Demonstrating the need for chemical exposure characterisation in a microplate test system: toxicity screening of sixteen pesticides on two marine microalgae

Toxicity of herbicides to the marine microalgae Tisochrysis lutea and Tetraselmis sp

Pesticides are ubiquitous in the catchments of the Great Barrier Reef (GBR) and regularly discharge into the nearshore waters. Effective management of pesticides requires suitable water quality guideline values (WQGVs), and further ecotoxicological data for many pesticides are needed to improve the reliability of environmental risk assessments. To help address this issue, toxicity thresholds were determined to two species of tropical marine microalgae Tisochrysis lutea and Tetraselmis sp. for a suite of herbicides detected in the GBR. Photosystem II (PSII) herbicides significantly reduced growth with no effect concentration (NEC) and 10% effect concentration (EC10) values spanning two orders of magnitude from 0.60 µg L-1 for diuron to 60 µg L-1 for simazine across both species. However, growth was insensitive to the non-PSII herbicides. The NEC/EC10 thresholds for most herbicide-microalgae combinations were greater than recent WQGVs intended to protect 99% of species (PC99); however, metribuzin was toxic to T. lutea at concentrations lower than the current PC99 value, which may have to be revisited. The toxicity thresholds for alternative herbicides derived here further inform the development of national and GBR-specific WQGVs, but more toxicity data is needed to develop WQGVs for the > 50 additional pesticides detected in catchments of the GBR.

Cytotoxicity of 19 Pesticides in Rainbow Trout Gill, Liver, and Intestinal Cell Lines

The rainbow trout gill cell line (RTgill-W1), via test guideline 249 of the Organisation for Economic Co-operation and Development, has been established as a promising New Approach Methodology, although to advance confidence in the method more case studies are needed that: 1) expand our understanding of applicability domains (chemicals with diverse properties); 2) increase methodological throughput (96-well format); and 3) demonstrate biological relevance (in vitro to in vivo comparisons; gill vs. other cells). Accordingly, the objective of our study was to characterize the cytotoxicity of 19 pesticides against RTgill-W1 cells, and also liver (RTL-W1) and gut epithelial (RTgutGC) cell lines, and then to compare the in vitro and in vivo data. Of the 19 pesticides tested, 11, 9, and 8 were cytotoxic to the RTgill-W1, RTL-W1, and RTgutGC cells, respectively. Six pesticides (carbaryl, chlorothalonil, chlorpyrifos, dimethenamid-P, metolachlor, and S-metolachlor) were cytotoxic to all three cell lines. Aminomethylphosphonic acid, chlorantraniliprole, dicamba, diquat, imazethapyr, and permethrin exhibited cell-line-specific toxicity. No cytotoxic responses were observed for three herbicides (atrazine, glyphosate, and metribuzin) and four insecticides (clothianidin, diazinon, imidacloprid, and thiamethoxam). When cytotoxicity was measured, there was a strong correlation (rs  = 0.9, p < 0.0001) between in vitro median effect concentration (EC50) values (based on predicted concentrations using the In Vitro Mass Balance Model Equilibrium Partitioning (IV-MBM EQP) Ver. 2.1) derived from RTgill-W1 and RTL-W1 cells with in vivo median lethal concentration (LC50) values from 96-h acute toxicity studies with trout. In all 28 cases, the in vitro EC50 was within 18-fold of the in vivo LC50. These data help increase our understanding of the ecotoxicological domains of applicability for in vitro studies using cultured rainbow trout cells, while also demonstrating that these assays performed well in a 96-well format and have promise to yield data of biological relevance. Environ Toxicol Chem 2024;00:1-13. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Pesticide soil microbial toxicity: setting the scene for a new pesticide risk assessment for soil microorganisms (IUPAC Technical Report)

Pesticides constitute an integral part of modern agriculture. However, there are still concerns about their effects on non-target organisms. To address this the European Commission has imposed a stringent regulatory scheme for new pesticide compounds. Assessment of the aquatic toxicity of pesticides is based on a range of advanced tests. This does not apply to terrestrial ecosystems, where the toxicity of pesticides on soil microorganisms, is based on an outdated and crude test (N mineralization). This regulatory gap is reinforced by the recent methodological and standardization advances in soil microbial ecology. The inclusion of such standardized tools in a revised risk assessment scheme will enable the accurate estimation of the toxicity of pesticides on soil microorganisms and on associated ecosystem services. In this review we (i) summarize recent work in the assessment of the soil microbial toxicity of pesticides and point to ammonia-oxidizing microorganisms (AOM) and arbuscular mycorrhizal fungi (AMF) as most relevant bioindicator groups (ii) identify limitations in the experimental approaches used and propose mitigation solutions, (iii) identify scientific gaps and (iv) propose a new risk assessment procedure to assess the effects of pesticides on soil microorganisms.

