Comparative sensitivity of green algae to herbicides using erlenmeyer flask and microplate growth-inhibition assays

The herbicide alachlor severely affects photosystem function and photosynthetic gene expression in the marine dinoflagellate Prorocentrum minimum

Alachlor is one of the most widely used herbicides and can remain in agricultural soils and wastewater. The toxicity of alachlor to marine life has been rarely studied; therefore, we evaluated the physiological and transcriptional responses in the marine dinoflagellate Prorocentrum minimum. The herbicide led to considerable decreases in P. minimum cell numbers and pigment contents. The EC₅₀ was determined to be 0.373 mg/L. Photosynthesis efficiency and chlorophyll autofluorescence dramatically decreased with increasing alachlor dose and exposure time. Real-time PCR analysis showed that the photosynthesis-related genes PmpsbA, PmatpB, and PmrbcL were induced the most by alachlor; the transcriptional level of each gene varied with time. PmrbcL expression increased after 30 min of alachlor treatment, whereas PmatpB and PmpsbA increased after 24 h. The PmpsbA expression level was highest (5.0 times compared to control) after 6 h of alachlor treatment. There was no significant change in PmpsaA expression with varying treatment time or concentration. Additionally, there was no notable change in the expression of antioxidant genes PmGST and PmKatG, or in ROS accumulation. These suggest that alachlor may affect microalgal photosystem function, with little oxidative stress, causing severe physiological damage to the cells, and even cell death.

Development and Application of a Microplate Assay for Toxicity Testing on Aquatic Cyanobacteria

Regulatory environmental risk assessment, applied to establish a protection limit for all bacterial diversity in surface waters, relies on a growth inhibition test performed on a single species of cyanobacteria and the activated sludge respiration inhibition test. Recently, the ability of this approach to protect adequately for bacteria that provide important ecosystem services has been questioned, and empirical data on additional species to further investigate the effectiveness of the environmental risk assessment are urgently required. We present the development and validation of a cost-effective and time-efficient microplate assay that is comparable to the traditional shake flask test for measurement of cyanobacteria growth rate after chemical exposure. The assay has been optimized to ensure that comparisons of cyanobacteria sensitivity under exponential growth are assessed across equivalent experimental conditions using phycocyanin fluorescence as a surrogate for cell density. The test system is validated using potassium dichromate, and the results are compared with those obtained in an Organisation for Economic Co-operation and Development (2011) test guideline 201 shake flask test system. This assay is suitable for the screening of new and legacy chemicals (including antibiotics) for which ecotoxicology data are lacking across a wide range of cyanobacteria, with the aim of developing more comprehensive environmental risk assessment. Environ Toxicol Chem 2020;39:705-720. © 2019 SETAC.

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

Pesticides used in viticulture create a potential risk for the aquatic environment due to drift during application, runoff and soil leaching. The toxicity of sixteen pesticides and one metabolite were evaluated on the growth of two marine microalgae, Tisochrysis lutea and Skeletonema marinoi, in 96-h exposure assays conducted in microplates. For each substance, concentrations of stock solutions were analytically measured and abiotic assays were performed to evaluate the chemical stability of pesticides in microplates. For two chemicals, microalgae exposures were run simultaneously in microplates and culture flasks to compare EC₅₀ calculated from the two exposure systems. Results from chemical analyses demonstrated the low stability of hydrophobic pesticides (log K_OW > 3). For such chemicals, EC₅₀ values calculated using measured pesticide concentrations were two-fold lower than those first estimated using nominal concentrations. Photosystem II inhibitors were the most toxic herbicides, with EC₅₀ values below 10 μg L^-1 for diuron and around double this for isoproturon. Chlorpyrifos-methyl was the only insecticide to significantly affect the growth of T. lutea, with an EC₅₀ around 400 μg L^-1. All fungicides tested were significantly toxic to both species: strobilurins showed low overall toxicity, with EC₅₀ values around 400 μg L^-1, whereas quinoxyfen, and spiroxamine, showed high toxicity to both species, especially to T. lutea, with an EC₅₀ below 1 μg L^-1 measured for spiroxamine in culture flasks. This study highlights the need to perform chemical analyses for reliable toxicity assessment and discusses the advantages and disadvantages of using microplates as a toxicity screening tool.

