THE ROLE OF REACTIVE OXYGEN SPECIES IN COPPER TOXICITY TO TWO FRESHWATER GREEN ALGAE(1)

Insertional mutagenesis of AIDA or CYP720B1 in the green alga Chlamydomonas reinhardtii confers copper(II) tolerance and increased biomass

The widespread use of copper (Cu) in industrial and agricultural settings leads to the accumulation of excess Cu within aquatic ecosystems, posing a threat to organism health. Microalgal bioremediation has emerged as a popular and promising solution to mitigate the risks. Nevertheless, the genetic underpinnings and engineering tactics involved in heavy metal bioremediation by microalgae remain inadequately elucidated. In this study, two mutants obtained from screening a Chlamydomonas reinhardtii (C. reinhardtii) mutant library were identified as insertional mutagenesis in the AIDA (Cre12.g487450) and CYP720B1 (Cre10.g426700) genes. Interestingly, these two mutants exhibited decreased cell size and ciliary length but increased cell growth rates. Under Cu(II) stress, the AIDA and CYP720B1 mutants presented dose-dependent tolerance to Cu(II), resulting in increased biomass and improved cellular morphology. Furthermore, the analysis for the antioxidant system suggested that increased Cu(II) tolerance was associated with a low-level response strategy to Cu(II) stress. Transmission electron microscopy images also revealed increased stress-related organelles (starch granules, acidocalcisomes, and plastoglobules) in these two mutants. Considering the excellent Cu(II) tolerance and biomass of these two mutants, our findings provide potential microalgal strains for further genetic modifications and performance mining to improve aquatic Cu(II) bioremediation through biomass enhancement.

Comparative growth inhibition of 6PPD and 6PPD-Q on microalgae Selenastrum capricornutum, with insights into 6PPD-induced phototoxicity and oxidative stress

Widespread environmental detection of tire additive N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its toxic metabolite 6PPD-Q has raised great concerns for their potential impact on aquatic biota. This study investigated the effects of 6PPD and 6PPD-Q on the model green microalgae (Selenastrum capricornutum). Results showed that 6PPD at the concentrations of 1-5 mg·L-1 stimulated S. capricornutum growth, while higher concentrations (10-50 mg·L-1) inhibited growth with an IC50(96 h) of 8.78 mg·L-1. However, at concentrations up to 10 mg·L-1, no toxicity was observed for S. capricornutum exposed to 6PPD-Q. Under the stress of 6PPD, S. capricornutum exhibited increased cellular membrane permeability and cell wall rupture, indicating structural damage to the algae cell. Microalgal oxidative stress was induced through the accumulation of reactive oxygen species (ROS), reaching levels of 1.65-5.29 times higher than the non-exposure cells, which altered enzymatic activities including superoxide dismutase (SOD) and catalase. Exposure to 6PPD at concentrations of 10-50 mg·L-1 resulted in photosynthetic toxicity as evidenced by decreased Chlorophyll a (Chl a) content and adverse effects on chlorophyll fluorescence parameters, such as maximum photochemical quantum yield (Fv/Fm), PSII (photosystem II) effective quantum yield [Y(II)], and photosynthetic electron transfer rate (ETR). While the concentrations employed may be higher than those typically found in the environment, this study uncovers a significant finding that 6PPD may demonstrate even greater toxicity to microalgae than its derivative, 6PPD-Q. This underscores the need for further investigation into the ecological risks of 6PPD, particularly in the context of primary producers like microalgae.

Integration of Copper Toxicity Mechanisms in Raphidocelis subcapitata: Advancing Insights at Environmentally Relevant Concentrations

This work aimed to characterize the impact of copper (Cu), at environmentally relevant concentrations, using the freshwater microalga Raphidocelis subcapitata. Algae were incubated with 33 or 53 µg/L Cu, in OECD medium, and toxic impacts were evaluated over 72 h, using different cellular and biochemical biomarkers. The exposure to 33 µg/L Cu had an algistatic effect: slowing growth and reducing algal population (53%, at 72 h) without compromising the cell membrane. This Cu concentration promoted a transient reduction in chlorophyll a (chla) content and typical markers of oxidative stress: increased levels of reactive oxygen species (ROS), augmented catalase (CAT) activity, and lipid peroxidation (malondialdehyde, MDA). Algae exposed to 53 µg/L Cu, suffered a severe effect with a 93% reduction in the number of cells, 50% decrease in chla content, and diminished (17%) maximum photochemical quantum yield of PSII (Fv/Fm). This population also presented increased levels of ROS and MDA, 33 and 20 times higher than the control, respectively, at 72 h, augmented CAT activity, and permeabilized cell membrane (5%, at 72 h). These findings provide valuable insights into Cu toxicity in aquatic ecosystems, highlighting the biochemical and physiological impacts at environmentally relevant concentrations.

