Influence of humic substances on the toxic effects of cadmium and zinc to the green alga Pseudokirchneriella subcapitata

Systematization of a toxicity screening method based on a combination of chemical analysis and the delayed fluorescence algal growth inhibition test for use in emergency environmental surveys

In recent years, heavy rainfall disasters linked to climate change have become more frequent, raising concerns about the release of chemicals stored in factories. Assessing chemical contamination during such emergencies therefore necessitates the development of a quick and easy method for evaluating hazardous contaminants in combination with toxicity testing. This study proposes a "toxicity screening" method that combines biological response testing and chemical analysis to systematically evaluate hazardous contaminants in emergency situations. The toxicity screening method evaluates the water quality in three steps, including water quality measurements and a delayed fluorescence (DF) assay, metal content measurements and a DF assay, and targeted screening analysis and a DF assay. The efficacy of this method was tested using industrial wastewater from 14 locations. Seven of the samples were non-toxic, while the other seven samples were toxic, displaying no observed effect concentration (NOEC) values ranging from 0.625 to 20%. Two toxic samples in the first phase possessed high total chlorine concentrations (0.4 mg L-1) and conductivities (2200 mS m-1), indicating that the main sources of toxicity were residual chlorine and a high salt concentration. In the second phase, metal content analysis identified metals as the toxicity cause in four samples. In the third phase, the organic contaminants were analyzed, and tri-n-octyl phosphate (TNOP) was detected at a concentration of 0.00027 mg L-1. The results of solid-phase extraction experiments and exposure tests with TNOP alone indicated that the contribution of TNOP to the toxicity was negligible and that chemicals not adsorbed on the solid-phase extraction cartridges were the cause of toxicity. The proposed method can therefore be considered effective for disaster-related water quality assessment, delivering results within 12 days.

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.

Impact of aquatic humic substances on speciation and toxicity of arsenic and cobalt to Ceriodaphnia dubia

Humic substances (HS) interact with trace metals such as As and Co, affecting their mobility and availability in aquatic systems. However, their combined effects on toxicity to aquatic organisms are not totally understood. The objective of this study was to evaluate the toxicity of Co(II) and As(III) to the water flea Ceriodaphnia dubia in the presence of HS, considering element speciation. Toxicity assays were performed in the presence and absence of HS at two different concentrations of As(III) (10 and 20 μg/L) and Co(II) (50 and 100 μg/L). The free As(III) and Co(II) (< 1 kDa, fraction most potentially bioavailable) in the test solutions were determined via ultrafiltration. While free Co(II) decreased by approximately 80% in the presence of HS, free As(III) decreased just by 1%. Despite the higher percentage of As(III) potentially bioavailable, the presence of HS reduced significantly the toxicity of As at 20 μg/L (no toxicity was observed at 10 μg/L). This was attributed to direct effects of HS such as hormesis, hormone-like effects of HS and/or formation of protective coating. These effects also stimulated the reproduction, including in the assays in the absence of As and Co. HS reduced the toxicity of Co(II) at both test concentrations. The results of this investigation support that HS should be considered when safe limits for As and Co are defined.

Photoinduced production of substances with humic-like fluorescence, upon irradiation of water samples from alpine lakes

Evidence is here provided that irradiation of some lake water samples can trigger the formation of fluorophores with humic-like properties, at the same time increasing water absorbance. This phenomenon is the opposite of photobleaching, which is often observed when natural waters are irradiated. The photoproduced humic-like fluorophores observed here would be of autochthonous rather than allochthonous origin, which marks a difference with the fraction of humic substances that derives from terrestrial sources. Photogeneration of humic-like compounds can be highlighted in water samples where the fluorescence signal of initially occurring humic substances is low, so that their photobleaching is minimised. Samples that are most likely to show photoinduced formation of humic-like fluorophores are in fact characterised by high values of protein-like vs. humic-like contribution ratios to fluorescence, as evidenced by parallel factor (PARAFAC) analysis. Mountain lakes in late summer appear to be suitable candidates to highlight the described phenomenon. In some cases, lake-water irradiation caused a decrease in the spectral slope of the absorbance that, together with increasing absorbance values, is consistent with an increase in molecular mass and aromaticity of organic matter. The absorbance increase triggered by irradiation might play a role in screening biologically harmful UV radiation, in mountain environments that would otherwise be characterised by very clear water that allows for easy transmission of UV light along the water column.

Bioleaching of rare earths elements from phosphate rock using Acidothiobacillus ferrooxidans

Phosphate rock containing rare earth elements (REEs) is considered one of the most promising potential secondary sources of REEs, as evidenced by large tonnages of phosphate rock mined annually. The bioleaching of REEs from phosphate rock using A. ferrooxidans was done for the first time in this study, and it was found to be greater than abiotic leaching and was more environmentally friendly. The result showed that the total leaching rate of REEs in phosphate rock was 28.46% under the condition of 1% pulp concentration and pH=2, and the leaching rates of four key rare earths, Y, La, Ce, and Nd, were 35.7%, 37.03%, 27.92%, and 32.53%, respectively. The bioleaching process was found to be accomplished by bacterial contact and Fe²⁺ oxidation. The blank control group which contained Fe²⁺ was able to leach some of the rare earths, indicating that the oxidation of Fe²⁺ may affect the leaching of rare earths. X-Ray Diffraction (XRD)analysis showed that the minerals were significantly altered and the intensity of the diffraction peaks of dolomite and apatite decreased significantly after microbial action compared to the blank control, and it was observed that bacteria adhere to the mineral surface and the minerals become smooth and angular after bioleaching by Scanning electron microscope (SEM), indicating that bacteria have a further effect on the rock based on Fe²⁺ oxidation.Finally.Fourier Transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission matrix (3DEEM) fluorescence spectra analysis showed that extracellular polymeric substances (EPS) participate in the bioleaching process.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

Differences in bioaccumulation of Ni and Zn by microalgae in the presence of fulvic acid

In the presence of dissolved organic matter, the mechanism of algal bioaccumulation of different metals is complex, and its significance goes far beyond the alga-metal binary system. In the presence of 10 and 20 mg L^-1 fulvic acid (FA), the maximum tolerance concentrations of Chlorella pyrenoidosa to Ni were 0.25 and 0.26 mmol L^-1, and to Zn were 0.62 and 0.68 mmol L^-1, respectively. Within the maximum tolerance concentration ranges, the bioaccumulation behaviors of Ni and Zn were systematically compared in the presence of FA. The presence of FA shortened the adsorption equilibrium time and decreased the maximum bioaccumulation capacity of Ni and Zn. The bioaccumulation mechanism of Ni by C. pyrenoidosa was more inclined to monolayer adsorption, while the bioaccumulation mechanism of Zn was more inclined to multilayer adsorption. More details were revealed after the bioaccumulated metals were separated into adsorption and internalization states by 0.01 M EDTA elution. The presence of FA decreased more adsorbed Zn than the adsorbed Ni, due to the different competitive roles of FA in the ternary system of Ni and Zn, but the presence of FA increased the internalized Ni might due to the stronger complexation of Ni-FA. This research indicated that algae had unique bioaccumulation mechanisms for different metals in the presence of FA, which is of great significance to accurately evaluate the ecological risk posed by heavy metals.