Effect of Bisphenol A on the extremophilic microalgal strain Picocystis sp. (Chlorophyta) and its high BPA removal ability

Bisphenol A (BPA) effects and removal by an alkaliphilic chlorophyta, Picocystis, were assessed. BPA at low concentrations (0-25 mg L^-1) did not inhibit the Picocystis growth and photosynthesis during 5 days of exposure. At higher BPA concentrations (50 and 75 mg L^-1), the growth inhibition did not exceed 43%. The net photosynthetic activity was dramatically reduced at high BPA concentrations while, the PSII activity was less affected. The exposure to increasing BPA concentrations induced an oxidative stress in Picocystis cells, as evidenced by increased malondialdehyde content and the over-expression of antioxidant activities (ascorbate peroxydase, gluthation-S-transferase and catalase). Picocystis exhibited high BPA removal efficiency, reaching 72% and 40% at 25 and 75 mg L^-1 BPA. BPA removal was ensured mainly by biodegradation/biotransformation processes. Based on these results, the extended tolerance and the high removal ability of Picocystis make her a promising specie for use in BPA bioremediation.

The use of multiwell culture plates in the duckweed toxicity test-A case study on Zn nanoparticles

Extensive production of nanomaterials of various properties needs to be coupled with rapid toxicity testing in order to provide information about their potential risks to the environment and human health. Miniaturization of toxicity tests may accelerate economical testing of nanomaterials, but is not a common practice. We describe a case study to miniaturize a commonly used toxicity test with plant duckweed Lemna minor. 6-well, 12-well and 24-well culture plates were used to assess their potential use for the duckweed toxicity test with potassium chloride as reference material. The results were compared to the standard test design using 100 mL glass beakers. The comparison showed that the best agreement was with the 6-well vessels. This set-up was further used for toxicity testing of zinc oxide nanoparticles (ZnO NP) and zinc chloride. Zinc was not adsorbed onto either glass or plastic walls of the miniaturized system. We assume that in both vessels a fast agglomeration and settling of ZnO NP took place. Linear regression and statistical testing indicated a good correlation between the toxicity results obtained in the standard test and miniaturized 6-well vessels. The miniaturization of the test system for assessing the biological effect of nanomaterials on Lemna minor could become an appropriate alternative to the traditionally used high volume vessels.

Combined effects of pharmaceuticals, personal care products, biocides and organic contaminants on the growth of Skeletonema pseudocostatum

Organisms in the environment are exposed to a number of pollutants from different compound groups. In addition to the classic pollutants like the polychlorinated biphenyls, polyaromatic hydrocarbons (PAHs), alkylphenols, biocides, etc. other compound groups of concern are constantly emerging. Pharmaceuticals and personal care products (PPCPs) can be expected to co-occur with other organic contaminants like biocides, PAHs and alkylphenols in areas affected by wastewater, industrial effluents and intensive recreational activity. In this study, representatives from these four different compound groups were tested individually and in mixtures in a growth inhibition assay with the marine algae Skeletonema pseudocostatum (formerly Skeletonema costatum) to determine whether the combined effects could be predicted by models for additive effects; the concentration addition (CA) and independent action (IA) prediction model. The eleven tested compounds reduced the growth of S. pseudocostatum in the microplate test in a concentration-dependent manner. The order of toxicity of these chemicals were irgarol>fluoxetine>diuron>benzo(a)pyrene>thioguanine>triclosan>propranolol>benzophenone 3>cetrimonium bromide>4-tert-octylphenol>endosulfan. Several binary mixtures and a mixture of eight compounds from the four different compound groups were tested. All tested mixtures were additive as model deviation ratios, the deviation between experimental and predicted effect concentrations, were within a factor of 2 from one or both prediction models (e.g. CA and IA). Interestingly, a concentration dependent shift from IA to CA, potentially due to activation of similar toxicity pathways at higher concentrations, was observed for the mixture of eight compounds. The combined effects of the multi-compound mixture were clearly additive and it should therefore be expected that PPCPs, biocides, PAHs and alkylphenols will collectively contribute to the risk in areas contaminated by such complex mixtures.

