Effect assessment of the herbicide paraquat on a green alga using differential gene expression and biochemical biomarkers

Harnessing the potential of microalgae-based systems for mitigating pesticide pollution and its impact on their metabolism

Due to increased pesticide usage in agriculture, a significant concentration of pesticides is reported in the environment that can directly impact humans, aquatic flora, and fauna. Utilizing microalgae-based systems for pesticide removal is becoming more popular because of their environmentally friendly nature, ability to degrade pesticide molecules into simpler, nontoxic molecules, and cost-effectiveness of the technology. Thus, this review focused on the efficiency, mechanisms, and factors governing pesticide removal using microalgae-based systems and their effect on microalgal metabolism. A wide range of pesticides, like atrazine, cypermethrin, malathion, trichlorfon, thiacloprid, etc., can be effectively removed by different microalgal strains. Some species of Chlorella, Chlamydomonas, Scenedesmus, Nostoc, etc., are documented for >90% removal of different pesticides, mainly through the biodegradation mechanism. The antioxidant enzymes such as ascorbate peroxidase, superoxide dismutase, and catalase, as well as the complex structure of microalgae cell walls, are mainly involved in eliminating pesticides and are also crucial for the defense mechanism of microalgae against reactive oxygen species. However, higher pesticide concentrations may alter the biochemical composition and gene expression associated with microalgal growth and metabolism, which may vary depending on the type of strain, the pesticide type, and the concentration. The final section of this review discussed the challenges and prospects of how microalgae can become a successful tool to remediate pesticides.

Ecotoxicology of the herbicide paraquat: effects on wildlife and knowledge gaps

Paraquat (PQ) is an organic herbicide introduced to the commercial market in 1962 and since linked to a variety of human health effects, including lung fibrosis, liver tumors, and Parkinson's disease. Although PQ is banned in the European Union, it is still frequently used in agricultural areas of the United States and Asia. The general mechanism of PQ's toxicity is the disruption of the redox cycle in cells. This mini-review summarizes our current understanding of PQ toxicity in non-target plants and animals. Among vertebrates, PQ sensitivity tends follow the pattern of fish > amphibians > mammals > birds. Aquatic plants are particularly vulnerable to PQ, with EC50 values ranging from ~28-280 μg/L. A number of convenient but non-specific biomarkers have been identified for non-target species, including the activities of antioxidant enzymes such as superoxide dismutase and catalase, histological changes in the gill structures of fish, and the upregulation of genes associated with the cytochrome p450 monooxygenase system. Significant literature gaps include a lack of data for environmentally realistic conditions (i.e., chronic, low concentration, multi-stressor), toxicity in reptiles, and population- and ecosystem-level effects. Although PQ is a useful herbicide, considering the many human and ecological health impacts, it may be time for regulators and the agricultural industry to reconsider its use.

Sub-lethal toxicity of five disinfection by-products on microalgae determined by flow cytometry - Lines of evidence for adverse outcome pathways

Standardised tests are often used to determine the ecotoxicity of chemicals and focus mainly on one or a few generic endpoints (e.g. mortality, growth), but information on the sub-cellular processes leading to these effects remain usually partial or missing. Flow cytometry (FCM) can be a practical tool to study the physiological responses of individual cells (such as microalgae) exposed to a stress via the use of fluorochromes and their morphology and natural autofluorescence. This work aimed to assess the effects of five chlorine-based disinfection by-products (DBPs) taken individually on growth and sub-cellular endpoints of the green microalgae Raphidocelis subcapitata. These five DBPs, characteristic of a chlorinated effluent, are the following monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), bromochloroacetic acid (BCAA) and 1,1-dichloropropan-2-one (1,1-DCP). Results showed that 1,1-DCP had the strongest effect on growth inhibition (EC50 = 1.8 mg.L-1), followed by MCAA, TCAA, BCAA and DCAA (EC50 of 10.1, 15.7, 27.3 and 64.5 mg.L-1 respectively). Neutral lipid content, reactive oxygen species (ROS) formation, red autofluorescence, green autofluorescence, size and intracellular complexity were significantly affected by the exposure to the five DBPs. Only mitochondrial membrane potential did not show any variation. Important cellular damages (>10%) were observed for only two of the chemicals (BCAA and 1,1-DCP) and were probably due to ROS formation. The most sensitive and informative sub-lethal parameter studied was metabolic activity (esterase activity), for which three types of response were observed. Combining all this information, an adverse outcome pathways framework was proposed to explain the effect of the targeted chemicals on R. subcapitata. Based on these results, both FCM sub-cellular analysis and conventional endpoint of algal toxicity were found to be complementary approaches.

Palmelloid-like phenotype in the alga Raphidocelis subcapitata exposed to pollutants: A generalized adaptive strategy to stress or a specific cellular response?

This work focuses on the formation of palmelloid-like phenotype in the freshwater alga Raphidocelis subcapitata (formerly known as Pseudokirchneriella subcapitata and Selenastrum capricornutum), when exposed to adverse conditions generated by the presence of organic [the antibiotic erythromycin (ERY) and the herbicide metolachlor (MET)] or inorganic [the heavy metals, cadmium (Cd) and zinc (Zn)] pollutants, at environmentally relevant concentrations. This alga in absence of stress or when exposed to ERY or Zn, up to 200 µg/L, essentially showed a single-nucleus state, although algal growth was reduced or stopped. R. subcapitata "switched" to a multinucleated state (palmelloid-like morphology) and accumulated energy-reserve compounds (neutral lipids) when stressed by 100-200 µg/L MET or 200 µg/L Cd; at these concentrations of pollutants, growth was arrested, however, the majority of the algal population (≥83 %) was alive. The formation of palmelloid-like phenotype, at sub-lethal concentrations of pollutants, was dependent on the pollutant, its concentration and exposure time. The multinucleated structure is a transitory phenotype since R. subcapitata population was able to revert to a single-nucleus state, with normal cell size, within 24-96 h (depending on the impact of the toxic in the alga), after being transferred to fresh OECD medium, without pollutants. The obtained results indicate that the formation of a palmelloid-like phenotype in R. subcapitata is dependent on the mode of action of toxics and their concentration, not constituting a generalized defense mechanism against stress. The observations here shown contribute to understanding the different strategies used by the unicellular alga R. subcapitata to cope with severe stress imposed by organic and inorganic pollutants.

Integrated physiological and metabolomic analysis reveals new insights into toxicity pathways of paraquat to Microcystis aeruginosa

Chemical pollutants, such as herbicides, released into the aquatic environment adversely affect the phytoplankton community structure. While majority of herbicides are specifically designed to target photosynthetic processes, they also can be toxic to phytoplankton; however, despite the photosynthetic toxicity, some herbicides can target multiple physiological processes. Therefore, a full picture of toxicity pathway of herbicide to phytoplankton is necessary. In the present study, the cyanobacterium Microcystis aeruginosa was exposed to two levels (17 μg L^-1 (EC₁₀) and 65 μg L^-1 (EC₅₀)) of paraquat for 72 h. The physiological and metabolic responses were analyzed to elucidate the toxicity pathway and establish the adverse outcome pathway of paraquat to M. aeruginosa. The results revealed that enhanced glycolysis (upregulation of pyruvic acid level) and tricarboxylic acid cycle (upregulation of the levels of malic acid, isocitric acid and citric acid) exposed to EC₁₀ level of paraquat, which probably acted as a temporary strategy to maintain a healthy energy status in M. aeruginosa cells. Meanwhile, the expressions of glutathione and benzoic acid were enhanced to scavenge the excessive reactive oxygen species (ROS). Additionally, the accumulation of pigments (chlorophyll a and carotenoid) might play a supplementary role in the acclimation to EC₁₀ level paraquat treatment. In cells exposed to paraquat by EC₅₀ level, the levels of SOD, CAT, glutathione and benzoic acid increased significantly; however, the ROS exceeded the tolerance level of antioxidant system in M. aeruginosa. The adverse effects were revealed by inhibition of chlorophyll a fluorescence, the decreases in several carbohydrates (e.g., glucose 1-phosphate, fructose and galactose) and total protein content. Consequently, paraquat-induced oxidative stress caused the growth inhibition of M. aeruginosa. These findings provide new insights into the mode of action of paraquat in M. aeruginosa.