Dynamic Mass Balance Modeling for Chemical Distribution Over Time in In Vitro Systems With Repeated Dosing

As toxicologists and risk assessors move away from animal testing and more toward using in vitro models and biological modeling, it is necessary to produce tools to quantify the chemical distribution within the in vitro environment prior to extrapolating in vitro concentrations to human equivalent doses. Although models predicting chemical distribution in vitro have been developed, very little has been done for repeated dosing scenarios, which are common in prolonged experiments where the medium needs to be refreshed. Failure to account for repeated dosing may lead to inaccurate estimations of exposure and introduce bias into subsequent in vitro to in vivo extrapolations. Our objectives were to develop a dynamic mass balance model for repeated dosing in in vitro systems; to evaluate model accuracy against experimental data; and to perform illustrative simulations to assess the impact of repeated doses on predicted cellular concentrations. A novel dynamic in vitro partitioning mass balance model (IV-MBM DP v1.0) was created based on the well-established fugacity approach. We parameterized and applied the dynamic mass balance model to single dose and repeat dosing scenarios, and evaluated the predicted medium and cellular concentrations against available empirical data. We also simulated repeated dosing scenarios for organic chemicals with a range of partitioning properties and compared the in vitro distributions over time. In single dose scenarios, for which only medium concentrations were available, simulated concentrations predicted measured concentrations with coefficients of determination (R2) of 0.85–0.89, mean absolute error within a factor of two and model bias of nearly one. Repeat dose scenario simulations displayed model bias &lt;2 within the cell lysate, and ∼1.5-3 in the medium. The concordance between simulated and available experimental data supports the predictive capacity of the IV-MBM DP v1.0 tool, but further evaluation as empirical data becomes available is warranted, especially for cellular concentrations.

A QSTR model for toxicity prediction of pesticides towards Daphnia magna

Because of the large amount of pesticides discharged into rivers, adverse effects could be induced to aquatic organisms. Daphnia magna is often used as an indicator organism to evaluate the toxicity of pesticides. In this study, a quantitative structure-toxicity relationship (QSTR) model was established based on norm descriptors for predicting the acute toxicity of pesticides to Daphnia magna. The model results showed the good predictability (R_training² = 0.8045, R_testing² = 0.8224). The validation results of internal validation, external validation, Y-randomization test and application domain analysis demonstrated the model's stability, reliability and robustness. Therefore, the above results indicate that norm descriptors might be universal for describing the relationship between the toxicity and structures of pesticides compounds. Moreover, some pesticides' toxicities without experimental data were also predicted by this model.

Update and Evaluation of a High-Throughput In Vitro Mass Balance Distribution Model: IV-MBM EQP v2.0

This study demonstrates the utility of an updated mass balance model for predicting the distribution of organic chemicals in in vitro test systems (IV-MBM EQP v2.0) and evaluates its performance with empirical data. The IV-MBM EQP v2.0 tool was parameterized and applied to four independent data sets with measured ratios of bulk medium or freely-dissolved to initial nominal concentrations (e.g., C24/C0 where C24 is the measured concentration after 24 h of exposure and C0 is the initial nominal concentration). Model performance varied depending on the data set, chemical properties (e.g., "volatiles" vs. "non-volatiles", neutral vs. ionizable organics), and model assumptions but overall is deemed acceptable. For example, the r² was greater than 0.8 and the mean absolute error (MAE) in the predictions was less than a factor of two for most neutral organics included. Model performance was not as good for the ionizable organic chemicals included but the r² was still greater than 0.7 and the MAE less than a factor of three. The IV-MBM EQP v2.0 model was subsequently applied to several hundred chemicals on Canada's Domestic Substances List (DSL) with nominal effects data (AC50s) reported for two in vitro assays. We report the frequency of chemicals with AC50s corresponding to predicted cell membrane concentrations in the baseline toxicity range (i.e., >20-60 mM) and tabulate the number of chemicals with "volatility issues" (majority of chemical in headspace) and "solubility issues" (freely-dissolved concentration greater than water solubility after distribution). In addition, the predicted "equivalent EQP blood concentrations" (i.e., blood concentration at equilibrium with predicted cellular concentration) were compared to the AC50s as a function of hydrophobicity (log octanol-water partition or distribution ratio). The predicted equivalent EQP blood concentrations exceed the AC50 by up to a factor of 100 depending on hydrophobicity and assay conditions. The implications of using AC50s as direct surrogates for human blood concentrations when estimating the oral equivalent doses using a toxicokinetic model (i.e., reverse dosimetry) are then briefly discussed.