Implementation of a minimal set of biological tests to assess the ecotoxic effects of effluents from land-based marine fish farms

Environmental monitoring plans (EMP) that include chemical analysis of water, a battery of bioassays and the study of local hydrodynamic conditions are required for land-based marine aquaculture. In this study, the following standardized toxicity tests were performed to assess the toxicity of effluents from eight land-base marine fish farms (LBMFFs) located on the northwest coast of Spain: bacterial bioluminescence (with Vibrio fischeri at 15 and 30 min), microalgal growth (with Phaeodactyllum tricornutum and Isochrysis galbana) and sea urchin larval development (with Paracentrotus lividus and Arbacia lixula). These bioassays were evaluated for inclusion in routine fish farm monitoring. Effective concentrations (EC(5), EC(10), EC(20), EC(50)) for each bioassay were calculated from dose-response curves, obtained by fitting the bioassay results to the best parametric model. Moreover, a graphical method of integrating the results from the battery of bioassays and classifying the toxicity was proposed, and the potential ecotoxic effects probe (PEEP) index was calculated. The bacterial bioluminiscence test at 30min, growth of I. galbana and larval development of A. lixula were found to be the most sensitive and useful tests. Graphical integration of these test results enabled definition of the ecotoxicological profiles of the different farms. The PEEP index, considering EC(20), efficiently reflected the toxic loading potential of LBMFF effluents. In conclusion, a battery of bioassays with species from different low trophic levels is recommended as a rapid and cost-effective methodology for assessing LBMFF discharges. The graphical integration method and the PEEP index are proposed for consideration in EMPs for such farms.

Endogenous 4-hydroxy-2-nonenal in microalga Chlorella kessleri acts as a bioactive indicator of pollution with common herbicides and growth regulating factor of hormesis

Oxidative stress, i.e. excessive production of reactive oxygen species (ROS), leads to lipid peroxidation and to formation of reactive aldehydes (e.g. 4-hydroxy-2-nonenal; HNE), which act as second messengers of free radicals. It was previously shown that herbicides can induce ROS production in algal cells. In the current paper, the unicellular green microalga Chlorella kessleri was used to study the effect of two herbicides (S-metolachlor and terbuthylazine) and hydrogen peroxide (H(2)O(2)) on oxidative stress induction, HNE formation, chlorophyll content and the cell growth. Production of HNE was detected in this study for the first time in the cells of unicellular green algae using the antibody specific for the HNE-histidine adducts revealing the HNE-histidine adducts even in untreated, control C. kessleri. Exposure of algal cells to herbicides and H(2)O(2) increased the ROS production, modifying production of HNE. Namely, 4h upon treatment the levels of HNE-histidine conjugates were below controls. However, their amount increased afterwards. The increase of HNE levels in algae was followed by their increased growth rate, as was previously described for human carcinoma cells. Hence, changes in the cellular HNE content upon herbicide treatment inducing lipid oxidative stress and alterations in cellular growth rate of C. kessleri resemble adaptation of malignant cells to the HNE treatment. Therefore, as an addition to the standard toxicity tests, the evaluation of HNE-protein adducts in C. kessleri might indicate environmental pollution with lipid peroxidation-inducing herbicides. Finally, C. kessleri might be a convenient experimental model to further study cellular hormetic adaptation to oxidative stress-derived aldehydes.