Paraquat induces different programmed cell death patterns in Microcystis aeruginosa and Chlorella luteoviridis

Although programmed cell death (PCD) has been reported in phytoplankton, knowledge of the characterization of the PCD pathway and cascade process in different phytoplankton species is still limited. In this study, PCD progression in cyanobacterium Microcystis aeruginosa and green algae Chlorella luteoviridis by paraquat-induced oxidative stress was monitored. The results showed that paraquat-induced PCD in the two species belonged to the caspase-dependent pathway. Dose- and time-dependent PCD characteristics in the two strains under paraquat included the increase in caspase-like activity, DNA fragmentation, and chromatin condensation. However, the signaling pathway and cascade events of PCD in M. aeruginosa and C. luteoviridis differed. In M. aeruginosa, the free Ca²⁺ concentration was rapidly increased at 8 h, followed by a significant elevation of the reactive oxygen species (ROS) level at 24 h, and eventual cell death. In C. luteoviridis, the mitochondrial apoptosis pathway, revealed by the depolarization of the mitochondrial membrane potential at 1 h and increase in the ROS level and caspase-like activity at 8 h, might contribute to cell death. In addition, the dynamics of ROS levels and metacaspase activity were synchronized, suggesting that paraquat-triggered PCD was ROS-mediated in both M. aeruginosa and C. luteoviridis. These results provide insights into PCD patterns in prokaryotic cyanobacteria and eukaryotic green algae under similar stress.

Applicability Evaluation of Soil Algae Pipe Assay in Silver Nanoparticle-Contaminated Soils

Due to pervasive and resilient soil contaminants, heterogeneously contaminated soil poses unpredictable potential threats to ecosystems. In this study, the extension of a previously developed soil algae pipe assay for evaluating heterogeneously contaminated soil under an open system is described. The assay can be used in soil that is heterogeneously contaminated with silver nanoparticles in combination with the examination of morphological changes (e.g., in vivo chlorophyll a, cell granularity, cell size, and mucilaginous sheath) and lipid contents. In addition, we attempted to extend the exposure duration under an open system. We evaluated the applicability of this soil algae pipe assay using green alga Chlamydomonas reinhardtii exposed to heterogeneous and homogeneous polyvinylpyrrolidone capping silver nanoparticles in contaminated soils. The results demonstrated that this method is an applicable bioassay that can be employed to better evaluate soil algal toxicity under an open system, with significant changes in the measured endpoints. The developed assay showed decent predictivity, which can be a useful tool when evaluating heterogeneous soil algae contamination.

Pharmaceuticals and personal care products in aquatic environments and their removal by algae-based systems

The consumption of pharmaceuticals and personal care products (PPCPs) has been widely increasing, yet up to 90-95% of PPCPs consumed by human are excreted unmetabolized. Moreover, the most of PPCPs cannot be fully removed by wastewater treatment plants (WWTPs), which release PPCPs to natural water bodies, affecting aquatic ecosystems and potentially humans. This study sought to review the occurrence of PPCPs in natural water bodies globally, and assess the effects of important factors on the fluxes of pollutants into receiving waterways. The highest ibuprofen concentration (3738 ng/L) in tap water was reported in Nigeria, and the highest naproxen concentration (37,700 ng/L) was reported in groundwater wells in Penn State, USA. Moreover, the PPCPs have affected aquatic organisms such as fish. For instance, up to 24.4 × 10³ ng/g of atenolol was detected in P. lineatus. Amongst different technologies to eliminate PPCPs, algae-based systems are environmentally friendly and effective because of the photosynthetic ability of algae to absorb CO₂ and their flexibility to grow in different wastewater. Up to 99% of triclosan and less than 10% of trimethoprim were removed by Nannochloris sp., green algae. Moreover, variable concentrations of PPCPs might adversely affect the growth and production of algae. The exposure of algae to high concentrations of PPCPs can reduce the content of chlorophyll and protein due to producing reactive oxygen species (ROS), and affecting expression of some genes in chlorophyll (rbcL, psbA, psaB and psbc).

Long-term acclimation to cadmium exposure reveals extensive phenotypic plasticity in Chlamydomonas

Increasing industrial and anthropogenic activities are producing and releasing more and more pollutants in the environment. Among them, toxic metals are one of the major threats for human health and natural ecosystems. Because photosynthetic organisms play a critical role in primary productivity and pollution management, investigating their response to metal toxicity is of major interest. Here, the green microalga Chlamydomonas (Chlamydomonas reinhardtii) was subjected to short (3 d) or chronic (6 months) exposure to 50 µM cadmium (Cd), and the recovery from chronic exposure was also examined. An extensive phenotypic characterization and transcriptomic analysis showed that the impact of Cd on biomass production of short-term (ST) exposed cells was almost entirely abolished by long-term (LT) acclimation. The underlying mechanisms were initiated at ST and further amplified after LT exposure resulting in a reversible equilibrium allowing biomass production similar to control condition. This included modification of cell wall-related gene expression and biofilm-like structure formation, dynamics of metal ion uptake and homeostasis, photosynthesis efficiency recovery and Cd acclimation through metal homeostasis adjustment. The contribution of the identified coordination of phosphorus and iron homeostasis (partly) mediated by the main phosphorus homeostasis regulator, Phosphate Starvation Response 1, and a basic Helix-Loop-Helix transcription factor (Cre05.g241636) was further investigated. The study reveals the highly dynamic physiological plasticity enabling algal cell growth in an extreme environment.

Single and Combined Effects of Cypermethrin and UVR Pre-Exposure in the Microalgae Phaeodactylum Tricornutum

Coastal marine microalgae are exposed to anthropogenic pollutants, including pesticides from aquaculture/agriculture/household uses. Some microalgae species, such as Phaeodactylum tricornutum, can induce and accumulate UV-absorbing compounds (UACs) upon ultraviolet radiation (UVR) exposure to prevent deleterious effects. Tolerance mechanisms activated by natural stressors might also protect organisms from anthropogenic stressors. This work assesses the effects of the insecticide cypermethrin (Cyp) and UVR in the marine microalgae P tricornutum. Considering the pro-oxidant properties of both stressors and UACs' induction in P tricornutum, lethal and sublethal effects of Cyp were tested in cultures with and without UVR acclimation. After a 24-h exposure to 10 μg L^-1 of technical Cyp or culture medium, UACs, growth, glutathione-S-transferase activity (GST), sulfhydryl groups (SH-g), and lipid peroxidation (LPO) were analyzed. Results showed differences in terms of growth between Cyp and Cyp + UVR pre-exposure. UACs' content was induced after UVR acclimation and diminished after 24 h of growth in control and UVR pre-treated cultures, while levels remained constant under Cyp exposure. A single Cyp exposure exerted GST induction, SH-g depletion, and LPO increments. In UVR-acclimatized treatments, oxidative stress responses showed similar or more pronounced effects than the single chemical exposure, suggesting a potential additive effect of the UVR acclimation. The contrasting effects of Cyp + UVR observed between growth and biochemical responses suggest different compensatory mechanisms that need to be further investigated. Also, it highlights the need to include both lethal and sublethal endpoints to understand microalgae's tolerance and its significance in the multiple stressors' context.