Effects of cadmium and cobalt mixtures on growth and photosynthesis of Raphidocelis subcapitata (Chlorophyceae)

Metals occur simultaneously in the environment, and therefore it is important to know their toxicity and mechanism of action when associated with another metal. Furthermore, anthropogenic actions increase their concentrations in the environment where they can interact and undergo transformations that can even increase their toxicity. This study aimed to evaluate the effects of cadmium (Cd) and cobalt (Co), isolated and combined, on the microalgae Raphidocelis subcapitata. Regarding the toxicity of isolated metals, the IC₅₀96 h was 0.08 mg L^-1 of Cd and 0.16 mg L^-1 of Co. Cell density decreased at all concentrations of the Cd tested. The parameters related to cell size, cell complexity and mean cell chlorophyll a (Chl a) fluorescence were significantly affected by both metals. According to species sensitivity curves (SSD), the microalgae R. subcapitata was the second most sensitive organism to Co exposure and the tenth concerning Cd. Metal mixture data were best fitted to the concentration addition (CA) model and dose-ratio dependence (DR) deviation, showing synergism at high concentrations of Co and low concentrations of Cd. Besides that, antagonism was observed at low concentrations of Co and high concentrations of Cd. Photosynthetic performance, assessed by maximum quantum yield (Φ_M) and oxygen evolving complex (OEC), presented antagonism effects for both analyzed parameters. Thus, the mixture of Cd and Co showed synergistic and antagonistic interactions for the parameters analyzed in R. Subcapitata, indicating the importance of understanding the mechanisms of toxicity of metal mixtures in phytoplankton.

A designed moderately thermophilic consortia with a better performance for leaching high grade fine lead-zinc sulfide ore

Unwieldy fine sulfide ores are produced during mining; without being appropriately disposed of, they can cause environmental pollution and waste resources. This study investigated the leaching performance of a moderately thermophilic consortia (Leptospirillum ferriphilum + Acidithiobacillus caldus + Sulfobacillus benefaciens) for fine lead-zinc sulfide raw ore. The results showed this microbial community created a low pH, high ORP, and high cell concentration environment for mineral leaching, improving bioleaching efficiency. Under the action of this consortia, the zinc leaching rate reached 96.44 in 8 days, and reached 100% after 12 days. EPS analysis indicated that the consortia could mediate the secretion of more polysaccharides to ensure leaching efficiency. EPS levels and amino acids were the main factors affecting bioleaching. An analysis of mineral surface characteristics showed the consortia effectively leached pyrite and sphalerite from the fine sulfide ore, and prevented the mineral surface forming the jarosite that could hinder bioleaching. This study found that bioleaching reduced the potential environmental toxicity of the minerals, providing an important reference for guiding the bioleaching of unwieldy fine sulfide raw ore.

Responses of benthic diatoms to waters affected by post-fire contamination

Wildfire effects go beyond direct impact in terrestrial ecosystems. Specifically, the periphytic communities of aquatic ecosystems standing within and downstream the burnt areas are relevant ecological receptors of post-fire runoff contamination. Nevertheless, the off-site impacts of wildfires in these communities are limitedly studied so far. The present study aimed to assess the effects of river water contaminated with ash-loaded runoff in the growth benthic diatom Navicula libonensis (Schoeman 1970). Four surface water samples were collected approximately one year after the wildfire for laboratory testing with the diatom: one was collected from a site upstream the burnt area, within the Unhais river (UU); three were collected from sites standing within the burnt area, one in the Unhais river (UB) and two in the Zêzere river (Z1 and Z2), reflecting different hydrological regimes. N. libonensis was proven able to discriminate among river sites affected and unaffected by wildfire runoff, reflecting, in general, the expected trends considering the physico-chemical characterization of the water samples. The water samples from the sites standing within the burnt area inhibited the biomass yield and growth rate of the tested diatom, ranking the samples regarding toxicity as follows: Z1 > UB > Z2 > UU. However, UB rather than Z1 presented the highest contaminant burden, namely metal elements, and some were found above widely accepted safety benchmarks (polycyclic aromatic hydrocarbons were not detected). This inconsistency can be linked to unknown interactions among metals within each water sample, to differential nutrient enrichment of samples, as well as hydrological factors. Overall, our results suggest that monospecific laboratory assays with sensitive diatoms can be valuable as cost-effective screening tools to prioritize sites affected by wildfires runoff requiring in-depth monitoring of negative effects in benthic producer communities.

Generation behavior of extracellular polymeric substances and its correlation with extraction efficiency of valuable metals and change of process parameters during bioleaching of spent petroleum catalyst

The vital functions of extracellular polymeric substances (EPS) have been well recognized in bioleaching of sulfide ores. However, no report is available about the role of EPS in bioleaching of spent catalyst. To completely and deeply understand the functions of EPS in bioleaching of spent catalyst, the generation behavior of EPS at various pulp densities during bioleaching was characterized by three-dimensional excitation-emission matrix (3DEEM), and its relevance with bioleaching performance and process parameters were analyzed using mathematical means. The results showed that the EPS contain humus-like substances as main component (>70%) and protein-like substances as minor component (<30%). Both total EPS and humus-like substances mainly keep growing over the whole duration of bioleaching at low pulp density of 5.0% or lower; whereas total EPS and humus-like fraction keep declining at high pulp density of 7.5% or higher. Among the total EPS and its components, humus-like substances only have a positive significant correlation with bioleaching efficiencies of both Co and Mo and affect bioleaching process more greatly due to greater correlation coefficient. Biofilm appears at the spent catalyst surface under 2.5% of pulp density mediated by EPS while no biofilm occurs at 10% of pulp density due to shortage of EPS, accounting for the great difference in bioleaching efficiencies between high and low pulp densities which are 48.3% for Mo and 50.0% for Co at 10% of pulp density as well as 75.9% for Mo and 78.8% for Co at 2.5% of pulp density, respectively.

Zinc biosorption by Dunaliella sp. AL-1: Mechanism and effects on cell metabolism

Phycoremediation is being considered as an eco-friendly and safe technology for toxics eradication from contaminated aquatic systems. The zinc biosorption capacity of Dunaliella sp. AL-1 was demonstrated. Zinc impacted cell growth and photosynthetic pigments accumulation showing exposure time and concentration-dependent effects. The investigation of the antioxidant protective response to zinc exposition proved a stimulation of guaiacol peroxidase (GPX) activity and an increased rate of total phenolics, flavonoids, condensed tannins and glutathione (GSH). The Box-Behnken design was used to optimize zinc removal conditions by Dunaliella sp. AL-1 strain. The maximum experimental zinc uptake was obtained when zinc concentration, algae dose, initial pH, and contact time were set at 25 mg/L, 0.5 g/L, 7.59 and 13 h 43 min, respectively. Under completely optimized conditions, the fraction of zinc removed intracellularly was much lower than the adsorbed on the cell surface. FTIR analysis Dunaliella sp. AL-1 biomass demonstrated that several functional groups as OH, CH₂, CO, PO, COO and CO may participate in the biosorption process. A comparative proteomic analysis through nano-HPLC coupled to LC-MS/MS, was performed from pre- and post-zinc treatments cells. Among 199 identified proteins, 60 were differentially expressed of which 41 proteins were down-regulated against 19 up-regulated ones. Target proteins have been demonstrated to be implicated in different metabolic processes mainly photosynthesis and antioxidant defenses.

Bioleaching for detoxification of waste flotation tailings: Relationship between EPS substances and bioleaching behavior

The production of large volumes of waste flotation tailings results in environmental pollution and presents a major ecological and environmental risk. This study investigates bioleaching of waste flotation tailings using Acidithiobacillus ferrooxidans. The experiments were performed with 5.00% solid concentration, pH 2.0 with 100 mL medium for 25 d in the lab. The pH, OPR, metal concentration, dissolved organic matter (DOM) in leachate and extracellular polymeric substances (EPS) were recorded. Bioleaching tailing materials were finally characterized. Results showed that microorganisms, acclimating with mine tailings, effectively accelerated the bioleaching process, achieving maximum Zn and Fe extraction efficiencies of 95.45% and 83.98%, respectively, after 25 days. Compared with raw mine tailings, bioleaching could reduce 96.36% and 95.84% leachable Zn and Pb, and Pb presented a low risk (4.13%), while Zn, Cu, and Cr posed no risk (0.34%, 0.64%, and 0%). Toxicity and environmental risk analysis revealed bioleaching process significantly reduced the environmental risk associated with mine tailings. EPS analysis indicated that the loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS) fractions contained different organic substances, which played different roles in the bioleaching process. Pearson correlation analysis revealed that EPS was highly correlated with bioleaching behavior (p < 0.05), and EPS was the main factor affecting the bioleaching process, promoting bioleaching in the LB-EPS and TB-EPS fractions.

