Bernard Vigneault

Metal bioavailability to phytoplankton--applicability of the biotic ligand model.

To elicit a biological response from a target organism and/or to accumulate within this organism, a metal must first interact with a cell membrane. For hydrophilic metal species, this interaction with the cell surface can be represented in terms of the formation of M-X-cell surface complexes, e.g. M(z+)+(-)X-cell<-->M-X-cell, where -X-cell is a cellular ligand present at the cell surface. According to the free-ion model, or its derivative the biotic ligand model (BLM), the biological response elicited by the metal will be proportional to [M-X-cell]. In this paper, using freshwater algae as our test species, we examine some of the key assumptions that underlie the BLM, namely that metal internalization is slow relative to the other steps involved in metal uptake (i.e. the M-X-cell complex is in equilibrium with metal species in solution), that internalization occurs via cation transport, and that internalization must occur for toxicity to appear. Recent experiments with freshwater algae are described, demonstrating anomalously high metal accumulation and/or toxicity in the presence of a common low molecular weight metabolite (alanine), or in the presence of an assimilable inorganic anion (thiosulfate). The possible implications of these findings for the application of the BLM to higher organisms are discussed.

Uptake of cadmium by freshwater green algae : Effects of pH and aquatic humic substances

The effects of humic substances and low pH on short‐term Cd uptake by Pseudokirchneriella subcapitata (Korshikov) Hindak and Chlamydomonas reinhardtii Dang were investigated under defined exposure conditions. The uptake experiments were run in the presence of either a synthetic organic ligand (nitrilotriacetate) or natural organic ligands (Suwannee River fulvic or humic acid). An ion‐exchange method was used to measure the free Cd2+ concentrations in the exposure solutions. At pH 5, measured free Cd2+ concentrations agreed with estimations made using the geochemical equilibrium model WHAM, but at pH 7 the model overestimated complexation by both Suwannee River fulvic and humic acids compared with the ion‐exchange measurements. Consistent with the metal internalization step being rate limiting for overall short‐term uptake, intracellular Cd uptake was linear for exposure times less than 20 min at pH 5 or pH 7 for both algal species. After taking into account complexation of Cd in solution, Suwannee River humic substances had no additional effects on cadmium uptake at pH 7, as would be predicted by the free ion model. This absence of effects other than complexation persisted at pH 5, where the tendency of humic substances to adsorb to the algal cell surface is favored. Changes in pH strongly influenced Cd uptake, with the intracellular flux of Cd being at least 20 times lower at pH 5 than at pH 7 for P. subcapitata. Our results support models such as the free ion model or the biotic ligand model, in which humic substances act indirectly on Cd uptake by reducing the bioavailability of Cd by complexation in solution.

GROWTH STIMULATION OF ALEXANDRIUM TAMARENSE (DINOPHYCEAE) BY HUMIC SUBSTANCES FROM THE MANICOUAGAN RIVER (EASTERN CANADA)1

In the St. Lawrence Estuary, annual recurrent blooms of the toxic dinoflagellate Alexandrium tamarense L. Balech are associated with brackish waters. Riverine inputs are suspected to favor bloom development by increasing water column stability and/or by providing growth stimulants such as humic substances (HS). A 17‐day culture experiment was conducted to evaluate the importance of HS as growth factors for A. tamarense. Nonaxenic cultures were exposed to four HS extracts from three different sources: humic and fulvic acids isolated from the Manicouagan River, Quebec, Canada; humic acids from the Suwannee River, Georgia, United States; and a desalted alkaline soil extract. For each extract, four concentrations were tested as supplements to the artificial Keller medium, a nitrate‐rich algal culture medium. Additions of HS from all sources significantly enhanced the overall growth rates relative to the controls. Concentrations of HS, estimated by UV spectrophotometry, remained constant throughout the exponential growth phase, suggesting that the HS were acting mainly as growth promoters during our experiment. Dose–response curves indicated that HS could increase the growth rate of A. tamarense even at low concentrations, such as those encountered in the St. Lawrence Estuary. Our results support the hypothesis that HS from the Manicouagan River plume can stimulate the development of toxic dinoflagellate blooms.