Growth and metabolic responses to methyl viologen (1,1'-dimethyl - 4,4'-bipyridinium dichloride) on Chlorella vulgaris

Aquatic environments are especially susceptible to being contaminated by pesticides used in agricultural fields. Methyl viologen (MV) is an herbicide with high effectiveness for the control of unwanted land plants; however, it also has a high toxicity towards the algae in the aquatic environment. The objective of this work was to describe the effect of MV on photosynthetic metabolism and its relationship with respiration, growth and the content of photosynthetic pigments of Chlorella vulgaris. The cultures of C. vulgaris were exposed for 72 h at different concentrations of methyl viologen. The results show that growth, pigment content and metabolic activity decrease as the concentration of MV increases. Analysis of the photochemical activity indicates that MV produces an inhibition of electron transport between quinone A and quinone B of photosystem II. The inhibition of photosynthetic electron transport is directly related to the reduction of metabolic activity and cell growth. The results found in this research show that methyl viologen can be a toxic pollutant for primary producers in aquatic environments.

Exogenous Abscisic Acid Confers Salinity Tolerance in Chlamydomonas reinhardtii During Its Life Cycle

The plant hormone abscisic acid (ABA) coordinates responses to environmental signals with developmental changes and is important for stress resilience and crop yield. However, fundamental questions remain about how this phytohormone affects microalgal growth and stress regulation throughout the different stages of their life cycle. In this study, the effects of ABA on the physiology of the freshwater microalga Chlamydomonas reinhardtii at its different life cycle stages were investigated. Exogenously added ABA enhanced the growth and photosynthesis of C. reinhardtii during the vegetative stage. The hormone also increased the tolerance of this alga to high-salinity stress during gamete formation under nutrient depletion, as well as it extended their survival. We show that the level of reactive oxygen species (ROS) generated in the ABA-treated cells was significantly less than that in the untreated cells under inhibiting NaCl concentrations. Cell size examination showed that ABA prevents cells from forming palmella when exposed to high salinity. All together, these results suggest that ABA can support the vitality and survival of C. reinhardtii under high salt conditions.

Morphological plasticity in Chlamydomonas reinhardtii and acclimation to micropollutant stress

Phytoplankton are characterized by a great phenotypic plasticity and amazing morphological variability, both playing a primary role in the acclimation to changing environments. However, there is a knowledge gap concerning the role of algal morphological plasticity in stress responses and acclimation to micropollutants. The present study aims at examining palmelloid colony formation of the green alga Chlamydomonas reinhardtii upon micropollutants exposure. Cells were exposed to four micropollutants (MPs, copper, cadmium, PFOS and paraquat) with different modes of action for a duration of 72 h. Effects of MPs on palmelloid formation, growth and physiological traits (chlorophyll fluorescence, membrane integrity and oxidative stress) were monitored by flow cytometry and fluorescence microscopy. Palmelloid formation was observed upon treatment with the four micropollutants. Number of palmelloid colonies and their size were dependent on MP concentration and exposure duration. Cells reverted to their unicellular lifestyle when colonies were harvested and inoculated in fresh medium indicating that palmelloid formation is a plastic response to micropollutants. No physiological effects of these compounds were observed in cells forming palmelloids. Palmelloid colonies accumulated lower Cd concentration than unicellular C. reinhardtii suggesting that colony formation protects the cells from MPs stress. The results show that colony formation in Chlamydomonas reinhardtii is a stress response strategy activated to face sub-lethal micropollutant concentrations.

Photoautotrophic cultivation of Chlamydomonas reinhardtii in open ponds of greenhouse

Chlamydomonas reinhardtii is one of the most characterized green algae. The open-pond cultivation can be challenging due to sensitivity of strain to fluctuating environmental conditions and unavailability of low-cost photoautotrophic media. In this study, the photoautotrophic growth of C. reinhardtii was evaluated in 1-m² open ponds placed in greenhouse. Sodium bicarbonate (NaHCO₃) was evaluated as an alternative buffering agent to tris. The effect of buffer and pH was tested. The growth was studied in the presence of various nitrogen [urea and ammonium bicarbonate (NH₄HCO₃)] sources. In the study, it was found that 125-ppm NaHCO₃ as an optimum concentration. The buffering agent in the media was found to have major impact on growth. Without buffering agent, culture did not grow, and pH drop was observed. The sodium bicarbonate-buffered media reported to have the lowest bacterial contamination (18.3%), highest AFDW per OD (0.39 ± 0.027 g/L) and higher Fv/Fm (0.714 ± 0.016), whereas these values were found to be 62%, 0.19 ± 0.02 g/L and 0.537 ± 0.053 for tris-grown culture, respectively. The pH 7.0-7.5 was determined as an optimum, whereas pH 6.5-7.0 and 8.0-8.5 were found to affect the growth and induce palmelloidy. The OD and AFDW of culture grown in NH₄HCO₃ were found equivalent to a standard nitrogen source (NH₄Cl), whereas culture shown poor growth in urea. Based on these data, NH₄HCO₃ media recipe and the optimized cultivation parameters were selected for photoautotrophic cultivation of Chlamydomonas in greenhouse open ponds.

Evaluation of the toxic response induced by BDE-47 in a marine alga, Phaeodactylum tricornutum, based on photosynthesis-related parameters

The pollution of polybrominated diphenyl ethers (PBDEs) is becoming a pressing environmental problem in aquatic environments, and its threat to aquatic organism has received much attention. In this study, Phaeodactylum tricornutum was treated with 0.8 and 4 mg L^-1 2,2',4,4'-tetrabrominated biphenyl ether (BDE-47), the most toxic PBDEs, for 96 h. BDE-47 inhibited cell growth in a time- and concentration-dependent manner. Observation of cell ultrastructure suggested the damage of the chloroplasts morphology. BDE-47 also decreased the chlorophyll content and the oxygen evolution rate, and altered the performance of photosystems. Transcriptomic analysis revealed differential expression of 62 genes related to photosynthesis in BDE-47 treatments (4 mg L^-1) and transcription suppression of 58 genes involved in chlorophyll synthesis, antenna proteins, oxygen evolution, electron transport and downstream carbon fixation, implying potential toxicity targets in cells. Additionally, the levels of reactive oxygen species (ROS) and lipid peroxidation increased under BDE-47 stress and were positively correlated with photosynthesis inhibition. Pretreatment with the ROS scavenger N-acetyl-l-cysteine reduced the extent of inhibition, suggesting that ROS was responsible for these effects. Another experiment with the electron transport chain inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea showed that the generation of ROS was partially blocked, primarily indicating that photosynthetic inhibition induced by BDE-47 contributed to ROS overproduction. Thus, BDE-47 inhibited the photosynthesis by down-regulating the gene expression. This change stimulated ROS production, further leading to chloroplast membrane damage to aggravate this inhibition via a feedback loop. These effects of BDE-47 had adverse outcomes on the entire physiological state and the population growth of the microalgae.

Effects of Bisphenol A on the microalga Chlamydomonas reinhardtii and the clam Corbicula fluminea

Bisphenol A (BPA) is used throughout the world and it could enter aquatic ecosystems causing harmful effects on humans, animals and plants. The current study relies on the investigation of the toxicity of this emerging pollutant on two freshwater species from different trophic levels: the microalga Chlamydomonas reinhardtii and the clam Corbicula fluminea. After 96 h of exposure to several concentrations of BPA, the growth of C. reinhardtii was affected, being the 96 h-EC₅₀ value for growth 30 mg L^-1. The toxicity and bioaccumulation of 30 mg L^-1 BPA in microalgae after 24 h of exposure were studied. Several cytotoxicity biomarkers such as vitality, oxidative stress and cytoplasmic membrane potential were altered in exposed cells and microalgae accumulated 0.16 pg BPA cell^-1. Regarding C. fluminea, four treatments were established: control without BPA (C); BPA in the food (microalgae pre-exposed for 24 h to 30 mg L^-1) (M); BPA in the water (7.5 mg L^-1) (W); BPA in both food and water (M + W). After one month of exposure, treated bivalves showed a significantly decrease in the filtration rate and increased lipid peroxidation levels, indicating fitness reduction and oxidative damage. Furthermore, the activities of catalase, glutathione reductase, Se-dependent and total glutathione peroxidase enzymes increased significantly in W and M + W treatments with respect to the control. Clams of the M + W treatment were the most affected, indicating that the little amount of BPA bioaccumulated by microalgae could increase the damage. Emerging contaminants may accumulate in several organisms, such as microalgae, and could have negative impacts on ecosystems.