Using multiple endpoints to assess the toxicity of cadmium and cobalt for chlorophycean Raphidocelis subcapitata

Metals may cause damage to the biota of contaminated environments. Moreover, using multiple endpoints in ecotoxicological studies is useful to better elucidate the mechanisms of toxicity of these compounds. Therefore, this study aimed to evaluate the effects of cadmium (Cd) and cobalt (Co) on growth, biochemical and photosynthetic parameters of the microalgae Raphidocelis subcapitata, through quantification of lipid classes composition, chlorophyll a (Chl a) content, maximum (Φ_M) and effective (Φ'_M) quantum yields and efficiency of the oxygen-evolving complex (OEC). Both metals affected the algal population growth, with an IC_50-96h of 0.67 and 1.53 μM of Cd and Co, respectively. Moreover, the metals led to an increase in the total lipid content and reduced efficiency of OEC and Φ_M. Cell density was the most sensitive endpoint to detect Cd toxicity after 96 h of treatment. Regarding Co, the photosynthetic parameters were the most affected and the total lipid content was the most sensitive endpoint as it was altered by the exposure to this metal in all concentrations. Cd led to increased contents of the lipid class wax esters (0.89 μM) and phospholipids (PL - at 0.89 and 1.11 μM) and decreased values of triglycerides (at 0.22 μM) and acetone-mobile polar lipids (AMPL - at 0.44 and 1.11 μM). The percentage of free fatty acids (FFA) and PL of microalgae exposed to Co increased, whereas AMPL decreased in all concentrations tested. We were able to detect differences between the toxicity mechanisms of each metal, especially how Co interferes in the microalgae at a biochemical level. Furthermore, to the best of our knowledge, this is the first study reporting Co effects in lipid classes of a freshwater Chlorophyceae. The damage caused by Cd and Co may reach higher trophic levels, causing potential damage to the aquatic communities as microalgae are primary producers and the base of the food chain.

Effect of humic acids on the toxicity of pollutants to Chlamydomonas reinhardtii: Investigation by a microscale algal growth inhibition test

Dissolved humic substances (DHSs) are the major components of organic matter in the aquatic environment. DHSs are well known to considerably affect the speciation, solubility, and toxicity of a wide variety of pollutants in the aquatic environment. In this study, the effects of the toxicity of heavy metals and hydrophobic organic pollutants (HOPs) on Chlamydomonas reinhardtii in the presence of humic acid (HA) were examined by a microscale algal growth inhibition (μ-AGI) test based on spectrophotometric detection. To clarify the relationship between the chemical properties of HAs and the toxicity change of pollutants, eight HAs from different sources were prepared and used. HAs were responsible for mitigating the toxicity of Hg, Cu, pesticides (γ-HCH, 2,4-D, and DDT), and polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (Nap), anthracene (Ant), and benzo[a]pyrene (BaP). In particular, an approximately 100-fold decrease in the toxicity of BaP was observed in the presence of 10 ppm HAs extracted from tropical peat. The results indicated that the carboxylic group content and the HA molecular weight are correlated to the changes in the heavy metal toxicity. For HOPs, the aromaticity and polarity of HAs are crucial for mitigating their toxicity. Furthermore, it was clearly shown that the lake water including a high concentration of DHSs collected from Central Kalimantan, Indonesia, reduced the toxicity of Hg and γ-HCH on Chlamydomonas reinhardtii. Graphical abstract.

Effects of chemical interaction of nutrients and EDTA on metals toxicity to Pseudokirckneriella subcapitata

We studied the effect of the chemical interaction of nutrients and the ethylenediamine tetraacetic acid (EDTA) on metals toxicity. Growth inhibition tests of Pseudokirchneriella subcapitata by nutrient metals copper (Cu) and zinc (Zn), and the non-nutrient metal lead (Pb), were performed. The high-enriched Bold's Basal medium (BBm) and two low-enriched standard media, recommended by the Organization for Economic Cooperation and Development (OECDm) and Environmental Protection Agency-algal assay procedure medium (AAPm), were used in this study. The metals toxicity was affected by the interaction of nutrients and EDTA. Cu⁺² was more toxic in the OECDm (EC₅₀ 20.3 μg/L), while Pb⁺² (EC₅₀ 23.1 μg/L) and Zn⁺² (EC₅₀ 99.4 μg/L) in the AAPm. Non-toxic effect of these metals was observed in BBm, but the exclusion of EDTA shifted it into a toxic medium. Finally, we found that the toxicity of the studied nutrient metals is mainly influenced by EDTA, which reduced the concentration of ionized metals, while the toxicity of the non-nutrient metal is affected by EDTA and phosphates.

Remediation of Cd, Pb and as Co-contaminated Paddy Soil by Applying Different Amendments

The current study investigated the efficiency of sepiolite (SE), sodium humate (HS), microbial fertilizer (JF) and SE combined with JF/HS in a ratio of 2:1 (w/w) (JF-2SE and HS-2SE) on Cd, Pb and As bioavailability in field trials with rice (Oryza sativa L.). The results showed that all the amendments remarkably decreased (p < 0.05) the contents of available Cd and available Pb in soil. Only JF-2SE treatment reduced available As concentration in soil. All the amendments were found to effectively reduce (p < 0.05) the contents of As in brown rice. Both JF-2SE and HS-2SE co-applications reduced the concentrations of Cd in brown rice to 0.108 and 0.135 mg kg^-1, and that of Pb reduced to 0.2 and 0.175 mg kg^-1, which met the national standard limit of China. Thus, the co-application of JF/HS-2SE can be a promising remediation strategy in Cd, Pb and As co-contaminated paddy soil.

Modifying effects of leaf litter extracts from invasive versus native tree species on copper-induced responses in Lemna minor

Invasive plant species tend to migrate from their native habitats under favourable climatic conditions; therefore, trophic and other relationships in ecosystems are changing. To investigate the effect of natural organic matter derived from native Alnus glutinosa tree species and from invasive in Lithuania Acer negundo tree species on copper toxicity in Lemna minor, we analysed the dynamics of Cu binding in aqueous leaf litter extracts (LLE) and plant accumulation, morphophysiological parameters, and antioxidative response. The results revealed that A. glutinosa LLE contained polyphenols (49 mg pyrogallol acid equivalent (PAE)/g DM) and tannins (7.5 mg PAE/g DM), while A. negundo LLE contained only polyphenols (23 mg PAE/g DM). The ability of LLE to bind Cu increased rapidly over 1.5-3 h to 61% and 49% of the total Cu concentration (6.0 ± 0.9 mg/L), respectively for A. glutinosa (AG) and A. negundo (AN), then remained relatively stable until 48 h. At the same time, L. minor accumulated 384, 241 or 188 µg Cu/g FW when plants were exposed to Cu (100 µM CuSO4), Cu with 100 mg/L dissolved organic carbon (DOC) from either AG LLE or AN LLE, accordingly. Catalase (CAT) and guaiacol peroxidase (POD) played a dominant role in hydrogen peroxide scavenging when plants were exposed to Cu and 10 or 100 mg/L DOCAG mixtures in both the first (up to 6h) and the second (6-48 h) response phases. Due to functioning of oxidative stress enzymes, the levels of the lipid peroxidation product malondialdehyde (MDA) reduced in concentration-dependent manner, compared to Cu treatment. When combining Cu and DOCAN treatments, the most sensitive enzymes were POD, ascorbate peroxidase and glutathione reductase. Their activities collectively with CAT were sufficient to reduce MDA levels to Cu-induced in the initial, but not the second response phase. These data suggest that leaf litter extracts of different phenolic compositions elicited different antioxidant response profiles resulting in different reductions of Cu stress, thus effecting L. minor frond and root development observed after seven days. The complex data from this study may be useful in modelling the response of the aquatic ecosystem to a changing environment.