Uptake of Neutral Metal Complexes by a Green Alga: Influence of pH and Humic Substances

We have examined the influence of pH and a natural humic acid on the short-term uptake (<40 min) of a neutral, lipophilic metal complex by a unicellular freshwater alga, Pseudokirchneriella subcapitata. Cadmium diethyldithiocarbamate ([Cd(DDC)2]0) was used as a model lipophilic metal complex and Suwannee River Humic Acid (SRHA) was chosen as a representative aquatic humic acid (6.5 mg C L−1). Under the experimental conditions virtually all the Cd was expected to be present as the lipophilic complex ([Cd]T = 0.38 nM; [DDC] 1 μM; [Cd2+] <10−15 M; pH 7.0, 6.0, or 5.5). Uptake of [Cd(DDC)2]0 proved to be sensitive to pH changes. It was lower at pH 6.0 and 5.5 than at pH 7.0. To our knowledge, this is the first demonstration of reduced uptake of a lipophilic metal complex at low pH. The presence of SRHA also affected uptake, either by binding the lipophilic complex in solution and reducing its bioavailability (pH 7.0) or by increasing the permeability of the algal membrane (pH 5.5).

Development of an In Situ Ion-Exchange Technique for the Determination of Free Cd, Co, Ni, and Zn Concentrations in Freshwaters

Abstract It is now well established that the bioavailability of metals toward aquatic organisms varies as a function of the free metal concentration. The ion-exchange technique (IET), which consists of equilibrating a calibrated cation-exchange resin with the water sample, is one of the few existing speciation methods that provide sufficient sensitivity and specificity to measure free metals in natural waters. In the present study, we developed an in situ IET (field-IET) in which the resin was directly equilibrated on site using dialysis cassettes. The field-IET was tested in six Canadian Shield lakes and in the Athabasca River (AB, Canada) for Cd2+, Co2+ (only in the Athabasca River), Ni2+, and Zn2+. Measurements were compared with those obtained on samples collected from the same sites with in situ diffusion samplers and analyzed in the laboratory (lab-IET). IET results were also compared with predictions from the Windermere Humic Aqueous Model [WHAM VII; the Co and Ni carbonato complex formation constants were updated according to the US National Institute of Science and Technology (NIST compilation)]. Good agreement was obtained between the field-IET and the lab-IET for Co2+ and Ni2+ concentrations, but field-IET Cd2+ and Zn2+ concentrations were systematically higher than the lab-IET results (factors of ~1.5× and ~2.4×, respectively). Uncertainties in the field-IET resin calibration and a significant Zn contamination could explain these results. WHAM VII predicted the free metal concentrations reasonably well, except for Ni2+ concentrations in the lakes, probably due to inappropriate formation constants for complexation of Ni with dissolved organic matter. This study showed that the field-IET, with its relative simplicity and its low detection limits, could be a useful method for the determination of free metal concentrations in acidic to neutral freshwaters.

Root length of aquatic plant, Lemna minor L., as an optimal toxicity endpoint for biomonitoring of mining effluents

Lemna minor, a free-floating macrophyte, is used for biomonitoring of mine effluent quality under the Metal Mining Effluent Regulations (MMER) of the Environmental Effects Monitoring (EEM) program in Canada and is known to be sensitive to trace metals commonly discharged in mine effluents such as Ni. Environment Canadas standard toxicity testing protocol recommends frond count (FC) and dry weight (DW) as the 2 required toxicity endpoints-this is similar to other major protocols such as those by the US Environmental Protection Agency (USEPA) and the Organisation for Economic Co-operation and Development (OECD)-that both require frond growth or biomass endpoints. However, we suggest that similar to terrestrial plants, average root length (RL) of aquatic plants will be an optimal and relevant endpoint. As expected, results demonstrate that RL is the ideal endpoint based on the 3 criteria: accuracy (i.e., toxicological sensitivity to contaminant), precision (i.e., lowest variance), and ecological relevance (metal mining effluents). Roots are known to play a major role in nutrient uptake in conditions of low nutrient conditions-thus having ecological relevance to freshwater from mining regions. Root length was the most sensitive and precise endpoint in this study where water chemistry varied greatly (pH and varying concentrations of Ca, Mg, Na, K, dissolved organic carbon, and an anthropogenic organic contaminant, sodium isopropyl xanthates) to match mining effluent ranges. Although frond count was a close second, dry weight proved to be an unreliable endpoint. We conclude that toxicity testing for the floating macrophyte should require average RL measurement as a primary endpoint.

