and community composition alterations was explored. Amplicon sequencing revealed a significant modification of the composition of microalgal, bacterial and fungal communities in the biofilms collected in the impacted reservoirs in comparison with those living in the uncontaminated control reservoir. The abundance corrected Simpson index showed lower richness and diversity in biofilms collected in the impacted reservoirs than in the control reservoir. The biofilm bacterial communities of the impacted reservoirs were characterized by the contaminant-tolerant Cyanobacteria and Bacteroidetes, whereas microalgal communities were predominantly composed of Bacillariophyta and fungal communities of Lecanoromycetes and Paraglomycetes. A principal component analysis revealed that major contaminants present in the waste water of the chlor-alkali production plant, i.e. Na⁺, Ca²⁺, Cl^- and Hg, were correlated with the alteration of biofilm community composition in the impacted reservoirs. However, the biofilm composition was also influenced by water quality variables such as NO₃^-, SO₄^2-, DOC and Zn from unknown sources. The results of the present study imply that, even when below the environmental quality standards, typical contaminants of chlor-alkali plant releases may affect biofilm composition and that their impacts on the microbial biodiversity might be currently overlooked.

Influence of chemical speciation and biofilm composition on mercury accumulation by freshwater biofilms

Mercury (Hg) is a pollutant of high concern for aquatic systems due to the biomagnification of its methylated form along the food chain. However, in contrast to other metals, gaining knowledge of its bioavailable forms for aquatic microorganisms remains challenging, making Hg risk assessment difficult. Ubiquitous and sessile freshwater biofilms are well known to accumulate and to transform Hg present in their ambient environment. The present study thus aims to evaluate whether non-extractable (proxy of intracellular) Hg accumulated by biofilms could be a good indicator of Hg bioavailability for microorganisms in freshwater. To that end, the link between Hg concentration and speciation, as well as biofilm composition (percentage of abiotic, biotic, chlorophyll and phycocyanin-fractions and abundance of dsrA, gcs, merA and hgcA bacterial genes) and biofilm Hg accumulation was examined. The studied biofilms were grown on artificial substrata in four reservoirs along the Olt River (Romania), which was contaminated by Hg coming from chlor-alkali plant effluents. The 0.45 μm-filterable Hg concentrations in ambient waters were measured and inorganic IHg speciation was modelled. Biofilms were analyzed for their non-extractable IHg and methylmercury (MeHg) contents as well as for their composition. The non-extractable IHg content was related, but not significantly, to the concentration of total IHg (r² = 0.88, p = 0.061) whereas a significant correlation was found with the predicted IHg concentration that is not bound to dissolved organic matter (r² = 0.95, p = 0.027), despite its extremely low concentrations (10^-25 M), showing a limitation of the thermodynamic Hg modelling to predict Hg bioavailability. The studied biofilms were different in biomass and composition and a principal component analysis showed that the non-extractable IHg content correlated with the abundance of the merA and hgcA genes, while MeHg accumulation was only linked with the abundance of the rRNA 16S gene. The present study suggests that non-extractable IHg concentrations in biofilms are a useful proxy of IHg bioavailable forms in waters whereas the hgcA and merA genes are good biomarkers of both biofilm IHg exposure and bioavailability.

Alternating Current-Dielectrophoresis Collection and Chaining of Phytoplankton on Chip: Comparison of Individual Species and Artificial Communities