Dipteryx alata, a tree native to the Brazilian Cerrado, is sensitive to the herbicide nicosulfuron

The expansion of land use for agricultural interests and the excessive use of herbicides are among the causes of biodiversity losses in the Brazilian Cerrado biome. Therefore, we aimed to test the hypothesis that Dipteryx alata Vogel, a common species in this biome, is sensitive to nicosulfuron because of its high phytotoxicity. We evaluated physiological, biochemical and morphological responses in D. alata plants exposed to increasing doses of the herbicide. Young plants were transplanted to 10 L pots containing substrate composed of soil and sand (2:1) after fertilization. After an acclimation period, the following doses of nicosulfuron were applied: 0 (control), 6, 12, 24, 48, and 60 g a.e. ha^-1. The experiment was conducted in a randomized block design factorial scheme with six doses of nicosulfuron, three evaluation times, and five replicates per treatment. The effects of the herbicide were assessed by measuring gas exchange, chlorophyll a fluorescence, photosynthetic pigments, membrane permeability, antioxidant enzymes and acetolactate synthase. Nicosulfuron altered the photosynthetic machinery and enzymatic metabolism of D. alata. Reductions in physiological traits, increased catalase and ascorbate peroxidase activities, enhanced malondialdehyde concentrations rate of electrolyte leakage and decreased acetolactate synthase activity in response to nicosulfuron all suggest that D. alata is sensitive to this herbicide.

Differential toxicity of the UV-filters BP-3 and BP-4 in Chlamydomonas reinhardtii: A flow cytometric approach

Due to the concern about the negative effects of exposure to sunlight, UV-filters are being introduced in all kind of cosmetic formulas. Wastewater treatment plants are not able to remove and/or degrade them; consequently they find their way into rivers, lakes and oceans. These chemicals are acquiring a concerning status due to their increasingly common use and the potential risk for the environment. Benzophenone-3 (BP-3) and Benzophenone-4 (BP-4) are broad-spectrum UV-filters used for the same purpose in personal care products, insecticides and plastic bags; however, after 96 h of exposure to several concentrations of these UV-filters, the growth of C. reinhardtii was more affected by BP-3 than by BP-4, being the 96 h-EC₅₀ for growth 5 mg L^-1 and 38 mg L^-1, respectively. Based on these values Chlamydomonas reinhardtii cultures were exposed during 24 h to 2.5, 5 and 10 mg L^-1 of BP-3 and 19, 38 and 76 mg L^-1 of BP-4. A cytometric panel was carried out to evaluate the effect of sublethal concentrations of these UV-filters, thus several cytotoxicity biomarkers were analysed, including chlorophyll a fluorescence, viability, metabolic activity, oxidative stress, cytoplasmic and mitochondrial membrane potentials, and intracellular pH. BP-3 and BP-4 affect C.reinhardtii cells in a different way, showing differences for three of the examined parameters. Chlorophyll a fluorescence and mitochondrial membrane potential showed a significant increase (p < 0.05) in BP-3 and a significant decrease in BP-4, whereas viability only decreased significantly in the highest concentrations of BP-3. Regarding to the other parameters analysed, a similar pattern of cytotoxicity was observed. Growth rate, vital population and metabolic activity (esterase activity) and intracellular pH decreased significantly and cytoplasmic membrane potential and ROS levels increased significantly in cultures exposed to both pollutants.

Oxidative stress potential of the herbicides bifenox and metribuzin in the microalgae Chlamydomonas reinhardtii

The widespread presence of herbicides in the aquatic environment has raised awareness about the need to develop further in depth ecotoxicological risk assessments, more specifically on potential effects on photosynthetic organisms as microalgae. The majority of the information available regarding the toxicity of herbicides towards microalgae is related to traditional toxicological and regulatory-relevant endpoints such as growth inhibition, leaving a significant gap on knowledge regarding underlying interactions and damage to biological targets. In this context, this study aimed to supplement the general toxicity information of bifenox and metribuzin in the microalgae Chlamydomonas reinhardtii using a battery of selected high-throughput methods. This multiple-endpoint approach included the measurement of formation of reactive oxygen species (ROS), alterations in reduced glutathione (GSH) content, formation of lipid peroxidation (LPO), photosystem II (PSII) performance and loss of photosynthetic pigments after 24 h exposure. Results obtained showed that both herbicides caused a concentration-dependent increase in ROS formation, with bifenox showing higher but less reactive ROS. This increase in ROS production by bifenox and metribuzin was followed by alterations in the antioxidant capacity of algae, oxidative damage in the form of LPO and alterations in pigment content. Furthermore, both herbicides impacted the photosynthetic activity of algae, as seen by alterations in the maximum and effective quantum efficiency of PSII, PSII photochemistry and energy dissipation pathways, impact in the water-splitting apparatus and reduction in the electron transport rate. The inhibitory effect of metribuzin on photosynthetic processes/components was larger than that seen for bifenox. The impact of bifenox and metribuzin in the photosynthetic processes of C. reinhardtii seems to be in close association with the formation of ROS and consequent oxidative stress and damage in algal cells. Overall, this study showed that the high-throughput methods developed could successfully characterise both potential Modes of Action and adverse effects of bifenox and metribuzin in C. reinhardtii.

Transketolase Is Identified as a Target of Herbicidal Substance α-Terthienyl by Proteomics

α-terthienyl is a natural phytotoxin isolated originally from Flaveria bidentis (L.) Kuntze. The bioassay presented here shows the strong herbicidal activity of α-terthienyl on Digitaria sanguinalis, Arabidopsis thaliana and Chlamydomonas reinhardtii. The α-terthienyl-induced response of A. thaliana at the protein level was analyzed at different times. Changes in the protein expression profiles were analyzed by two-dimensional gel electrophoresis and liquid chromatography tandem mass spectrometry (LC-MS/MS) mass spectrometry. Sixteen protein spots were identified that showed reproducible changes in the expression of at least 2-fold when compared to the control. Among these 16 spots, three were up-regulated and 13 were down-regulated. The decreased expression of several proteins associated with energy production and carbon metabolism suggested that these processes were affected by α-terthienyl. To search for the candidate proteins in this screen, A. thaliana T-DNA mutants of the candidate proteins were used to test their susceptibility to α-terthienyl. Amongst the others, attkl1, a mutant of transketolase, exhibited a significantly lower sensitivity to α-terthienyl when hit compared with Col-0. Based on the identification of the proteins associated with the response to α-terthienyl by proteomics, a candidate target protein transketolase was identified.