Considerations when using microplates and Neubauer counting chamber in ecotoxicity tests on microplastics

The ubiquity of microplastics raises issues regarding contamination control measures and laboratory practices. The objective was to adapt the use of counting chambers and plastic microplates on the ecotoxicity evaluation of microplastics. Counting chambers, originally used to quantify cells, can also be used to count high concentrations of microplastics (<100 μm) used in laboratory assays. By decontaminating the chamber and mixing the test solution with Nile Red (1:1), fluorescent particles can be easily counted under optical microscopy. Microplate wells, due to their composition, can be contaminated or release microplastics to the test medium, which can interfere with the results of ecotoxicity assays or spectroscopy readings. A cleaning method based on ethanol was developed, which effectively removed particles by 91% without interfering with microalgae yield. Besides providing practical applications that can improve ecotoxicity assays, this work intends to raise awareness on the need to adapt laboratory practices when working with microplastics.

Remediation of emerging environmental pollutants: A review based on advances in the uses of eco-friendly biofabricated nanomaterials

The increased environmental pollutants due to anthropogenic activities are posing an adverse effects and threat on various biotic forms on the planet. Heavy metals and certain organic pollutants by their toxic persistence in the environment are regarded as significant pollutants worldwide. In recent years, pollutants exist in various forms in the environment are difficult to eliminate by traditional technologies due to various drawbacks. This has lead to shifting of research for the development of cost-effective and efficient technologies for the remediation of environmental pollutants. The adaption of adsorption phenomenon from the traditional technologies with the modification of adsorbents at nanoscale is the trended research for mitigating the environmental pollutants with petite environmental concerns. Over the past decade, the hidden potentials of biological sources for the biofabrication of nanomaterials as bequeathed rapid research for remediating the environmental pollution in a sustainable manner. The biofabricated nanomaterials possess an inimitable phenomenon such as photo and enzymatic catalysis, electrostatic interaction, surface active site interactions, etc., contributing for the detoxification of various pollutants. With this background, the current review highlights the emerging biofabricated nano-based adsorbent materials and their underlying mechanisms addressing the environmental remediation of persistent organic pollutants, heavy metal (loid)s, phytopathogens, special attention to the reduction of pathogen-derived toxins and air pollutants. Each category is illustrated with suitable examples, fundamental mechanism, and graphical representations, along with societal applications. Finally, the future and sustainable development of eco-friendly biofabricated nanomaterial-based adsorbents is discussed.

Predicting exposure concentrations of chemicals with a wide range of volatility and hydrophobicity in different multi-well plate set-ups

Quantification of chemical toxicity in small-scale bioassays is challenging owing to small volumes used and extensive analytical resource needs. Yet, relying on nominal concentrations for effect determination maybe erroneous because loss processes can significantly reduce the actual exposure. Mechanistic models for predicting exposure concentrations based on distribution coefficients exist but require further validation with experimental data. Here we developed a complementary empirical model framework to predict chemical medium concentrations using different well-plate formats (24/48-well), plate covers (plastic lid, or additionally aluminum foil or adhesive foil), exposure volumes, and biological entities (fish, algal cells), focusing on the chemicals’ volatility and hydrophobicity as determinants. The type of plate cover and medium volume were identified as important drivers of volatile chemical loss, which could accurately be predicted by the framework. The model focusing on adhesive foil as cover was exemplary cross-validated and extrapolated to other set-ups, specifically 6-well plates with fish cells and 24-well plates with zebrafish embryos. Two case study model applications further demonstrated the utility of the empirical model framework for toxicity predictions. Thus, our approach can significantly improve the applicability of small-scale systems by providing accurate chemical concentrations in exposure media without resource- and time-intensive analytical measurements.