The capability of alternating current (AC) dielectrophoresis (DEP) for on-chip capture and chaining of the three species representative of freshwater phytoplankton was evaluated. The effects of the AC field intensity, frequency and duration on the chaining efficiency and chain lengths of green alga Chlamydomonas reinhardtii, cyanobacterium Synechocystis sp. and diatom Cyclotella meneghiniana were characterized systematically. C. reinhardtii showed an increase of the chaining efficiency from 100 Hz to 500 kHz at all field intensities; C. meneghiniana presented a decrease of chaining efficiency from 100 Hz to 1 kHz followed by a significant increase from 1 kHz to 500 kHz, while Synechocystis sp. exhibited low chaining tendency at all frequencies and all field intensities. The experimentally-determined DEP response and cell alignment of each microorganism were in agreement with their effective polarizability. Mixtures of cells in equal proportion or 10-times excess of Synechocystis sp. showed important differences in terms of chaining efficiency and length of the chains compared with the results obtained when the cells were alone in suspension. While a constant degree of chaining was observed with the mixture of C. reinhardtii and C. meneghiniana, the presence of Synechocystis sp. in each mixture suppressed the formation of chains for the two other phytoplankton species. All of these results prove the potential of DEP to discriminate different phytoplankton species depending on their effective polarizability and to enable their manipulation, such as specific collection or separation in freshwater.

Silver nanoparticle behaviour in lake water depends on their surface coating

The present study examines the stability of silver nanoparticles (AgNPs) of three different coatings - citrate (CIT), polyvinyl pyrrolidone (PVP) and lipoic acid (LIP) and two sizes - 20 and 50nm in lake water (LW) over time. Using a combination of asymmetric flow field-flow fractionation (AsFlFFF), surface plasmon resonance (SPR), and single particle inductively coupled plasma mass spectrometry (SP-ICP-MS), the influence of size, surface coating, exposure time, as well as the presence and nature of dissolved organic matter (DOM) on the transformation of AgNPs at low environmental concentrations was thoroughly investigated. The results revealed that the AgNP stability in lake water are complex interplay between the surface coating characteristics, exposure time and presence and nature of DOM. Among the studied variables surface coating was found to play the major role of determining AgNPs behaviour in lake water. PVP-coated AgNPs agglomerated to a lesser extent as compared with the CIT- and LIP-AgNPs. For a given surface coating, DOM of pedogenic and aquagenic origin increased the stability of the AgNPs (LW+EPS>LW+SRHA>LW). Moreover, extracellular polymeric substances (EPS; aquagenic origin) stabilized lipoic acid-coated AgNPs more effectively than Suwannee River Humic Acids (SRHA; pedogenic origin), showing that DOM nature has to be also considered for improved understanding the AgNP stability in aquatic environment.

Role of cellular compartmentalization in the trophic transfer of mercury species in a freshwater plant-crustacean food chain

Mercury (Hg) represents an important risk for human health through the food webs contamination. Macrophytes bioaccumulate Hg and play a role in Hg transfer to food webs in shallow aquatic ecosystems. Nevertheless, the compartmentalization of Hg within macrophytes, notably major accumulation in the cell wall and its impact on trophic transfer to primary consumers are overlooked. The present work focusses on the trophic transfer of inorganic Hg (IHg) and monomethyl-Hg (MMHg) from the intracellular and cell wall compartments of the macrophyte Elodea nuttallii - considered a good candidate for phytoremediation - to the crustacean Gammarus fossarum. The results demonstrated that Hg accumulated in both compartments was trophically bioavailable to gammarids. Besides IHg from both compartments were similarly transferred to G. fossarum, while for MMHg, uptake rates were ∼2.5-fold higher in G. fossarum fed with the cell wall vs the intracellular compartment. During the depuration phase, Hg concentrations in G. fossarum varied insignificantly suggesting that both IHg and MMHg were strongly bound to biological ligands in the crustacean. Our data imply that cell walls have to be considered as an important source of Hg to consumers in freshwater food webs when developing procedures for enhancing aquatic environment protection during phytoremediation programs.