A new strategy on biomining of low grade base-metal sulfide tailings

This study investigated the effect of designed microbial consortia on biomining of low grade base-metal sulfide tailings. The results show the amount of recycled metals were equal if the tailings were leached by mixed cultures of Leptospirillum ferriphilum and Acidithiobacillus thiooxidans at three different ratios or by pure culture of L. ferriphilum, which was better than the pure culture of Acidithiobacillus ferrooxidans. qPCR (quantitative polymerase chain reaction) demonstrated only L. ferriphilum functioned in the mixtures at initial stage. The results of extracellular polymeric substances (EPS) via three-dimensional excitation-emission matrix combined with parallel factor analysis (3DEEM-PARAFAC) collected from mixed or pure cultures indicated there were no interactions between two strains. Secondary minerals were formed, but did not influence the leaching process. A new strategy for tailings biomining was proposed: only ferrous oxidizers should be added during the initial and middle biomining stage, while sulfur oxidizers should be added at the end.

Divalent Mercury in Dissolved Organic Matter Is Bioavailable to Fish and Accumulates as Dithiolate and Tetrathiolate Complexes

The freshwater cyprinid Tanichthys albonubes was used to assess the bioavailability of divalent mercury (Hg(II)) complexed in dissolved organic matter (DOM) to fish. The fish acquired 0.3 to 2.2 μg Hg/g dry weight after 8 weeks in aquaria containing DOM from a Carex peat with complexed mercury at initial concentrations of 14 nM to 724 nM. Changes in the relative proportions of dithiolate Hg(SR)₂ and nanoparticulate β-HgS in the DOM, as quantified by high energy-resolution XANES (HR-XANES) spectroscopy, indicate that Hg(SR)₂ complexes either produced by microbially induced dissolution of nanoparticulate β-HgS in the DOM or present in the original DOM were the forms of mercury that entered the fish. In the fish with 2.2 μg Hg/g, 84 ± 8% of Hg(II) was bonded to two axial thiolate ligands and one or two equatorial N/O electron donors (Hg[(SR)₂+(N/O)_1-2] coordination), and 16% had a Hg(SR)₄ coordination, as determined by HR-XANES. For comparison, fish exposed to Hg²⁺ from 40 nM HgCl₂ contained 10.4 μg Hg/g in the forms of dithiolate (20 ± 10%) and tetrathiolate (23 ± 10%) complexes, and also Hg _xS _y clusters (57 ± 15%) having a β-HgS-type local structure and a dimension that exceeded the size of metallothionein clusters. There was no evidence of methylmercury in the fish or DOM within the 10% uncertainty of the HR-XANES. Together, the results indicate that inorganic Hg(II) bound to DOM is a source of mercury to biota with dithiolate Hg(SR)₂ complexes as the immediate species bioavailable to fish, and that these complexes transform in response to cellular processes.

Influence of humic substances on the toxic effects of cadmium and SDBS to the green alga Scenedesmus obliquus

The joint toxicity of chemicals mixture in the aquatic environment was still not well clear. To clarify the joint toxicity of the mixtures of metals and organic pollutants, as well as the influence of dissolved organic matter (DOM) in field water-body on their toxic effects, we conducted the toxicity tests with cadmium (Cd) and sodium dodecyl benzene sulfonate (SDBS) on Scenedesmus obliquus (S. obliquus) with or without the presence of fulvic acid (FA), a typical of DOM. Our results showed Cd was more toxic to S. obliquus than SDBS, and the effects of fulvic acid on SDBS were greater than Cd. The joint toxicity of Cd and SDBS expressed a synergistic effect on S. obliquus, which was observed to be increased with the presence of FA. Our results gave an example for the joint toxicity investigations of organics and metals, aiding to understanding the toxicity of pollutant mixtures in field water bodies containing DOM.

Microalgal whole-cell biomarkers as sensitive tools for fast toxicity and pollution monitoring of urban wet weather discharges

Urban wet weather discharge (UWWD) management is an important issue. UWWD often represents a significant source of pollution in all aquatic bodies. The occurrence of this pollution is difficult to predict due to the variability of storm events and the unknown contents of urban watershed leached out by rain. Previous studies have tried to demonstrate the ecotoxic impact of UWWD. However, most of them merely highlight the limitations of classic monospecific bioassays, given the high dilution of micropollutants or the presence of nutrients masking toxic effects. Overcoming this problem is therefore of great interest. In this study, we demonstrated the utility of a battery of biomarkers (e.g. membrane permeability, chlorophyll fluorescence, esterase and alkaline phosphatase activities) on the microalgae Chlorella vulgaris to detect the toxic effects of 7 UWWD samples after short exposures (2 and 24 h). These biomarkers are linked to microalgal life traits. Complementarily, monospecific bioassays were carried on Pseudokirchneriella subcapitata, Chlorella vulgaris, Daphnia magna and Heterocypris incongruens to compare their sensitivity to the UWWD samples. No toxic effect was detected in any of the bioassays. Yet, algal biomarkers indicated a disturbance in microalgae physiology, and particularly a perturbation of chlorophyll fluorescence, which was observed in all of the samples tested. While algal membrane permeability was affected by only one UWWD, these two enzymatic activities were stimulated or inhibited depending on the sample. Finally, this study demonstrates the sensitivity of algal biomarkers and the need to develop new, fruitful approaches to characterizing UWWD toxicity.

Pb (II) bioavailability to algae (Chlorella pyrenoidosa) in relation to its complexation with humic acids of different molecular weight

Humic acid (HA) has a major influence on the environmental fate of metal ions due to its heterogeneity in chemical compositions, structure and functional groups. In this study, we investigated the effect of humic acid (HA) with different molecular weight (Mw) on the bioavailability of Pb for a representative algae-Chlorella pyrenoidosa. The results showed that HA with larger Mw had stronger inhibitory effects on the bioavailability of Pb to algae, and the biosorption capacity of Pb decreased with increasing Mw, which is in accordance with the variations of complexation capacities of Pb for HA fraction. In addition, we found that HA with Mw lower than 10 kDa could increase the biosorption capacity of Pb. The considerable differences among the Mw fractions on Pb biosorption were mainly attributed to their properties and corresponding complexation capacities. Phenolic groups were responsible for the variations of binding capacities among different Mw fractions, and it could also better explain the bioaccumulation of Pb to the membranes of algae. By using NICA-Donnan model, we found that over 60% of Pb ions were bound by HAs through specific binding, and the formation of Pb-HAs complex were non-bioavailable to algae, which was proved by the considerably decreasing percentage of internalized Pb. This study provided further insight into the bioavailability of Pb to algae as influenced by the complexation of HA with metal ion such as Pb.

Aerobic and Anaerobic Bacterial Mercury Uptake is Driven by Algal Organic Matter Composition and Molecular Weight

The biological mobilization of mercury (Hg) into microbes capable of Hg methylation is one of the limiting steps in the formation of the neurotoxin methylmercury (MeHg). Although algal dissolved organic matter (DOM) has been associated with increased MeHg production, the relationship between bacterial Hg uptake and algal DOM remains unexplored. In this study, we aimed to address how the quantity and quality of DOM, freshly harvested from several algae, affected the bacterial uptake of Hg with the use of a biosensor capable of functioning both aerobically and anaerobically. We combined biosensor measurements with high-resolution mass spectrometry and field-flow fractionation to elucidate how DOM composition and molecular weight influenced microbial Hg uptake. We showed that freshly harvested DOM from Chlorophyte and Euglena mutabilis strongly inhibited aerobic and anaerobic Hg uptake, whereas DOM harvested from Euglena gracilis did not exhibit this same pronounced effect. Once fractionated, we found that amino acids and polyamines, most abundant in Euglena gracilis DOM, were positively correlated to increase Hg uptake, suggesting that these molecules are potentially underappreciated ligands affecting Hg bioavailability. As water quality is affected by eutrophication, algal community assemblages will change, leading to variations in the nature of autochthonous DOM released in aquatic systems. Our results highlight that variations in the emergent properties of DOM originating from varying algal species can have a profound effect on bacterial Hg uptake and thus methylation.