Influence of polyethylene-type microplastics on long-term exposure to heavy metals in freshwater phytoplankton

The use of plastic materials has brought about significant social benefits but has also led to negative consequences, particularly their accumulation in aquatic environments. Studies have shown that small plastic particles, known as microplastics (MPs), can carry various harmful pollutants, such as heavy metals (HMs). Therefore, the aim of this research is to investigate the impact of polyethylene-type MPs on the long-term exposure of different HMs on freshwater microalgae Scenedesmus armatus and cyanobacteria Microcystis aeruginosa, in both isolated cultures and phytoplanktonic community conditions. Over a period of 28 days, the strains were subjected to concentrations of Ag+, Cu+2, and Cr+6 corresponding to their respective 72 h-EC10, with or without the presence of MPs. Throughout this period, the growth cell ratio, photosynthetic activity, and reactive oxygen species (ROS) were monitored. The findings indicated a substantial inhibitory impact on cell growth during the initial 7-14 days of exposure, followed by a reduction until reaching values like the controls after 28 days of exposure. There was a disturbance in photosynthetic activity during the first 72 h of exposure, which gradually returned to control levels, mainly significantly affected the respiration phase. Reactive oxygen species (ROS) activity was also affected during the initial 14 days of exposure. The presence or absence of MPs in the culture medium did not significantly alter the observed effects. However, interspecies competition created a more favorable environment for M. aeruginosa over the freshwater microalgae S. armatus. These findings suggest that the formation of MP-HMs complexes may have a limited impact on reducing the adverse effects of HMs in long-term exposures. However, because the impact depends on the specific HM involved, further studies are needed to gain a better understanding of the interaction between these pollutants.

Interactive effects of nanoplastics, multi-contaminants, and environmental conditions on prairie aquatic ecosystems: A factorial composite toxicity analysis within a Canadian context

Limited data exist on the interactions between nanoplastics (NPs) and co-contaminants under diverse environmental conditions. Herein, a factorial composite toxicity analysis approach (FCTA) was developed to analyze the time-dependent composite effects of NPs (0 ∼ 60 mg/L), copper (Cu, 0.2 ∼ 6 mg/L) and phenanthrene (PHE, 0.001 ∼ 1 mg/L) on microalgae under diverse pH (6.7 ∼ 9.1), dissolved organic matter (DOM, 1.5 ∼ 25.1 mg/L), salinity (1 ∼ 417 mg/L) and temperature (23 ∼ 33 °C) within the Canadian prairie context. The toxic mechanism was revealed by multiple toxic endpoints. The combined toxicity of NPs, Cu and PHE within prairie aquatic ecosystems was assessed by the developed FCTA-multivariate regression model. Contrary to individual effects, NPs exhibited a promotional effect on microalgae growth under complex environmental conditions. Although Cu and PHE were more hazardous, NPs mitigated their single toxicity. Environmental conditions and exposure times significantly influenced the main effects and interactions of NPs, Cu and PHE. The synergistic effect of NPs*Cu and NPs*PHE on microalgae growth became antagonistic with increased pH or DOM. Microalgae in the Souris River, Saskatchewan, were projected to suffer the most toxic effects. Our findings have significant implications for the risk management of NPs.

Physiological and transcriptomic response of dinoflagellate Gymnodinium catenatum to nitrate deficiency

The paralytic shellfish toxin producing dinoflagellate Gymnodinium catenatum is a globally distributed species and often forms massive blooms. However, the physiological and molecular responses of G. catenatum to nitrate starvation have not been thoroughly investigated. Our results showed that multiple forms of N could be utilized by G. catenatum under nitrate-deficient conditions. Nitrate deficiency adversely affected the growth, cellular Chlorophyll a (Chl a) content, and toxin production of G. catenatum. Transcriptomic analysis revealed significant down-regulation of gene expressions involved in the light reaction of photosynthesis, while genes related to fatty acids synthesis and antioxidation were significantly upregulated in the N-depleted cultures. Our results suggested that excess carbon was channeled into lipid synthesis for energy storage, and antioxidant reactions were upregulated to eliminate toxic peroxides caused by nitrate limitation. These findings highlight the adaptative strategy of G. catenatum in low-nitrate environments, which are crucial factors driving its bloom formation.