Quantification of silver nanoparticle toxicity to algae in soil via photosynthetic and flow-cytometric analyses

Soil algae, which have received attention for their use in a novel bioassay to evaluate soil toxicity, expand the range of terrestrial test species. However, there is no information regarding the toxicity of nanomaterials to soil algae. Thus, we evaluated the effects of silver nanoparticles (0-50 mg AgNPs/kg dry weight soil) on the soil alga Chlamydomonas reinhardtii after six days, and assessed changes in biomass, photosynthetic activity, cellular morphology, membrane permeability, esterase activity, and oxidative stress. The parameters measured were markedly affected by AgNP-induced stress at 50 mg AgNPs/kg dry weight soil, where soil algal biomass, three measures of photosynthetic activity (area, reaction center per absorption flux, and reaction center per trapped energy flux), and esterase activity decreased. AgNPs also induced increases in both cell size and membrane permeability at 50 mg AgNPs/kg dry weight soil. In addition to the increase in cell size observed via microscopy, a mucilaginous sheath formed as a protective barrier against AgNPs. Thus, the toxicity of AgNPs can be effectively quantified based on the physiological, biochemical, and morphological responses of soil algae, where quantifying the level of toxicity of AgNPs to soil algae could prove to be a useful method in terrestrial ecotoxicology.

Effects of diuron and carbofuran and their mixtures on the microalgae Raphidocelis subcapitata

In aquatic environments, organisms are often exposed to mixtures of several pesticides. In this study, the effects of carbofuran and diuron and their mixtures on the microalgae Raphidocelis subcapitata were investigated. For this purpose, toxicity tests were performed with the single compounds (active ingredients and commercial formulations) and their combinations (only active ingredients). According to the results, the toxicity of active ingredients and their commercial formulations to R. subcapitata was similar. In the single exposures, both carbofuran and diuron inhibited significantly the R. subcapitata growth and caused physiological (chlorophyll a content) and morphological (complexity and cell size) changes in cells, as captured by flow cytometry single-cell properties. Regarding the mixture toxicity tests, data fitted to both reference models, concentration addition (CA) and independent action (IA), and evidenced significant deviations. After the CA fitting, dose-ratio dependent deviation had the best fit to the data, demonstrating synergism caused mainly by diuron and antagonism caused mainly by carbofuran. After fitting the IA model, a synergistic deviation represented the best fit for the diuron and carbofuran mixtures. In general, the two reference models indicated the occurrence of synergism in the mixtures of these compounds, especially when diuron was the dominant chemical in the combinations. The increased toxicity caused by the mixture of these pesticides could pose a greater environmental risk for phytoplankton. Thus, exposure to diuron and carbofuran mixtures must also be considered in risk assessments, since the combination of these compounds may result in more severe effects on algae population growth than single exposures.

Oxidative stress in the algae Chlamydomonas reinhardtii exposed to biocides

The toxicity of biocides can be associated with the formation of reactive oxygen species (ROS) and subsequent oxidative damage, interfering with the normal function of photosynthetic organisms. This study investigated the formation and effects of ROS in the unicellular green algae Chlamydomonas reinhardtii exposed to three environmentally relevant biocides, aclonifen, dichlofluanid and triclosan. After a first screening to identify which biocides induced ROS, a 24h multi-endpoint analysis was used to verify the possible consequences. A battery of high-throughput methods was applied in algae for measuring ROS formation, reduced glutathione (GSH), lipid peroxidation (LPO), photosystem (PS) II performance and pigments (chlorophylls a, b and carotenoids). Results show that only aclonifen induced ROS after the first 6h exposure, with the other two biocides not showing any ROS formation. Aclonifen, a Protox and carotenoid inhibitor, induced a concentration-dependent ROS formation, LPO and interfered with algae pigments content, while no alterations were detected in GSH content. A significant effect was also seen in the photosynthetic process, especially a reduction in the maximum and effective quantum yields, accompanied by alterations in energy dissipation in PSII reaction centers and the impairment of the electron transport rate. This study demonstrated the successful use of a battery of high-throughput methods for quickly screening biocides capacity to induce the formation of ROS and the subsequent effects in C. reinhardtii, thus revealing their mode of action (MoA) at concentrations before an impact on growth can become effective.

Sensitivity of the green algae Chlamydomonas reinhardtii to gamma radiation: Photosynthetic performance and ROS formation

The aquatic environment is continuously exposed to ionizing radiation from both natural and anthropogenic sources, making the characterization of ecological and health risks associated with radiation of large importance. Microalgae represent the main source of biomass production in the aquatic ecosystem, thus becoming a highly relevant biological model to assess the impacts of gamma radiation. However, little information is available on the effects of gamma radiation on microalgal species, making environmental radioprotection of this group of species challenging. In this context, the present study aimed to improve the understanding of the effects and toxic mechanisms of gamma radiation in the unicellular green algae Chlamydomonas reinhardtii focusing on the activity of the photosynthetic apparatus and ROS formation. Algal cells were exposed to gamma radiation (0.49-1677mGy/h) for 6h and chlorophyll fluorescence parameters obtained by PAM fluorometry, while two fluorescent probes carboxy-H₂DFFDA and DHR 123 were used for the quantification of ROS. The alterations seen in functional parameters of C. reinhardtii PSII after 6h of exposure to gamma radiation showed modifications of PSII energy transfer associated with electron transport and energy dissipation pathways, especially at the higher dose rates used. Results also showed that gamma radiation induced ROS in a dose-dependent manner under both light and dark conditions. The observed decrease in photosynthetic efficiency seems to be connected to the formation of ROS and can potentially lead to oxidative stress and cellular damage in chloroplasts. To our knowledge, this is the first report on changes in several chlorophyll fluorescence parameters associated with photosynthetic performance and ROS formation in microalgae after exposure to gamma radiation.

Time-Dependent Effects in Algae for Chemicals with Different Adverse Outcome Pathways: A Novel Approach

Chemicals affect unicellular algae as a result of toxicokinetic and toxicodynamic processes. The internal concentration of chemicals in algae cells typically reaches equilibrium within minutes, while damage cumulatively increases over hours. The time gap between the steady state of internal exposure and damage development is thus suspected to span up to hours, mainly due to toxicodynamic processes. The quantification of rate-limited toxicodynamic processes, aggregated as a progressive effect from an initiating molecular event through biological key events toward the adverse outcome on algae growth inhibition, might discriminate between different adverse outcome pathways (AOPs). To support our hypothesis, we selected six chemicals according to different physicochemical properties and three distinctly dissimilar AOPs. The time courses of internal concentrations were linked to the observed affected Scenedesmus vacuolatus growth using toxicokinetic-toxicodynamic modeling. Effects on cell growth were explained by effect progression and not by the time to reach internal equilibrium concentration. Effect progression rates ranged over 6 orders of magnitude for all chemicals but varied by less than 1 order of magnitude within similar AOP (photosystem II inhibitors > reactive chemicals > lipid biosynthesis inhibitors), meaning that inhibitors of photosystem II advance an effect toward algae growth fastest compared to reactive chemicals and inhibitors of lipid biosynthesis.

Early alterations on photosynthesis-related parameters in Chlamydomonas reinhardtii cells exposed to atrazine: A multiple approach study

Chlamydomonas reinhardtii cells were exposed to a sublethal concentration of the widespread herbicide atrazine for 3h. Physiological cellular parameters, such as chlorophyll a fluorescence and oxidative stress monitored by flow cytometry and pigments levels were altered in microalgal cells exposed to 0.25 μM of atrazine. Furthermore, the effects of this herbicide on C. reinhardtii were explored using "omics" techniques. Transcriptomic analyses, carried out by RNA-Seq technique, displayed 9 differentially expressed genes, related to photosynthesis, between control cultures and atrazine exposed cultures. Proteomic profiles were obtained using iTRAQ tags and MALDI-MS/MS analysis, identifying important changes in the proteome during atrazine stress; 5 proteins related to photosynthesis were downexpressed. The results of these experiments advance the understanding of photosynthetic adjustments that occur during an early herbicide exposure. Inhibition of photosynthesis induced by atrazine toxicity will affect the entire physiological and biochemical states of microalgal cells.