Measuring the distribution of trace elements amongst dissolved colloidal species as a fingerprint for the contribution of tributaries to large boreal rivers

Organic and inorganic colloids play important roles governing the speciation, transport, and bioaccessibility of trace elements in aquatic systems. These carriers are especially important in the boreal zone, where rivers that contain high concentrations of iron and organic matter are prevalent. The distribution of trace elements amongst different colloidal species (or "speciation profile") can therefore be useful as a fingerprint to detect different trace element sources and for tracking colloid transformations, with implications for bioaccessibility. Asymmetrical flow field-flow fractionation coupled to an inductively coupled plasma mass spectrometer was applied to detect the source of trace elements based on their speciation profile along a 125-km stretch of a large river in the Canadian boreal forest. Both the concentration and proportion of bound trace elements were increased by tributary inputs: bound As, Co, Fe, Mn, Pb, U, and Zn increased monotonically from upstream to downstream, increasingly resembling the speciation profile of tributaries. Principal component (PC) analysis also revealed tributary contributions of bound Cu, Ni, Th, V, and Y reflecting their higher concentrations in tributaries, and PC scores also increased monotonically from upstream-downstream. Monotonically decreasing concentrations of mainly ionic and small (i.e. <ca. 300 Da) As, Ba, Mo, and U species were also observed from upstream-downstream.

Relating Cd2+ binding by humic acids to molecular weight: A modeling and spectroscopic study

Molecular weight (Mw) is a fundamental property of humic acids (HAs), which considerably affect the mobility and speciation of heavy metals in the environment. In this study, soil humic acid (HA) extracted from Jinyun Mountain, Chongqing was ultra-filtered into four fractions according to the molecular weight, and their properties were characterized. Complexation of cadmium was investigated by titration experiments. For the first time, Langmuir and non-ideal competitive adsorption-Donna (NICA-Donnan) models combined with fluorescence excitation-emission matrix (EEM) quenching were employed to elucidate the binding characteristics of individual Mw fractions of HA. The results showed that the concentration of acidic functional groups decreased with increasing Mw, especially the phenolic groups. The humification degree and aliphaticity increased with increasing Mw as indicated by elemental composition analysis and FT-IR spectra. The binding capacity of Cd²⁺ to Mw fractions of HA followed the order UF1 (<5kDa)>UF2 (5-10kDa)>UF4 (>30kDa)>UF3 (10-30kDa). Moreover, the distribution of cadmium speciation indicated that the phenolic groups were responsible for the variations in binding of Cd²⁺ among different Mw fractions. The results of fluorescence quenching illustrated that the binding capacity of Cd²⁺ to Mw fractions was controlled by the content of functional groups, while the binding affinity was largely influenced by structural factors. The results provide a better understanding of the roles that different HA Mw fractions play in heavy metal binding, which has important implications in the control of heavy metal migration and bio-toxicity.

A freshwater diatom challenged by Zn: Biochemical, physiological and metabolomic responses of Tabellaria flocculosa(Roth) Kützing

Freshwater ecosystems are under threatening anthropogenic pressures worldwide, namely by metals. Diatoms are used as water quality indicators, but the influence of micronutrients such as Zn and its impacts are poorly understood. Thus, our study aimed to elucidate the tolerance level, the cellular targets and the responses to counteract Zn toxicity of freshwater diatoms by exposing Tabellaria flocculosa, isolated from a Zn contaminated stream. Biochemical, physiological and metabolomic approaches were used. It was demonstrated that Zn is toxic to T. flocculosa at concentrations occurring in contaminated environments. At low stress (30 μg Zn/L) few alterations in the metabolome were observed, but the enzymatic (SOD, CAT) and molecular (GSH, GSSG) antioxidant systems were induced, protecting cells from oxidative stress. At moderate stress (500 μg Zn/L) the main changes occurred in the metabolome (increases in fatty acids, amino acids, terpenoids, glycerol and phosphate, decreases in sucrose and lumichrome) with a moderate increase in cell damage (LPO and PC). The concerted action of all these mechanisms resulted in a non-significant decrease of growth, explaining the survival of this T. flocculosa strain in an environment with this Zn concentration. At the highest stress level (1000 μg Zn/L) the metabolome was identical to 500 μg Zn/L, and the induction of antioxidant systems and extracellular ion chelation (exopolysaccharides, frustulins) were the main responses to the increase of Zn toxicity. However, these mechanisms were unable to effectively abrogate cellular damage and growth reduction was observed. Moreover, the decrease in sucrose and especially in lumichrome should be tested as new specific markers of Zn toxicity. The information obtained in this study can assist in environmental risk assessment policies, support the prediction of diatom behaviour in highly impacted Zn environments, such as mining scenarios, and may help develop new indices, which include alterations induced by metals.

Hybrid process, electrocoagulation-biofiltration for landfill leachate treatment

Landfill leachates are known for their high and complex composition of organic, inorganic and microbial pollutants. As a result, it is quite challenging to treat these effluents by using only one treatment process. A combining approach is generally required to treat efficiently these wastewaters and comply with the discharge standards. In this present study, electrocoagulation (EC) and biofiltration (BF) processes were sequentially used to treat landfill leachate. EC process has been able to remove 37 ± 2% of the initial total COD. A fractionation of organic compounds showed that EC was particularly efficient to remove insoluble COD and humic acids. In addition, other pollutants such as turbidity, true color, Zn and phosphorus were significantly reduced by EC with 82 ± 2.7%, 60 ± 13%, 95 ± 2.6% and 82 ± 5.5% of removal respectively. The subsequent treatment by BF process led to completely removal of ammonia pollution (>99% of NH₄ removal) and a partial removal of dissolved organic compounds (42 ± 7% of COD removal). The hybrid process EC/BF could form the basis of a process capable of removing organic and inorganic pollutants from many refractory wastewaters (mature landfill leachates, industrial and municipal wastewaters).

Impact of humic acid on the accumulation of metals by microalgae

Indirect impact of humic acid (HA) on metal accumulation and toxicity (Cd, Ni, Pb, and Hg; 100 μM; 24 h of exposure) in Scenedesmus quadricauda was studied. Algae were pre-cultured on solid (10 and 100 mg HA/L) or in liquid media (1, 5, and 10 mg HA/L) over 30 days and then exposed to metals mentioned above. Accumulation of applied metals irrespective of pre-culture increased in the order Ni < Cd < Pb < Hg. Algae pre-cultured on solid HA-enriched media accumulated more Cd (+ 46% at 10 mg HA/L), Ni (+ 50 and + 81% at 10 and 100 mg HA/L, respectively), and Pb (+ 15% at 100 mg HA/L) but the impact on Hg amount was not detected. Potassium and calcium decreased in response to all metals (K strongly under Hg excess) and HA had negligible impact. Interestingly, fluorescence microscopy detection of reactive oxygen species/nitric oxide (ROS/NO) balance showed that HA pre-culture suppressed ROS signal and stimulated NO signal in response to Cd (indicating positive impact of HA) while ROS signal in Ni and Pb treatments rather increased but NO signal decreased as expected from elevated Ni and Pb accumulation. Hg had clearly the most toxic impact on the ROS/NO balance. Algae pre-cultured in liquid HA-enriched media showed significantly increased Ni accumulation only (+ 14% at a dose 10 mg HA/L). Present study for the first time showed that humic acid may indirectly affect accumulation of metals and that solid HA-enriched medium used for pre-culture is more suitable to increase accumulation of metals by algae.