Progress on the usage of the rotifer Brachionus plicatilis in marine ecotoxicology: A review

The rotifer, Brachionus plicatilis, is a widely used model species in marine ecotoxicology for evaluating pollutions, toxins, and harmful algae. In this paper, the marine ecotoxicology of Brachionus plicatilis was reviewed, including toxicity measurements of harmful algae species and environmental stresses. In addition, marine pollution involving pesticides, heavy metals, drugs, petroleum, and petrochemicals were addressed. Methods for measuring toxicity were also discussed. The standard acute lethal assay and the chronic population dynamics test were indicated as common methods of toxicity evaluating using B. plicatilis. Research on other biomarkers, such as behaviour, enzyme activity, or gene expression, are also reported here, with potential applications for fast detection or the scientific exploration of underlying molecular mechanisms. It is suggested that the methods selected should reflect the experimental purpose. Additionally, series assays should be conducted for comprehensive evaluation of ecotoxicity as well as to elucidate the correct mechanisms. Genetic methods, such as transcriptomics, were suggested as useful tools for exploring the toxicity mechanism using the rotifer B. plicatilis.

Influence of eugenol on algal growth, cell physiology of cyanobacteria Microcystis aeruginosa and its interaction with signaling molecules

Essential oils (EOs) are naturally occurring substances that have shown great prospect in the field of antimicrobial, antioxidant and pest control by nontoxic mechanisms. In this regard, EOs are considered the promising and eco-friendly approach for controlling harmful algae. In this study, the anti-cyanobacterial activity of EOs eugenol against Microcystis aeruginosa are evaluated from the perspective of photosynthetic efficiency, the behavior of extracellular organic matter (EOM), endogenous plant hormone synthesis, and nitric oxide signaling pathway. Results showed that the photosynthetic activity of M. aeruginosa decreased significantly after eugenol treatments. Eugenol treatment resulted in cells rupture and the release of EOM. Levels of endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA) were enhanced separately by 2.32 and 2.01 times after 4 d of exposure to eugenol. And the inhibition of SA and JA biosynthesis further promotes the inhibitory effects of eugenol on algae. Additionally, the signaling molecule nitric oxide (NO) increased significantly by 3.78-fold. Furthermore, the influence of NO on microalgae exposed to eugenol was also determined, suggesting that the inhibitory effect of eugenol stress might be associated with NO generation in M. aeruginosa. These findings will be helpful for the understanding of the fate and potential of eugenol in harmful algae control.

Efficient fungal and bacterial facilitated remediation of thiencarbazone methyl in the environment

Triazole herbicide, Thiencarbazone-methyl (TCM) applied on different crops for weedicidal activity is associated with an inherent toxicity towards bladder and urinary functionality. TCM has been first time explored for its biodegradative behavior utilizing microbes, previously isolated from soils. Simulated bio-transformation assemblies of five fungal strains; Aspergillus flavus (AF), Penicillium chrysogenum (PC), Aspergillus niger (AN), Aspergillus terrus (AT), Aspergillus fumigatus (AFu) and two bacterial strains: Xanthomonas citri (XC), Pseudomonassyringae (PS), were utilized. 10 mg/L TCM concentration was set up utilizing each microbe and analysed for 42 days. TCM bio-degradation was evaluated by UV-Visible spectrophotometery and gas chromatography mass spectroscopy. Aspergillus terrus (R² = 0.86) and Penicillium chrysogenum (R² = 0.88) exhibited highest capability to metabolize TCM while forming intermediate metabolites including; 2,4-dihydro-[1,2,4] triazol-3-one, semicarbazide and urea, methyl 4-isocyanatosulfonyl-5-methylthiophene-3-carboxylate. TCM degradation by all strains AF, AFu, AN, PC, AT, PS and XC was found to be 74, 74, 81, 95, 98, 90 and 95%, respectively after 42 days elucidating the effectiveness of all the utilized strains in degrading TCM. Current investigations can impact vital bioremediation approaches for pesticides mitigation from the ecological compartments. Furthermore, present research can be extended to the optimization of the bio-deteriorative assays to be employed on the practical scale for the successful management of environment through sustainable and cost effective ways.