Modelling the effect of exposing algae to pulses of S-metolachlor: How to include a delay to the onset of the effect and in the recovery

In agriculture, herbicides are applied to improve crop productivity. During and after rain event, herbicides can be transported by surface runoff in streams and rivers. As a result, the exposure pattern in creeks is time-varying, i.e., a repeated pollution of aquatic system. In previous studies, we developed a model to assess the effects of pulse exposure patterns on algae. This model was validated for triazines and phenylureas, which are substances that induce effects directly after exposure with no delay in recovery. However, other herbicides display a mode of action characterized by a time-dependency effect and a delay in recovery. In this study, we therefore investigate whether this previous model could be used to assess the effects of pulse exposure by herbicides with time delay in effect and recovery. The current study focuses on the herbicide S-metolachlor. We showed that the effect of the herbicide begins only after 20 h of exposure for the alga Scenedesmus vacuolatus based on both the optical density and algal cells size measurements. Furthermore, the duration of delay of the recovery for algae previously exposed to S-metolachlor was 20 h and did not depend on the pulse exposure duration or the height of the peak concentration. By accounting for these specific effects, the measured and predicted effects were similar when pulse exposure of S-metolachlor is tested on the alga S. vacuolatus. However, the sensitivity of the alga is greatly modified after being previously exposed to a pulse of S-metolachlor. In the case of scenarios composed of several pulses, this sensitivity should be considered in the modelling. Therefore, modelling the effects of any pulse scenario of S-metolachlor on an alga is feasible but requires the determination of the effect trigger, the delay in recovery and the possible change in the sensitivity of the alga to the substance.

Acute effects of a prooxidant herbicide on the microalga Chlamydomonas reinhardtii: Screening cytotoxicity and genotoxicity endpoints

Since recent evidence has demonstrated that many types of chemicals exhibit oxidative and/or genotoxic potential on living organisms, reactive oxygen species (ROS) formation and DNA damage are currently the best accepted paradigms to assess the potential hazardous biological effects of a wide range of contaminants. The goal of this study was to evaluate the sensitivity of different cytotoxicity and genotoxicity responses on the model microalga Chlamydomonas reinhardtii exposed to the prooxidant herbicide paraquat. In addition to the growth endpoint, cell viability, mitochondrial membrane potential and presence of reactive oxygen species (ROS) were assayed as potential markers of cytotoxicity using flow cytometry (FCM). To study the effects of paraquat on C. reinhardtii DNA, several genotoxicity approaches were implemented for the first time in an ecotoxicological study on microalgae. Oxidative DNA base damage was analysed by measuring the oxidative DNA lesion 8-OHdG by FCM. DNA fragmentation was analysed by different methods: comet assay, and cell cycle analysis by FCM, with a particular focus on the presence of subG1-nuclei. Finally, effects on morphology of nuclei were monitored through DAPI staining. The evaluation of these endpoints showed that several physiological and biochemical parameters reacted to oxidative stress disturbances with greater sensitivity than integrative parameters such as growth rates or cell viability. The experiments revealed concentration-dependent cytotoxicity (ROS formation, depolarization of mitochondrial membrane), genotoxicity (oxidative DNA damage, DNA strand breakage, alterations in nuclear morphology), and cell cycle disturbances (subG1-nuclei, decrease of 4N population) in paraquat-treated cells. Overall, the genotoxicity results indicate that the production of ROS caused by exposure to paraquat induces oxidative DNA damage followed by DNA single- and double-strand breaks and cell cycle alterations, possibly leading to apoptosis in C. reinhardtii cells. This is supported by the observation of typical hallmarks of apoptosis, such as mitochondrial membrane depolarization, alterations in nuclear morphology and subG1 nuclei in cells exposed to the highest assayed concentrations. To our knowledge, this is the first study that provides a comprehensive analysis of oxidative DNA base damage in unicellular algal cells exposed to a prooxidant pollutant, as well as of its possible relation with other physiological effects. These results reinforce the need for additional studies on the genotoxicity of environmental pollutants on ecologically relevant organisms such as microalgae that can provide a promising basis for the characterization of potential pollutant hazards in the aquatic environment.

Chlamydomonas reinhardtii cells adjust the metabolism to maintain viability in response to atrazine stress

Chlamydomonas reinhardtii cells were exposed to a sublethal concentration of the widespread herbicide atrazine for 3 and 24h. Physiological parameters related to cellular energy status, such as cellular activity and mitochondrial and cytoplasmic membrane potentials, monitored by flow cytometry, were altered in microalgal cells exposed to 0.25μM of atrazine. Transcriptomic analyses, carried out by RNA-Seq technique, displayed 12 differentially expressed genes between control cultures and atrazine-exposed cultures at both tested times. Many cellular processes were affected, but the most significant changes were observed in genes implicated in amino acid catabolism and respiratory cellular process. Obtained results suggest that photosynthesis inhibition by atrazine leads cells to get energy through a heterotrophic metabolism to maintain their viability.

Toxic effects of individual and combined effects of BTEX on Euglena gracilis

BTEX is a group of volatile organic compounds consisting of benzene, toluene, ethylbenzene and xylenes. Environmental contamination of BTEX can occur in the groundwater with their effects on the aquatic organisms and ecosystem being sparsely studied. The aim of this study was to evaluate the toxic effects of individual and mixed BTEX on Euglena gracilis (E. gracilis). We examined the growth rate, morphological changes and chlorophyll contents in E. gracilis Z and its mutant SMZ cells treated with single and mixture of BTEX. BTEX induced morphological change, formation of lipofuscin, and decreased chlorophyll content of E. gracilis Z in a dose response manner. The toxicity of individual BTEX on cell growth and chlorophyll inhibition is in the order of xylenes>ethylbenzene>toluene>benzene. SMZ was found more sensitive to BTEX than Z at much lower concentrations between 0.005 and 5 μM. The combined effect of mixed BTEX on chlorophyll contents was shown to be concentration addition (CA). Results from this study suggested that E. gracilis could be a suitable model for monitoring BTEX in the groundwater and predicting the combined effects on aqueous ecosystem.

Tolerance to clomazone herbicide is linked to the state of LHC, PQ-pool and ROS detoxification in tobacco (Nicotiana tabacum L.)

In this study, plantlets of two tobacco (Nicotiana tabacum L.) varieties that are clomazone-tolerant (cv. Xanthi) and clomazone-sensitive (cv. Virginie vk51) were subjected to low concentration of clomazone herbicide. The oxygen-evolving rate of isolated chloroplasts, chlorophyll a fluorescence transients, JIP-test responses, hydrogen peroxide contents, antioxidant enzyme activities, cytohistological results and photosynthetic pigment contents were recorded. The results indicated that the carotenoid content was 2-fold higher in Virginie, which had greater clomazone sensitivity than Xanthi. Virginie exhibited noticeable decreases in the LHC content (Chl a/b ratio), the maximum photochemical quantum efficiency of PSII (Fv/Fm), the performance index on the absorption basis (PIabs), and the electron flux beyond the first PSII QA evaluated as (1-VJ) with VJ=(FJ-F0)/(Fm-F0) as well as increases in the rate of photon absorption (ABS/RC) and the energy dissipation as heat (DI0/RC). These results suggest that PSII photoinhibition occurred as a consequence of more reduced PQ-pool and accumulated QA(-). The oxygen evolution measurements indicate that PSI electron transport activity was not affected by clomazone. The more significant accumulation of H2O2 in Virginie compared to Xanthi was due to the absence of ROS-scavenging enzymes, and presumably induced programmed cell death (PCD). The symptoms of PCD were observed by cytohistological analysis, which also indicated that the leaf tissues of clomazone-treated Virginie exhibited significant starch accumulation compared to Xanthi. Taken together, these results indicate that the variable tolerance to clomazone observed between Virginie and Xanthi is independent of the carotenoid content and could be related to the state of the LHC, the redox state of the PQ-pool, and the activity of detoxification enzymes.