Metribuzin impairs the unicell-colony transformation in the green alga Scenedesmus obliquus

Active growth is a prerequisite for the formation of grazing-protective, mostly eight-celled colonies by the ubiquitous green alga Scenedesmus in response to chemical cues from zooplankton. Colonies can also be evoked by chemically quite similar manmade anionic surfactants, such as FFD-6. In this study, it was hypothesized that growth-inhibiting concentrations of the herbicide metribuzin impair the ability of Scenedesmus obliquus to form colonies in response to the surfactant morphogen FFD-6. The results confirmed that the formation of colonies in S. obliquus was hampered by metribuzin. EC50 values of metribuzin for colony inhibition (approximately 11 μg L(-1)) were similar to those for growth and photosynthesis inhibition (12-25 μg metribuzinL(-1)). In the absence of the colony-inducing surfactant FFD-6, S. obliquus populations were comprised of 92% unicells, having on average 1.2 cells per colony at all tested metribuzin concentrations (0-100 μg L(-1)). In contrast, in the presence of FFD-6 and at low metribuzin concentrations (0 and 5 μg L(-1)), S. obliquus had more than five cells per colony with a high portion of eight-celled colonies. However, increasing concentrations of metribuzin decreased the number of colonies in the FFD-6-exposed populations and caused them to remain mostly unicellular at the highest concentrations (50 and 100 μg L(-1)). This study revealed that metribuzin impeded growth and by doing so, also obstructed the possibility for unicellular Scenedesmus to form colonies. Consequently, an increase in mortality of Scenedesmus from grazing is expected.

Rapid algal toxicity assay using variable chlorophyll fluorescence for Chlorella kessleri (chlorophyta)

Three methods of algal assays--the standard assay, microassay, and the proposed fluorescence assay--are compared from the point of view of reliability of EC50 detection, the minimum required time for the detection, sensitivity of individual measurement, i.e. at which cell density the particular assay can be used for EC50 estimation, and the time stability of the EC50 values. The assays were performed with green alga Chlorella kessleri strain LARG/1 growing in potassium dichromate solution in Z-medium ranging from 0.01 to 100 mg Cr L⁻¹. The inoculation cell density was set according to the standards to 10⁴ cells mL⁻¹ and according to spectrophotometer/plate reader detection limit. The average EC50 ranged from 0.096 to 0.649 mg Cr L⁻¹ and there were no significant differences in EC50 between the assay type and the inoculation methods with the exception of the significant difference between EC(c)50₇₂ (EC50 established from biomass measured as chlorophyll a concentration after 72 h of cultivation) in the standard assay and EC(r)50 (EC50 derived from growth rate) in the microassay in the standard inoculation experiment due to low variability of their values. The EC(f)50 (EC50 derived from variable fluorescence measurement) values correspond to EC50 values derived from the growth rates. Fluorescence measurement revealed the toxic effect of the chromium after 24 h of exposure at cell density of 5 x 10⁴ cells mL⁻¹, less by half than other used assay methods. The positive correlation of EC(f)50 and time was found in the standard inoculation experiment but opposite effect was observed at the spectrophotometric one.

Caffeine and agricultural pesticide concentrations in surface water and groundwater on the north shore of Kauai (Hawaii, USA)

Caffeine has been associated with wastewater pollution in temperate and subtropical locations, but environmental caffeine concentrations in tropical locations have not been reported. The objectives of this study were to measure caffeine and agricultural pesticide (carbaryl, metalaxyl, and metribuzin) concentrations in environmental waters on the tropical north shore of Kauai (Hawaii, USA) and assess whether patterns in caffeine concentration were consistent with a wastewater caffeine source. Groundwater, river, stream and coastal ocean samples were collected in August 2006 and February 2007. Caffeine was detected in all August 2006 samples and in 33% of February 2007 samples at concentrations up to 88ngL(-1). Metribuzin was detected in five samples collected in February 2007. Carbaryl and metalaxyl were not detected in any sample. Caffeine was not detected in offshore ocean samples or river samples upstream of human development. A positive correlation between caffeine and enterococci suggested a possible wastewater caffeine source.