Elodea nuttallii exposure to mercury exposure under enhanced ultraviolet radiation: Effects on bioaccumulation, transcriptome, pigment content and oxidative stress

The hypothesis that increased UV radiation result in co-tolerance to Hg toxicity in aquatic plants was studied at the physiological and transcriptomic level in Elodea nuttallii. At the transcriptomic level, combined exposure to UV+Hg enhanced the stress response in comparison with single treatments, affecting the expression level of transcripts involved in energy metabolism, lipid metabolism, nutrition, and redox homeostasis. Single and combined UV and Hg treatments dysregulated different genes but with similar functions, suggesting a fine regulation of the plant to stresses triggered by Hg, UV and their combination but lack of co-tolerance. At the physiological level, UV+Hg treatment reduced chlorophyll content and depleted antioxidative compounds such as anthocyanin and GSH/GSSG in E. nuttallii. Nonetheless, combined exposure to UV+Hg resulted in about 30% reduction of Hg accumulation into shoots vs exposure to Hg alone, which was congruent with the level of expression of several transporter genes, as well as the UV effect on Hg bioavailability in water. The findings of the present work underlined the importance of performing experimentation under environmentally realistic conditions and to consider the interplay between contaminants and environmental variables such as light that might have confounding effects to better understand and anticipate the effects of multiple stressors in aquatic environment.

Environmental quality assessment of reservoirs impacted by Hg from chlor-alkali technologies: case study of a recovery

Mercury (Hg) pollution legacy of chlor-alkali plants will be an important issue in the next decades with the planned phase out of Hg-based electrodes by 2025 within the Minamata convention. In such a context, the present study aimed to examine the extent of Hg contamination in the reservoirs surrounding the Oltchim plant and to evaluate the possible improvement of the environmental quality since the closure of its chlor-alkali unit. This plant is the largest chlor-alkali plant in Romania, which partly switched to Hg-free technology in 1999 and definitely stopped the use of Hg electrolysis in May 2012. Total Hg (THg) and methylmercury (CH₃Hg) concentrations were found to decrease in the surface waters and sediments of the reservoirs receiving the effluents of the chlor-alkali platform since the closure of Hg units. Hence, calculated risk quotients (RQ) indicated no adverse effect of Hg for aquatic organisms from the ambient water exposure. RQ of Hg in sediments were mostly all higher than 1, showing important risks for benthic organisms. However, ecotoxicity testing of water and sediments suggest possible impact of other contaminants and their mixtures. Hg hotspots were found in soils around the platform with RQ values much higher than 1. Finally, THg and CH₃Hg concentrations in fish were below the food safety limit set by the WHO, which contrasts with previous measurements made in 2007 revealing that 92 % of the studied fish were of high risk of consumption. Discontinuing the use of Hg electrodes greatly improved the surrounding environment of chlor-alkali plants within the following years and led to the decrease environmental exposure to Hg through fish consumption. However, sediment and soil still remained highly contaminated and problematic for the river reservoir management. The results of this ecological risk assessment study have important implications for the evaluation of the benefits as well as limits of the Minamata Convention implementation.

A Multimethod Approach for Investigating Algal Toxicity of Platinum Nanoparticles

The ecotoxicity of platinum nanoparticles (PtNPs) widely used in for example automotive catalytic converters, is largely unknown. This study employs various characterization techniques and toxicity end points to investigate PtNP toxicity toward the green microalgae Pseudokirchneriella subcapitata and Chlamydomonas reinhardtii. Growth rate inhibition occurred in standard ISO tests (EC₅₀ values of 15-200 mg Pt/L), but also in a double-vial setup, separating cells from PtNPs, thus demonstrating shading as an important artifact for PtNP toxicity. Negligible membrane damage, but substantial oxidative stress was detected at 0.1-80 mg Pt/L in both algal species using flow cytometry. PtNPs caused growth rate inhibition and oxidative stress in P. subcapitata, beyond what was accounted for by dissolved Pt, indicating NP-specific toxicity of PtNPs. Overall, P. subcapitata was found to be more sensitive toward PtNPs and higher body burdens were measured in this species, possibly due to a favored binding of Pt to the polysaccharide-rich cell wall of this algal species. This study highlights the importance of using multimethod approaches in nanoecotoxicological studies to elucidate toxicity mechanisms, influence of NP-interactions with media/organisms, and ultimately to identify artifacts and appropriate end points for NP-ecotoxicity testing.