Effects of paraquat on photosynthetic pigments, antioxidant enzymes, and gene expression in Chlorella pyrenoidosa under mixotrophic compared with autotrophic conditions

Only limited information is available on herbicide toxicity to algae under mixotrophic conditions. In the present study, we studied the effects of the herbicide paraquat on growth, photosynthetic pigments, antioxidant enzymes, and gene expression in Chlorella pyrenoidosa under mixotrophic compared with autotrophic conditions. The mean measured exposure concentrations of paraquat under mixotrophic and autotrophic conditions were in the range of 0.3-3.4 and 0.6-3.6 μM, respectively. Exposure to paraquat for 72 h under both autotrophic and mixotrophic conditions induced decreased growth and chlorophyll (Chl) content, increased superoxide dismutase and peroxidase activities, and decreased transcript abundances of three photosynthesis-related genes (light-independent protochlorophyllide reductase subunit, photosystem II protein D1, and ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit [rbcL]). Compared with autotrophic conditions, the inhibition percentage of growth rate under mixotrophic conditions was lower at 0.8 μM paraquat, whereas it was greater at 1.8 and 3.4 μM paraquat. With exposure to 0.8-3.4 μM paraquat, the inhibition rates of Chl a and b content under mixotrophic conditions (43.1-52.4% and 54.6-59.7%, respectively) were greater compared with autotrophic conditions, whereas the inhibition rate of rbcL gene transcription under mixotrophic conditions (35.7-44.0%) was lower. These data showed that similar to autotrophic conditions, paraquat affected the activities of antioxidant enzymes and decreased Chl synthesis and transcription of photosynthesis-related genes in C. pyrenoidosa under mixotrophic conditions, but a differential susceptibility to paraquat toxicity occurred between autotrophically versus mixotrophically grown cells.

Lysine acetylsalicylate increases the safety of a paraquat formulation to freshwater primary producers: a case study with the microalga Chlorella vulgaris

Large amounts of herbicides are presently used in the industrialized nations worldwide, with an inexorable burden to the environment, especially to aquatic ecosystems. Primary producers such as microalgae are of especial concern because they are vital for the input of energy into the ecosystem and for the maintenance of oxygen in water on which most of other marine life forms depend on. The herbicide paraquat (PQ) is known to cause inhibition of photosynthesis and irreversible damage to photosynthetic organisms through generation of reactive oxygen species in a light-dependent manner. Previous studies have led to the development of a new formulation of PQ containing lysine acetylsalicylate (LAS) as an antidote, which was shown to prevent the mammalian toxicity of PQ, while maintaining the herbicidal effect. However, the safety of this formulation to primary producers in relation to commercially available PQ formulations has hitherto not been established. Therefore, the aim of this study was to evaluate the toxicity of the PQ+LAS formulation in comparison with the PQ, using Chlorella vulgaris as a test organism. Effect criterion was the inhibition of microalgal population growth. Following a 96 h exposure to increasing concentrations of PQ, C. vulgaris growth was almost completely inhibited, an effect that was significantly prevented by LAS at the proportion used in the formulation (PQ+LAS) 1:2 (mol/mol), while the highest protection was achieved at the proportion of 1:8. In conclusion, the present work demonstrated that the new formulation with PQ+LAS has a reduced toxicity to C. vulgaris when compared to Gramoxone(®).

Two calcium-dependent protein kinases from Chlamydomonas reinhardtii are transcriptionally regulated by nutrient starvation

We report here, the transcriptional regulation of 2 Calcium Dependent Protein Kinases in response to nutrient starvation of Chlamydomonas reinhardtii vegetative cells. The CDPK proteins, CDPK1 and CDPK3; share 53% identity among themselves, a maximum of 57% and 52% to higher plants respectively and 42% to apicomplexan protozoans. We expressed a CDPK1-GFP fusion protein in the C. reinhardtii vegetative cells and showed its distribution both in the cell body and the membrane-matrix fraction of the flagella. The fusion protein exhibits mobility shift in the presence of Ca (2+), confirming its Ca (2+)-binding properties. To the best of our knowledge, this is the first report of transcriptional regulation of CDPKs from a unicellular chlorophyte in response to nutrient starvation namely acetate (A), phosphorus (P), and nitrogen (N).

A label-free microfluidic biosensor for activity detection of single microalgae cells based on chlorophyll fluorescence

Detection of living microalgae cells is very important for ballast water treatment and analysis. Chlorophyll fluorescence is an indicator of photosynthetic activity and hence the living status of plant cells. In this paper, we developed a novel microfluidic biosensor system that can quickly and accurately detect the viability of single microalgae cells based on chlorophyll fluorescence. The system is composed of a laser diode as an excitation light source, a photodiode detector, a signal analysis circuit, and a microfluidic chip as a microalgae cell transportation platform. To demonstrate the utility of this system, six different living and dead algae samples (Karenia mikimotoi Hansen, Chlorella vulgaris, Nitzschia closterium, Platymonas subcordiformis, Pyramidomonas delicatula and Dunaliella salina) were tested. The developed biosensor can distinguish clearly between the living microalgae cells and the dead microalgae cells. The smallest microalgae cells that can be detected by using this biosensor are 3 μm ones. Even smaller microalgae cells could be detected by increasing the excitation light power. The developed microfluidic biosensor has great potential for in situ ballast water analysis.

Assessing benzene-induced toxicity on wild type Euglena gracilis Z and its mutant strain SMZ

Benzene is a representative member of volatile organic compounds and has been widely used as an industrial solvent. Groundwater contamination of benzene may pose risks to human health and ecosystems. Detection of benzene in the groundwater using chemical analysis is expensive and time consuming. In addition, biological responses to environmental exposures are uninformative using such analysis. Therefore, the aim of this study was to employ a microorganism, Euglena gracilis (E. gracilis) as a putative model to monitor the contamination of benzene in groundwater. To this end, we examined the wild type of E. gracilis Z and its mutant form, SMZ in their growth rate, morphology, chlorophyll content, formation of reactive oxygen species (ROS) and DNA damage in response to benzene exposure. The results showed that benzene inhibited cell growth in a dose response manner up to 48 h of exposure. SMZ showed a greater sensitivity compared to Z in response to benzene exposure. The difference was more evident at lower concentrations of benzene (0.005-5 μM) where growth inhibition occurred in SMZ but not in Z cells. We found that benzene induced morphological changes, formation of lipofuscin, and decreased chlorophyll content in Z strain in a dose response manner. No significant differences were found between the two strains in ROS formation and DNA damage by benzene at concentrations affecting cell growth. Based on these results, we conclude that E. gracilis cells were sensitive to benzene-induced toxicities for certain endpoints such as cell growth rate, morphological change, depletion of chlorophyll. Therefore, it is a potentially suitable model for monitoring the contamination of benzene and its effects in the groundwater.

Omics methods for probing the mode of action of natural and synthetic phytotoxins

For a little over a decade, omics methods (transcriptomics, proteomics, metabolomics, and physionomics) have been used to discover and probe the mode of action of both synthetic and natural phytotoxins. For mode of action discovery, the strategy for each of these approaches is to generate an omics profile for phytotoxins with known molecular targets and to compare this library of responses to the responses of compounds with unknown modes of action. Using more than one omics approach enhances the probability of success. Generally, compounds with the same mode of action generate similar responses with a particular omics method. Stress and detoxification responses to phytotoxins can be much clearer than effects directly related to the target site. Clues to new modes of action must be validated with in vitro enzyme effects or genetic approaches. Thus far, the only new phytotoxin target site discovered with omics approaches (metabolomics and physionomics) is that of cinmethylin and structurally related 5-benzyloxymethyl-1,2-isoxazolines. These omics approaches pointed to tyrosine amino-transferase as the target, which was verified by enzyme assays and genetic methods. In addition to being a useful tool of mode of action discovery, omics methods provide detailed information on genetic and biochemical impacts of phytotoxins. Such information can be useful in understanding the full impact of natural phytotoxins in both agricultural and natural ecosystems.

