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

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

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

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

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

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

As and Sb are more labile and toxic to water spinach (Ipomoea aquatica) in recently contaminated soils than historically co-contaminated soils.

Elevated concentrations of As and Sb impact environmental quality and human health. In this study total and bioavailable As and Sb were measured from recently and historically contaminated soils and the phytotoxicity of these soils was evaluated with Ipomoea aquatica (35-d exposure from germination) using biomass, length of plant tissues and photosynthetic efficiency. As and Sb were both present within the soil (co-contaminated). The bioavailable As and Sb in soils were determined by a Sequential Extraction Procedure (SEP) and compared to total soil concentrations and bioaccumulation in the edible parts of I. aquatica. For both As and Sb, bioavailable concentrations increased proportionally with the total soil concentrations and greater bioavailability in recently contaminated soil was observed. Tissue dry mass and length drastically reduced with increasing total and SEP-bioavailable As and Sb soil concentrations. The total soil concentration was a less sensitive measure of the phytotoxicity of As and Sb than the bioavailable fraction. Shoot length was inhibited by 50% (EC50) at bioavailable As concentrations of 80-96 mg kg-1 in both recently and historically contaminated soils; however, bioavailable Sb EC50 for shoot length was achieved at lower bioavailable concentrations, 96 (42-219) and 12 (7-19) mg kg-1 in recently contaminated soils and historically contaminated soils, respectively. Shoot biomass was inhibited by 50% (EC50) at bioavailable As concentrations of 11 (4-30) and 49 (37-65) mg kg-1 in recently and historically contaminated soils, respectively whereas this occurred at much lower bioavailable Sb concentrations, 2-5 mg kg-1 in both recently and historically contaminated soils. Aging is important in contaminated soils, it decreases the lability of As and Sb and hence their bioavailability to agricultural plants, thus posing a lower risk of exposure of these metalloids to humans through agricultural plants grown in contaminated soils.

Impact of hydrocarbons from a diesel fuel on the germination and early growth of subantarctic plants.

Special Antarctic Blend (SAB) is a diesel fuel dominated by aliphatic hydrocarbons that is commonly used in Antarctic and subantarctic regions. The past and present use of SAB fuel at Australia's scientific research stations has resulted in multiple spills, contaminating soils in these pristine areas. Despite this, no soil quality guidelines or remediation targets have been developed for the region, primarily due to the lack of established indigenous test species and subsequent biological effects data. In this study, twelve plant species native to subantarctic regions were collected from Macquarie Island and evaluated to determine their suitably for use in laboratory-based toxicity testing, using germination success and seedling growth (shoot and root length) as endpoints. Two soil types (low and high organic carbon (OC)) were investigated to reflect the variable OC content found in soils on Macquarie Island. These soils were spiked with SAB fuel and aged for 14 days to generate a concentration series of SAB-contaminated soils. Exposure doses were quantified as the concentration of total petroleum hydrocarbons (TPH, nC9-nC18) on a soil dry mass basis. Seven species successfully germinated on control soils under laboratory conditions, and four of these species (Colobanthus muscoides Hook.f., Deschampsia chapmanii Petrie, Epilobium pendunculare A.Cunn. and Luzula crinita Hook.f.) showed a dose-dependent inhibition of germination when exposed to SAB-contaminated soils. Contaminated soils with low OC were generally more toxic to plants than high organic carbon soils. Increasing soil-TPH concentrations significantly inhibited shoot and root growth, and root length was identified as the most sensitive endpoint. Although the test species were tolerant to SAB-contaminated soils in germination assays, development of early life stages (up to 28 days) were generally more sensitive indicator of exposure effects, and may be more useful endpoints for future testing.