Fate, toxicity and bioconcentration of cadmium on Pseudokirchneriella subcapitata and Lemna minor in mid-term single tests

In the frame of a project which consists in modeling a laboratory microcosm under cadmium pressure, we initiated this study on the fate and effects of cadmium in the presence of either the microalga Pseudokirchneriella subcapitata or the duckweed Lemna minor, two organisms of the microcosm. For each organism, growth inhibition tests on a duration of 2-3 weeks were carried out with the objective of linking effects with total dissolved, ionic and internalized forms of cadmium. Numbers of organisms (algal cells or duckweed fronds) in 2-L beakers filled with synthetic nutritive medium containing EDTA were counted during the course of assays, while cadmium concentrations in the water and in the organisms were measured. Free cadmium fraction was calculated using PHREEQC, a computer program for chemical speciation. Results showed that cadmium toxicity to microalgae could be correlated to the free divalent fraction of this metal, limited by the presence of EDTA, and to its concentration in the organisms. Bioconcentration factors for our medium were suggested for P. subcapitata (111,000 on the basis of free cadmium concentration) and L. minor (17,812 on the basis of total dissolved concentration). No effect concentrations were roughly estimated around 400 µg/g for Pseudokirchneriella subcapitata and 200-300 µg/g for Lemna minor. This study is a first step towards a fate model based on chemical speciation for a better understanding of microcosm results.

Leonardite-derived humic substances are great adsorbents for cadmium

Adsorption is an important mechanism to immobilize cadmium (Cd) in soil, for which humic substances have a potential. However, commercial humic substances are either very acidic (pH = 2) or alkaline/Na⁺-enriched, making them less suitable for use in acid and saline soils. Here, we used leonardite to produce humic adsorbents HA (pH = 4.02), Ca-HA (pH = 10.9), and Ca-CPAM-HA (pH = 9.62) by using HCl, CaCl₂, or CaCl₂-polyacrylamide as a flocculant. Their elemental compositions, acidity, and spectroscopic properties were determined, and their Cd adsorption characteristics were assessed by batch kinetic and thermodynamic experiments at environmentally relevant concentrations. Further, HA was mixed with Cd-contaminated soils and incubated for a month to assess its effect on Cd immobilization. Good fitting of kinetic adsorption data into pseudo-second-order model, together with FTIR spectroscopic data, suggested the chemisorption mechanism by forming Cd(II)-carboxyl complexes. The maximum adsorption capacity derived from the Langmuir equation was 129, 114, and 110 mg Cd(II)/g for HA, Ca-HA, and Ca-CPAM-HA, respectively. These values are almost the same on carbon-normalized basis. HA reduced acetic acid extractable Cd by 31% or more. Besides their high propensity for Cd adsorption, humic adsorbents are inexpensive, safe, and beneficial to soil quality.

Protective role of humic acids against picloram-induced genomic instability and DNA methylation in Phaseolus vulgaris

Picloram (4-amino-3,5,6-trichloropicolinic acid) is a liquid auxinic herbicide used to control broad-leaved weeds. Picloram is representing a possible hazard to ecosystems and human health. Therefore, in this study, DNA methylation changes and DNA damage levels in Phaseolus vulgaris exposed to picloram, as well as whether humic acid (HA) has preventive effects on these changes were investigated. Random amplified polymorphic DNA (RAPD) techniques were used for identification of DNA damage and coupled restriction enzyme digestion-random amplification (CRED-RA) techniques were used to detect the changed pattern of DNA methylation. According to the obtained results, picloram (5, 10, 20, and 40 mg/l) caused DNA damage profile changes (RAPDs) increasing, DNA hypomethylation and genomic template stability (GTS) decreasing. On the other hand, different concentrations of applied HA (2, 4, 6, 8, and 10%) reduced hazardous effects of picloram. The results of the experiment have explicitly indicated that HAs could be an alternative for reducing genetic damage in plants. In addition to the alleviate effects of humic acid on genetic damage, its epigenetic effect is hypomethylation.

Effect of Fe and EDTA on Freshwater Cyanobacteria Bloom Formation

Due to the fact that not all eutrophic lakes have cyanobacteria blooms, we hypothesized Fe may be another important limiting factor which regulates cyanobacteria bloom formation. We tested the hypothesis by batch cultures of bloom-forming Cyanobacterium, Microcystis aeruginosa with different ethylenediaminetetraacetic acid (EDTA)-Fe concentrations (0.5–6.0 mg/L), three levels of initial biomass, and excessive N and P (N = 4.2 mg/L, P = 0.186 mg/L) to simulate dynamically a cyanobacteria bloom in eutrophic conditions. The effect of EDTA and Fe uptake kinetics by M. aeruginosa were also examined. Results showed M. aeruginosa growth rate positively correlated with EDTA-Fe concentration and negatively correlated with biomass. Maximal biomass of M. aeruginosa was determined by Fe availability and initial biomass. EDTA could decrease both Fe availability and toxicity. Based on experimental results, a conceptual model of how Fe availability regulates cyanobacterial biomass in eutrophic lakes was developed. This study demonstrated bioavailable Fe is a potential limiting factor in eutrophic lakes that should be included in eutrophication management strategies.

Evaluation of a hollow fiber supported liquid membrane device as a chemical surrogate for the measurements of zinc (II) bioavailability using two microalgae strains as biological references

The environmental bioavailability of zinc (II), i.e., the uptake of the element by an organism, was determined using two microalgae species, Scenedesmus acutus and Pseudokirchneriella subcapitata, and estimated using hollow fiber supported liquid membrane (HF-SLM) device as the chemical surrogate. Several experimental conditions were studied including the presence of organic matter, inorganic anions and concomitant cations and pH. The results show strong positive correlation coefficients between the responses given by the HF-SLM and the microalgae species (r = 0.900 for S. acutus and r = 0.876 for P. subcapitata) in multivariate environments (changes in pH, calcium, humic and citrate concentrations). The maximum amount of zinc (II) retained by the HF-SLM (4.7 × 10^-8 mol/cm²) was higher than those for P. subcapitata and S. acutus (9.4 × 10^-11 mol/cm² and 6.2 × 10^-11 mol/cm², respectively). The variation in pH (pH 5.5-9) was the variable with the greatest effect on zinc internalization in all systems, increasing approximately 2.5 times for P. subcapitata and 5.5 times for S. acutus respect to pH = 5.5, while the presence of humic acids did not affect the response. The species' concentration analysis of the experimental design at pH = 5.5 indicated that the amount of internalized zinc (II) by the HF-SLM and both microalgae species is strongly dependent on the free zinc concentration (r = 0.910 for the HF-SLM, r = 0.922 for S. acutus and r = 0.954 for P. subcapitata); however, at pH = 9.0, the amount of internalized zinc (II) is strongly dependent on the sum of free zinc and labile species (r = 0.912 for the HF-SLM, r = 0.947 for S. acutus and r = 0.900 for P. subcapitata). The presence of inorganic ligands (chloride, sulfate, phosphate, carbonate, and nitrate) and metal ions (cobalt (II), copper (II), nickel (II), chromium (VI), lead (II) and cadmium (II)) produced different behaviors both in the chemical surrogate and the biological references. The results showed that the synthetic device can mimic biological uptake in the presence of humic acids, nitrate, sulfate, and phosphate, and pH within the range 5.5-9 when S. acutus was used as the biological reference, considering the simultaneous contribution of the Zn²⁺ and ZnOH⁺ labile species depending on the chemical composition of the medium.