Inhibition of three algae species using chemicals released from barley straw

Algal blooms are a significant problem in the UK, particularly in water sources that supply potable water treatment works. A wide range of methods to control algae have been tested and, whilst many are effective, they all have disadvantages. The use of barley straw to control algal growth in reservoirs is one option that is gaining popularity, but little is known about its mode of action. One suggested mechanism is that, as the straw is broken down, algastatic chemicals such as phenolics are released. Here we have used an algae inhibition test to evaluate the effect of chemicals reported to be released from straw on three common algal species: Chlorella vulgaris, Microcystis aeruginosa and Scenedesmus subspicatus. It was shown that, of the chemicals assessed, many produced an algastatic effect on the growth of the three algal species tested, with 2 phenyl-phenol being the most effective, whilst p-cresol and benzaldehyde were shown to be effective at concentrations similar to those that have been reported downstream of rotted straw. Scenedesmus subspicatus proved to be much more resistant to the chemicals tested than the other species.

Compatibility of hydroxypropyl-beta-cyclodextrin with algal toxicity bioassays

Numerous reports have indicated that hydrophobic organic compound bioaccessibility in sediment and soil can be determined by extraction using aqueous hydroxypropyl-beta-cyclodextrin (HPCD) solutions. This study establishes the compatibility of HPCD with Selenastrum capricornutum and assesses whether its presence influences the toxicity of reference toxicants. Algal growth inhibition (72 h) showed no significant (P>0.05) difference at HPCD concentrations up to and including 20 mM. HPCD presence did not influence the toxicity of the inorganic reference toxicant (ZnSO(4)), with IC50 values of 0.82 microM and 0.85 microM, in the presence and absence of HPCD (20 mM), respectively. However, HPCD presence (20 mM) reduced the toxicity of 2,4-dichlorophenol and the herbicides diuron and isoproturon. These reductions were attributed to inclusion complex formation between the toxicants and the HPCD cavity. Liberation of complexed toxicants, by sample manipulation prior to toxicity assessment, is proposed to provide a sensitive, high throughput, bioassay that reflects compound bioaccessibility.

Effects of cold-dark storage on growth of Cylindrotheca closterium and its sensitivity to copper

Cylindrotheca closterium cells were maintained at low temperature (4+/-1 degrees C) and dark conditions up to 21 weeks to assess the effect on survival and physiological status. From a control culture under standard conditions, three densities were prepared: (A) 2 x 10(4), (B) 10 x 10(4), and (C) 25 x 10(4) cells ml(-1). Weekly, inoculums of each stored density were exposed to continuous light and at 20+/-1 degrees C. Sensitivity to copper for microalgal cultures was evaluated in order to assess possible changes in cells sensitivity due to storage. Concurrently, assays with a control culture were carried out in order to assess the sensitivity of C. closterium to copper and to be able to generate a standard sensitivity control chart with a mean value of EC50-72 h+/-2SD (standard deviation). Density-C presented higher cell yield values, between 40% and 80% relative to control culture. Cell density showed to be important feature that may be taken into account in cell storage experiments. There was an increase in sensitivity of cells submitted to storage; however results always kept in the range established as standard sensitivity with no statistically significant difference with regards to control culture. EC50-72 h mean value for the control culture was 29+/-10 mug Cul(-1), while for densities-A, B and C were 22+/-7; 23+/-9 and 23+/-8 microg Cul(-1), respectively. In spite of drastic changes in the environmental conditions due to storage, it is concluded that C. closterium cells stored during 5 months remained metabolically active and with no significant change in its sensitivity.