Toxicity and metal speciation in acid mine drainage treated by passive bioreactors

Sulfate-reducing passive bioreactors treat acid mine drainage (AMD) by increasing its pH and alkalinity and by removing metals as metal sulfide precipitates. In addition to discharge limits based on physicochemical parameters, however, treated effluent is required to be nontoxic. Acute and sublethal toxicity was assessed for effluent from 3.5-L column bioreactors filled with mixtures of natural organic carbon sources and operated at different hydraulic retention times (HRTs) for the treatment of a highly contaminated AMD. Effluent was first tested for acute (Daphnia magna and Oncorhynchus mykiss) and sublethal (Pseudokirchneriella subcapitata, Ceriodaphnia dubia, and Lemna minor) toxicity. Acute toxicity was observed for D. magna, and a toxicity identification evaluation (TIE) procedure was then performed to identify potential toxicants. Finally, metal speciation in the effluent was determined using ultrafiltration and geochemical modeling for the interpretation of the toxicity results. The 10-d HRT effluent was nonacutely lethal for O. mykiss but acutely lethal for D. magna. The toxicity to D. magna, however, was removed by 2 h of aeration, and the TIE procedure suggested iron as a cause of toxicity. Sublethal toxicity of the 10-d HRT effluent was observed for all test species, but it was reduced compared to the raw AMD and to a 7.3-d HRT effluent. Data regarding metal speciation indicated instability of both effluents during aeration and were consistent with the toxicity being caused by iron. Column bioreactors in operation for more than nine months efficiently improved the physicochemical quality of highly contaminated AMD at different HRTs. The present study, however, indicated that design of passive treatment should include sufficient HRT and posttreatment aeration to meet acute toxicity requirements.

Effect of pH and environmental ligands on accumulation and toxicity of Ni2+ to Lemna minor

Environmental context Predicting metal toxicity is an important tool for effective and efficient risk assessment and regulation of metal pollution in the environment. The present study aims to provide scientific support for the development of a predictive Ni toxicity model for aquatic plants that is particularly applicable to mining-affected natural waters. We show that the effects of pH and natural organic ligands on Ni accumulation and toxicity can be modelled, but further research is required to understand the effects of flotation ligands used in the mining industry. Abstract Effects of water chemistry and metal speciation on metal uptake and toxicity to aquatic plants such as Lemna minor are not fully understood. The present study examined the effect of pH and environmental ligands (dissolved organic carbon (DOC) and mining related flotation ligands diethylenetriamine (DETA), triethylenetetramine (TETA), sodium isopropyl xanthate), on Ni toxicity to L. minor. Exposure and tissue residue toxicity thresholds were assessed to validate the use of a Biotic Ligand Model (BLM) or a Tissue Residue Approach (TRA) as a framework for predicting Ni toxicity. An increase in the activity of H+ non-linearly decreased the toxicity of free Ni ion activity, whereas Ni accumulation kinetics indicated that the mechanism of Ni2+ and H+ interaction was not competitive inhibition as expected by the BLM framework. The effect of DOC on the toxicity of total Ni concentration was relatively small (toxicity decreased by less than a factor of 2) and was explained solely by the complexation of Ni2+ by DOC. Alternatively, the protective effect of flotation ligands (DETA and TETA) was much less than expected based on estimated Ni complexation. Overall, a TRA model was directly applicable in the presence of organic ligands but not to varying pH, whereas a BLM-type model was applicable with changes in pH and DOC but not in the presence of the lesser studied flotation ligands. Such mechanistic information is essential for the development of reliable Ni toxicity models that would aid in risk assessment and regulation of Ni in the environment, particularly in mining-affected regions.

Green Mines green energy : establishing productive land on mine tailings

The CANMET Mining and Mineral Sciences Laboratories (CANMET-MMSL) of Natural Resources Canada, Ottawa, has established a consortium initiative entitled “Green Mines Green Energy”. The consortium is composed of representatives from mining, forestry, government, academia and the private sector. The goal is to expand the practice of mine reclamation by furthering the use of organic residuals for the rehabilitation of mine sites to the extent that they can be used to establish feedstock for the production of biofuels. There is increasing pressure on industry and municipalities to divert clean organic waste materials from landfill and to establish productive uses for them. In many areas of Canada, these materials (particularly municipal SSO compost) are in reasonably close proximity to mines, and may offer a relatively stable, long term, beneficial disposal strategy. Laboratory studies are underway to investigate the potential impact of these materials on tailings oxidation and effluent chemistry, as well as the effect of biosolids/compost-derived dissolved organic carbon on effluent treatability and toxicity. The construction of several half-hectare field plots began on mine sites in 2008. This paper provides an overview of the Green Mines Green Energy initiative and a summary of laboratory and field testing undertaken to date. Overall, preliminary results from the column study suggest that sulphate reduction at the tailings – biosolids interface is occurring, and that steady state has not yet been reached after more than 1 year of testing. Furthermore, the addition DOC has had no significant effect on effluent treatability or toxicity.