Time-Resolved Freely Dissolved Concentrations of Semivolatile and Hydrophobic Test Chemicals in In Vitro Assays-Measuring High Losses and Crossover by Headspace Solid-Phase Microextraction

In vitro assays are normally conducted in plastic multiwell plates open to exchange with the ambient air. The concentration of test substances freely available to cells is often not known, can change over time, and is difficult to measure in the small volumes in microplates. However, even a well-characterized toxicological response is of limited value if it cannot be linked to a well-defined exposure level. The aim of this study was to develop and apply an approach for determining time-resolved freely dissolved concentrations of semivolatile and hydrophobic organic chemicals (SVHOCs) in in vitro assays: (1) free fractions were measured by a new medium dilution method and (2) time-resolved loss curves were obtained by measurements of total concentrations in 96-well plates during incubations at 37 °C. Headspace solid-phase microextraction was used as an analytical technique for 24 model chemicals spanning 6 chemical groups and 4-5 orders of magnitude in K_ow and K_aw. Free fractions were >30% for chemicals with log K_ow < 3.5 and then decreased with increasing log K_ow. Medium concentrations declined significantly (>50%) within 24 h of incubation for all 20 chemicals having log K_ow > 4 or log K_aw > -3.5 in serum-free medium. Losses of chemicals were lower for medium containing 10% fetal bovine serum, most significantly for chemicals with log K_ow > 4. High crossover to neighboring wells also was observed below log K_ow of 4 and log K_aw of -3.5. Sealing the well plates had limited effect on the losses but clearly reduced crossover. The high losses and crossover of most tested chemicals question the suitability of multiwell plates for in vitro testing of SVHOCs and call for (1) test systems that minimize losses, (2) methods to control in vitro exposure, (3) analytical confirmation of exposure, and (4) exposure control and confirmation being included in good in vitro reporting standards.

Toxicity of binary mixtures of pesticides to the marine microalgae Tisochrysis lutea and Skeletonema marinoi: Substance interactions and physiological impacts

This study screened binary mixtures of pesticides for potential synergistic interaction effects on growth of the marine microalgae Tisochrysis lutea and Skeletonema marinoi. It also examined the single and combined effects of three of the most toxic substances on microalgal physiology. Single substances were first tested on each microalgal species to determine their respective EC₅₀ and concentration-response relationships. The toxicity of six and seven binary mixtures was then evaluated in microplate experiments on the growth of T. lutea and S. marinoi, respectively, using two mixture modelling approaches: isobolograms and the MIXTOX tool, based on Concentration Addition (CA) or Independent Action (IA) models. Significant cases of antagonism (for both species) and synergism (for S. marinoi) were observed for the mixtures of isoproturon and spiroxamine, and isoproturon and metazachlor, respectively. These two mixtures, together with that of isoproturon and diuron, for which additivity was observed, were further studied for their impacts on the physiology of each species. Exposures were thus made in culture flasks at three concentrations, or concentration combinations for mixtures, selected to cause 25%, 50% and 75% growth rate inhibition. The effects of the selected pesticides singly and in combination were evaluated at three perceived effect concentrations on esterase metabolic activity, relative lipid content, cytoplasmic membrane potential and reactive oxygen species (ROS) content by flow cytometry, and on photosynthetic quantum yield (ϕ'_M) by PAM-fluorescence. Isoproturon and diuron singly and in mixtures induced 20-40% decreases in ϕ'_M which was in turn responsible for a significant decrease in relative lipid content for both species. Spiroxamine and metazachlor were individually responsible for an increase in relative lipid content (up to nearly 300% for metazachlor on S. marinoi), as well as cell depolarization and increased ROS content. The mixture of isoproturon and metazachlor tested on S. marinoi caused a 28-34% decrease in ϕ'_M that was significantly higher than levels induced by each of substances when tested alone. This strong decrease in ϕ'_M could be due to a combined effect of these substances on the photosynthetic apparatus, which is likely the cause of the synergy found for this mixture.