Influences of size-fractionated humic acids on arsenite and arsenate complexation and toxicity to Daphnia magna

The intrinsic physicochemical properties of dissolved organic matter (DOM) may affect the mobility and toxicity of arsenic in aquatic environments. In the present study, the humic acid (HA) was ultra-filtered into five fractions according to molecular weight, and their physicochemical properties were characterized. Complexation of HA fractions with arsenite and arsenate was first determined by differential pulse polarography (DPP). The influences of HA fractions on arsenic toxicity were then examined using Daphnia magna as a model organism. As(V) had a higher affinity with HA than As(III), and their complexation was dependent on the total acidity and fluorescence characteristics of DOM. We demonstrated that the acidity and fluorescence also better explained the As toxicity to daphnids than UV absorbance and hydraulic diameter. Arsenic speciation determined by DPP significantly affected the toxicity of arsenite and arsenate. The results extended the free-ion activity model application to the case of arsenic. The present study clearly indicated that DOM with different molecular weights has distinct physicochemical properties, and could influence the speciation and toxicity of As to different extent.

Effect of titanium dioxide nanoparticles on copper toxicity to Daphnia magna in water: Role of organic matter

Inevitably released into natural water, titanium dioxide nanoparticles (nano-TiO₂) may affect the toxicity of other contaminants. Ubiquitous organic matter (OM) may influence their combined toxicity, which has been rarely reported. This study investigated the effect of nano-TiO₂ on Cu toxicity to Daphnia magna and the role of OM (dissolved or particle surface bound) in inducing combined effects. The effect of nano-TiO₂ on heavy metal accumulation depended on the adsorption capacity for heavy metals of nano-TiO₂ and the uptake of nano-TiO₂-metal complexes by organisms. Nano-TiO₂ significantly decreased Cu accumulation in D. magna, but the reducing effect of nano-TiO₂ was eliminated in the presence of humic acid (HA, a model OM). In the Cu and HA solution, nano-TiO₂ slightly affected the bioavailability of Cu²⁺ and Cu-HA complexes and thus slightly influenced Cu toxicity. The nanoparticle surface-bound HA reduced the effect of nano-TiO₂ on the speciation of the accumulated Cu; therefore, the combined effects of nano-TiO₂ and Cu on biomarkers similarly weakened. HA-altered Cu speciation may be the main factor responsible for the influence of HA on the combined effects of nano-TiO₂ and Cu. This study provides insights into the combined effects of nano-TiO₂ and heavy metals in natural water.

Cadmium availability and uptake by radish (Raphanus sativus) grown in soils applied with wheat straw or composted pig manure

Soil cadmium (Cd) availability and uptake by cherry-red radish (Raphanus sativus) grown in Cd-contaminated soils after addition with wheat straw or composted pig manure were studied. The results indicated that wheat straw application promoted radish growth until the second harvest, while pig manure application improved radish biomass in Acid Ferralsols regardless of harvesting seasons. Application with pig manure might be more effective in lowering the Cd uptake by radish than wheat straw. Especially when pig manure of 11.9 g TOC kg(-1) amended into Acid Ferralsols, Cd contents in leaves and roots of radish decreased by 89.2 and 95.7 % at the second harvest, respectively. The changes in Cd fractions distribution in soils after application were contributed to the decline of Cd availability. Furthermore, significantly negative linear correlation (P < 0.05) between the ratio of humic acid (HA) and fulvic acid (FA) in soils and exchangeable Cd was also observed. However, the significantly negative relationship (P < 0.01) between soil pH and exchangeable Cd was merely found in pig manure-treated Acid Ferralsols. The increases in HA/FA ratio or pH values in soils after adding organic materials were also responsible for the decrease of Cd availability in soils and uptake by radish. Thus, it is recommended to stabilize soil Cd and reducing plant uptake by application with composted manure without or slightly contaminated with metals.

Influence of dissolved organic matter on dissolved vanadium speciation in the Churchill River estuary (Manitoba, Canada)

Diffusive gradients in thin films (DGT) devices were used to investigate the temporal and spatial changes in vanadium (V) speciation in the Churchill estuary system (Manitoba). Thirty-six DGT sets and 95 discrete water samples were collected at 8 river and 3 estuary sites during spring freshet and summer base flow. Dissolved V concentration in the Churchill River at summer base flow was approximately 5 times higher than those during the spring high flow (27.3 ± 18.9 nM vs 4.8 ± 3.5 nM). DGT-labile V showed an opposite trend with greater values found during the spring high flow (2.6 ± 1.8 nM vs 1.4 ± 0.3 nM). Parallel factor analysis (PARAFAC) conducted on 95 excitation-emission matrix spectra validated four humic-like (C1C4) and one protein-like (C5) fluorescent components. Significant positive relationship was found between protein-like DOM and DGT-labile V (r = 0.53, p < 0.05), indicating that protein-like DOM possibly affected the DGT-labile V concentration in Churchill River. Sediment leachates were enriched in DGT-labile V and protein-like DOM, which can be readily released when river sediment began to thaw during spring freshet.

Modelling algae-duckweed interaction under chemical pressure within a laboratory microcosm

Contaminant effects on species are generally assessed with single-species bioassays. As a consequence, interactions between species that occur in ecosystems are not taken into account. To investigate the effects of contaminants on interacting species dynamics, our study describes the functioning of a 2-L laboratory microcosm with two species, the duckweed Lemna minor and the microalgae Pseudokirchneriella subcapitata, exposed to cadmium contamination. We modelled the dynamics of both species and their interactions using a mechanistic model based on coupled ordinary differential equations. The main processes occurring in this two-species microcosm were thus formalised, including growth and settling of algae, growth of duckweeds, interspecific competition between the two species and cadmium effects. We estimated model parameters by Bayesian inference, using simultaneously all the data issued from multiple laboratory experiments specifically conducted for this study. Cadmium concentrations ranged between 0 and 50 μg·L(-1). For all parameters of our model, we obtained biologically realistic values and reasonable uncertainties. Only duckweed dynamics was affected by interspecific competition, while algal dynamics was not impaired. Growth rate of both species decreased with cadmium concentration, as well as competition intensity showing that the interspecific competition pressure on duckweed decreased with cadmium concentration. This innovative combination of mechanistic modelling and model-guided experiments was successful to understand the algae-duckweed microcosm functioning without and with contaminant. This approach appears promising to include interactions between species when studying contaminant effects on ecosystem functioning.

Statistical evaluation of biogeochemical variables affecting spatiotemporal distributions of multiple free metal ion concentrations in an urban estuary

Free metal ion concentrations have been recognized as a better indicator of metal bioavailability in aquatic environments than total dissolved metal concentrations. However, our understanding of the determinants of free ion concentrations, especially in a metal mixture, is limited, due to underexplored techniques for measuring multiple free metal ions simultaneously. In this work, we performed statistical analyses on a large dataset containing repeated measurements of free ion concentrations of Cu, Zn, Pb, Ni, and Cd, the most commonly measured metals in seawater, at five inshore locations in Boston Harbor, previously collected using an in-situ equilibrium-based multi-metal free ion sampler, the 'Gellyfish'. We examined correlations among these five metals by season, and evaluated effects of 10 biogeochemical variables on free ion concentrations over time and location through multivariate regressions. We also explored potential clustering among the five metals through a principal component analysis. We found significant correlations among metals, with varying patterns over season. Our regression results suggest that instead of dissolved metals, pH, salinity, temperature and rainfall were the most significant determinants of free metal ion concentrations. For example, a one-unit decrease in pH was associated with a 2.2 (Cd) to 99 (Cu) times increase in free ion concentrations. This work is among the first to reveal key contributors to spatiotemporal variations in free ion concentrations, and demonstrated the usefulness of the Gellyfish sampler in routine sampling of free ions within metal mixtures and in generating data for statistical analyses.