A BolA-like morphogene from the alga Chlamydomonas reinhardtii changes morphology and induces biofilm formation in Escherichia coli

Escherichia coli BolA protein is a stress-inducible morphogene, regulates transcription, forms biofilms and interacts with monothiol glutaredoxins. Its presence has been documented in plants but its role remains enigmatic. This study attempts to functionally dissect the role of a BolA-domain-containing protein in the alga Chlamydomonas reinhardtii. Of the five C. reinhardtii bolA-like genes annotated for the presence of BolA-domain, the open reading frame with the highest similarity to algal systems was cloned and the protein over-expressed in E. coli. This over-expression did not affect E. coli growth but induced biofilm formation and changed its morphology, indicating functional conservancy. This is the first compelling evidence depicting the role of a plant BolA-like protein in morphogenetic pathway and biofilm formation. The implications of the phenotypic consequences of this heterologous expression are discussed.

Linking proteome responses with physiological and biochemical effects in herbicide-exposed Chlamydomonas reinhardtii

Exposure to a toxicant causes proteome alterations in an organism. In ecotoxicology, analysis of these changes may allow linking them to physiological and biochemical endpoints, providing insights into subcellular exposure effects and responses and, ultimately mechanisms of action. Based on this, useful protein markers of exposure can be identified. We investigated the proteome changes induced by the herbicides paraquat, diuron, and norflurazon in the green alga Chlamydomonas reinhardtii. Shotgun proteome profiling and spectral counting quantification in combination with G-test statistics revealed significant changes in protein abundance. Functional enrichment analysis identified protein groups that responded to the exposures. Significant changes were observed for 149-254 proteins involved in a variety of metabolic pathways. While some proteins and functional protein groups responded to several tested exposure conditions, others were affected only in specific cases. Expected as well as novel candidate markers of herbicide exposure were identified, the latter including the photosystem II subunit PsbR or the VIPP1 protein. We demonstrate that the proteome response to toxicants is generally more sensitive than the physiological and biochemical endpoints, and that it can be linked to effects on these levels. Thus, proteome profiling may serve as a useful tool for ecotoxicological investigations in green algae.

Multiple-endpoint assay provides a detailed mechanistic view of responses to herbicide exposure in Chlamydomonas reinhardtii

The release of herbicides into the aquatic environment raises concerns about potential detrimental effects on ecologically important non-target species, such as unicellular algae, necessitating ecotoxicological risk assessment. Algal toxicity tests based on growth, a commonly assessed endpoint, are integrative, and hence do not provide information about underlying toxic mechanisms and effects. This limitation may be overcome by measuring more specific biochemical and physiological endpoints. In the present work, we developed and applied a novel multiple-endpoint assay, and analyzed the effects of the herbicides paraquat, diuron and norflurazon, each representing a specific mechanism of toxic action, on the single celled green alga Chlamydomonas reinhardtii. The endpoints added to assessment of growth were pigment content, maximum and effective photosystem II quantum yield, ATP content, esterase and oxidative activity. All parameters were measured at 2, 6 and 24h of exposure, except for growth and pigment content, which were determined after 6 and 24h only. Effective concentrations causing 50% of response (EC50s) and lowest observable effect concentrations (LOECs) were determined for all endpoints and exposure durations where possible. The assay provided a detailed picture of the concentration- and time-dependent development of effects elicited by the analyzed herbicides, thus improving the understanding of the underlying toxic mechanisms. Furthermore, the response patterns were unique to the respective herbicide and reflected the different mechanisms of toxicity. The comparison of the endpoint responses and sensitivities revealed that several physiological and biochemical parameters reacted earlier or stronger to disturbances than growth. Overall, the presented multiple-endpoint assay constitutes a promising basis for investigating stressor and toxicant effects in green algae.

Flow cytometric analysis to evaluate physiological alterations in herbicide-exposed Chlamydomonas moewusii cells

Investigation of herbicide toxicology in non-target aquatic primary producers such as microalgae is of great importance from an ecological point of view. In order to study the toxicity of the widely used herbicide paraquat on freshwater green microalga Chlamydomonas moewusii, physiological changes associated with 96 h-exposures to this pollutant were monitored using flow cytometry (FCM) technique. Intracellular reactive oxygen species concentration, cytoplasmic membrane potential, metabolic activity and cell protein content were monitored to evaluate the toxicological impact of paraquat on algal physiology. Results showed that herbicide paraquat induced oxidative stress in C. moewusii cells, as it indicated the increase of both superoxide anion and hydrogen peroxide levels observed in non-chlorotic cells of cultures exposed to increasing herbicide concentrations. Furthermore, a progressive increase in the percentage of depolarised cells and a decrease in the metabolic activity level were observed in response to paraquat when non-chlorotic cells were analysed. Chlorotic cells were probably non-viable cells, based on the cytoplasmic membrane depolarisation, its metabolically non-active state and its drastically reduced protein content. In view of the obtained results, we have concluded that a range of significant physiological alterations, detected by flow cytometry, occur when C. moewusii, an ubiquitous microalga in freshwater environments, is challenged with environmentally relevant concentrations of the herbicide paraquat.

Comparison of three Chlamydomonas strains which show distinctive oxidative stress tolerance

Methyl viologen (MV) causes severe oxidative stress by generating superoxide in the photosystem. The marine Chlamydomonas strain W80 is highly tolerant to MV (inhibitory concentration 50% [IC₅₀]=110 μM), and another marine Chlamydomonas strain HS5 shows also relatively a high tolerance (IC₅₀=12 μM). These two marine strains and a freshwater Chlamydomonas reinhardtii, which is highly sensitive to MV (IC₅₀=0.03 μM), were compared with respect to their reactive oxygen species (ROS) eliminating enzymes (superoxide dismutase, catalase, glutathione peroxidase, and ascorbate peroxidase), intracellular free amino acids, and antioxidant activities of the cell extracts. The marked difference between the marine Chlamydomonas strains and C. reinhardtii is the much higher (more than 5 fold) ascorbate peroxidase (APX) activity in the marine strains. The marine strains also kept the high APX activities (more than 100% of non-stressed condition) under the MV stressed condition, while the APX activity in C. reinhardtii was significantly decreased (36% of non-stressed condition) under the stressed condition, indicating that APX activity potentially contributes to the oxidative stress tolerance in Chlamydomonas. In addition, the levels of intracellular free proline, which is supposed to ameliorate oxidative stress, were several tens of times higher in the marine Chlamydomonas strains than in C. reinhardtii.

Oxidative stress induced by a commercial glyphosate formulation in a tolerant strain of Chlorella kessleri

We studied the toxicity of a glyphosate formulation and provide evidence of metabolic alterations due to oxidative stress caused in a Chlorella kessleri tolerant strain by exposure to the herbicide. After 96 h of exposure to increasing concentrations of the herbicide (0-70 mg L(-1)) with alkylaryl polyglycol ether surfactant, growth was inhibited (EC50-96 h 55.62 mg L(-1)). Glyphosate increased protein and malondialdehyde content which was significantly higher than in the control at 50-70 mg L(-1). Superoxide dismutase and catalase activities and reduced glutathione levels increased in a concentration-dependant manner. Morphological studies showed increases in vacuolisation and in cell and sporangia sizes. The glyphosate formulation studied has a cytotoxic effect on C. kessleri through a mechanism that would involve the induction of oxidative stress. Upon glyphosate exposure, oxidative stress parameters such as SOD and CAT activities and MDA level could be more sensitive biomarkers than usually tested growth parameters in C. kessleri.