Evaluation of the measurement of Cu(II) bioavailability in complex aqueous media using a hollow-fiber supported liquid membrane device (HFSLM) and two microalgae species (Pseudokirchneriella subcapitata and Scenedesmus acutus)

The environmental bioavailability of copper was determined using a hollow-fiber supported liquid membrane (HFSLM) device as a chemical surrogate and two microalgae species (Scenedesmus acutus and Pseudokirchneriella subcapitata). Several experimental conditions were studied: pH, the presence of organic matter, inorganic anions, and concomitant cations. The results indicated a strong relationship between the response given by the HFSLM and the microalgae species with free copper concentrations measured by an ion selective electrode (ISE), in accordance with the free-ion activity model (FIAM). A significant positive correlation was evident when comparing the bioavailability results measured by the HFSLM and the S. acutus microalga species, showing that the synthetic device may emulate biological uptake and, consequently, be used as a chemical test for bioavailability measurements using this alga as a biological reference.

Effect of Metals, Metalloids and Metallic Nanoparticles on Microalgae Growth and Industrial Product Biosynthesis: A Review

Microalgae are a source of numerous compounds that can be used in many branches of industry. Synthesis of such compounds in microalgal cells can be amplified under stress conditions. Exposure to various metals can be one of methods applied to induce cell stress and synthesis of target products in microalgae cultures. In this review, the potential of producing diverse biocompounds (pigments, lipids, exopolymers, peptides, phytohormones, arsenoorganics, nanoparticles) from microalgae cultures upon exposure to various metals, is evaluated. Additionally, different methods to alter microalgae response towards metals and metal stress are described. Finally, possibilities to sustain high growth rates and productivity of microalgal cultures in the presence of metals are discussed.

Community structure and nutrient level control the tolerance of autotrophic biofilm to silver contamination

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. Since microbial species differ in their sensitivity to stressors, biofilms have long been proposed for assessing the quality of aquatic ecosystems. Among the many stressors impacting aquatic ecosystems, eutrophication and metal pollution are certainly the most common. Despite that these stressors often occur together, their effects on biofilms have been far much studied separately than interactively. In this study, we evaluated the interactive effects of silver (Ag), a reemerging contaminant, and phosphorus (P), a nutrient often associated with freshwater eutrophication, on the structure and functioning of two types of autotrophic biofilms, one dominated by diatoms and another one dominated by cyanobacteria. We hypothesized that P would alleviate the toxic effects of Ag, either directly, through the contribution of P in metal detoxification processes, or indirectly, through P-mediated shifts in biofilm community compositions and associated divergences in metal tolerance. Results showed that Ag impacted biofilm community structure and functioning but only at unrealistic concentrations (50 μg/L). P availability led to significant shifts in biofilm community composition, these changes being more pronounced in diatom- than those in cyanobacteria-dominated biofilm. In addition, P tended to reduce the impact of Ag but only for the cyanobacteria-dominated biofilm. More generally, our results highlight the preponderant role of the initial community structure and nutrient level on biofilm response to metallic pollutants.

Influence of dissolved organic nitrogen on Ni bioavailability in Prorocentrum donghaiense and Skeletonema costatum

Dissolved organic nitrogen (DON) is an important nutrient in the aquatic environment. This study examined the influence of DON addition on the adsorption, absorption, and distribution in macromolecular forms of environmentally deleterious trace metal (Ni) in Prorocentrum donghaiense and Skeletonema costatum over eight days. Ni adsorption and absorption of two species increased with the addition of urea, while Ni adsorption and absorption of two species in the presence of humic substances (HS) decreased. Meanwhile, Ni adsorption and absorption of P. donghaiense were higher than that of S. costatum. Furthermore, Ni contents in the protein fraction of the cells, both in P. donghaiense and S. costatum, were increased with both urea and HS addition. Thus, urea and HS input could impact Ni biogeochemistry and bioavailability, and then affect the biodynamics thereafter.

Natural humic substances effects on the life history traits of Latonopsis australis SARS (1888) (Cladocera--Crustacea)

Cultivation medium is one of the first aspects to be considered in zooplankton laboratory cultivation. The use of artificial media does not concern to reproduce natural conditions to the cultivations, which may be achieved by using natural organic compounds like humic substances (HS). This study aimed to evaluate the effects of a concentrate of dissolved organic carbon (DOC) from the Negro River (NR(1)) and an extraction of humic acids (HA) from humus produced by Eisenia andrei on the life history traits of laboratory-based Latonopsis australis SARS (1888). A cohort life table approach was used to provide information about the effectiveness of NR and HA as supplements for the artificial cultivation of L. australis. Additionally, we seek to observe a maximization of L. australis artificial cultivation fitness by expanding the range of HS concentrations. The first experiment demonstrated that the females of L. australis reared under NR10 (mgDOCL(-1)) may have experienced an acceleration of the population life cycle, as the females have proportionally reproduced more and lived shorter than controls. By contrast, the use of the HA did not improve life history traits considered. The expansion of the concentration range (5, 10, 20 and 50 mgDOCL(-1)) corroborated the patterns observed on the first assay. Results for the fitness estimates combined with shorter lifespans than controls demonstrated trade-offs between reproductive output and female longevity reared under NR conditions, with NR20 been suggested as the best L. australis cultivation medium. This response might be associated with hormone-like effects.

Dissolved organic matter kinetically controls mercury bioavailability to bacteria

Predicting the bioavailability of inorganic mercury (Hg) to bacteria that produce the potent bioaccumulative neurotoxin monomethylmercury remains one of the greatest challenges in predicting the environmental fate and transport of Hg. Dissolved organic matter (DOM) affects mercury methylation due to its influence on cell physiology (as a potential nutrient) and its influence on Hg(II) speciation in solution (as a complexing agent), therefore controlling Hg bioavailability. We assessed the role of DOM on Hg(II) bioavailability to a gram-negative bacterium bioreporter under oxic pseudo- and nonequilibrium conditions, using defined media and field samples spanning a wide range of DOM levels. Our results showed that Hg(II) was considerably more bioavailable under nonequilibrium conditions than when DOM was absent or when Hg(II) and DOM had reached pseudoequilibrium (24 h) prior to cell exposure. Under these enhanced uptake conditions, Hg(II) bioavailability followed a bell shaped curve as DOM concentrations increased, both for defined media and natural water samples, consistent with bioaccumulation results in a companion paper (this issue) observed for amphipods. Experiments also suggest that DOM may not only provide shuttle molecules facilitating Hg uptake, but also alter cell wall properties to facilitate the first steps toward Hg(II) internalization. We propose the existence of a short-lived yet critical time window (<24 h) during which DOM facilitates the entry of newly deposited Hg(II) into aquatic food webs, suggesting that the bulk of mercury incorporation in aquatic food webs would occur within hours following its deposition from the atmosphere.

Do natural rubber latex condoms pose a risk to aquatic systems?

The presence and potential adverse effects of plastic-polymers in the environment are receiving increasing attention in the popular and scientific press. However, quantifying emissions, exposure and effects of these materials remains a challenge. This paper describes the application of a questionnaire survey to quantify emissions of condom material from the domestic household to the sewage waste stream. Condoms are an important mainstay for birth control and the reduction of sexually transmitted infections. Survey participants were estimated to flush condoms down the toilet 2.96% of the time, and emissions were calculated as 0.99 mg of condom material per person per day. Using information on screening efficiencies at sewage treatment plants, the questionnaire data was combined with a GIS-based water quality model (LF2000-WQX) to predicted environmental concentrations (PEC) in a UK river basin catchment. Annual average PECs of condom material were 0.08-0.2 μg L(-1), under the model scenario used. To put these PECs into context, rubber latex condom material was degraded in outdoor microcosms. This resulted in the formation of a complex mixture of substances including chemical degradation products and particles in the nano range. The direct effects of the degradation mixture were investigated using two freshwater organisms with different life cycle traits, the water column crustacean Daphnia magna and the sediment-dwelling larval of Chironomus riparius. Ecotoxicity tests investigated both acute and chronic endpoints and were shown to exhibit no toxic effects. This precluded the derivation of a genuine no-effect concentration. Hence, the results suggest that limited risk to invertebrates is associated with latex condom degradation products to the organisms tested. Future studies should extend this risk framework to assess risks of condoms to other taxonomic groups as well as the risks of other polymer materials.