Unraveling the risks of nAl2O3 on harmful algal blooms: Insights from paralytic shellfish toxins production of Alexandrium tamarense

As one of the commonly used and cost-effective nanomaterials, nanosized aluminum oxide (nAl2O3) posses unique properties and chemical stability. However, its extensive use and resultant dissemination into aquatic ecosystems prompt concerns over the proliferation and repercussions of harmful algal blooms, particularly those caused by dinoflagellates producing toxins. This study investigated the sub-chronic effects of nAl2O3 on growth, physiological activities, and paralytic shellfish toxins (PSTs) production in Alexandrium tamarense. Results showed dose-dependent inhibition in growth (EC50 = 20.6 mg L-1), esterase activity, and photosynthetic efficiency (Fv/Fm) during the sub-chronic exposure (13-day). The internalization of nAl2O3 in microalgal cells and the significant decrease in the total cellular PSTs content were observed under high nAl2O3 concentrations (>40 mg L-1). The study also demonstrated a clear decrease in the content of some derivatives of PSTs (GTX5, C1/2, and GTX2/3) with the increase in nAl2O3 concentrations, accompanied by the induction of an unknown derivative. Excessive ROS production, dissolved Al, and physical inhibition were suggested as potential mechanisms for nAl2O3 toxicity and changes in PSTs toxin profile. Overall, this research enhances our understanding of the potentiated risks and threats on the possible concurrent events of toxic dinoflagellate, such as Alexandrium species and nanoparticles in aquatic environments.

Modulation in growth, oxidative stress, photosynthesis, and morphology reveals higher toxicity of alpha-cypermethrin than chlorpyrifos towards a non-target green alga at high doses

Considering the frequent detection of pesticides in the aquatic environment, the ecotoxicological effects of Chlorpyrifos (CHP), an organophosphate, and alpha-cypermethrin (ACM), a pyrethroid, on freshwater microalgae were compared for the first time in this study. High concentrations of both CHP and ACM significantly suppressed the growth of test microalga Graesiella emersonii (p < 0.05). The 96-h EC50 of CHP and ACM were 54.42 mg L-1 and 29.40 mg L-1, respectively. Sub-inhibitory doses of both pesticides increased ROS formation in a concentration-dependent manner, which was accompanied by changes in antioxidant enzymes activities, lipid peroxidation, and variations in photosynthetic pigment concentration. Furthermore, both pesticides influenced photosystem II performance, oxygen-evolving complex efficiency and, intracellular ATP levels. Scanning electron microscopy analysis revealed that high concentrations of both CHP and ACM caused considerable morphological changes in the microalga. In comparison, CHP was more toxic than ACM at low concentrations, whereas ACM was more toxic at high concentrations.

Evaluation of short-term copper toxicity in a co-culture of Synechococcus sp., Chaetoceros gracilis and Pleurochrisys cf. roscoffensis exposed to changes in temperature and salinity levels

Metals such as copper (Cu) enter marine environments from natural and anthropogenic sources, causing changes in the biodiversity of marine microalgae and cyanobacteria. Cu plays a dual role as either a micronutrient or toxicant depending on the environmental concentration. Many studies have summarized the potential of Cu to become more toxic to microalgae under environmental stress (for instance climate change). Most of the data available on Cu toxicity concerning microalgae and cyanobacteria have been produced using single-species laboratory tests, and there is still a significant gap in the information concerning the behavior of a group of algae exposed to environmental stressors. Thus, the objective of this study was to evaluate the toxicity of Cu at two concentrations (C1 = 2 μg L-1 and C2 = 5 μg L-1) in multispecies bioassays using three phytoplankton species (one cyanobacteria, Synechococcus sp., and two microalgae, Chaetoceros gracilis and Pleurochrisys cf. roscoffensis). Combinations of two temperatures (20 and 23 °C) and two salinities (33 and 36 PSU), were applied in a 96 h study using flow cytometry analysis (FCM). Algal growth and reactive oxygen species (ROS) production by 2'7'-dichlorofluorescein (DCFH) were monitored by FCM. The results indicated that Synechococcus sp. was more sensitive than C. gracilis and P. roscoffensis to Cu stress at a temperature 23 °C and salinity of 36 PSU under both concentrations of Cu. Chlorophyll a fluorescence showed a significant decrease (p < 0.05) in Synechococcus sp. under 5 μg L-1 of Cu in the combined treatment of 20 °C and 33 PSU; however, there was a significant increase in P. roscoffensis in all combinations at C2 = 5 μg L-1 compared to the control with no Cu, indicating a potential hormetic response to Cu for P. roscoffensis. ROS levels were triggered in a combination of 23 °C and 33 PSU and 5 μg L-1 of Cu, which was higher than all the other combinations studied. Our study resulted in data concerning the potential impacts caused by possible future climate change scenarios in aquatic habitats chronically exposed to metals.

Optimizing Analysis Methods: Rapid and Accurate Determination of Cuaminosulfate Residues with LC-MS/MS Based on Box-Behnken Design Study

In view of the defects in the previous detection of cuaminosulfate, which only focused on the analysis of copper ions, there is currently no analysis method available to determine the actual state of cuaminosulfate as chelated or bound. In order to investigate the dissipation and terminal residues in soil and watermelon of cuaminosulfate for food safety and environmental risk, a highly effective technique was developed to detect cuaminosulfate residues in watermelon and soil, and field experiments were conducted in China. After single-factor experiments, residual cuaminosulfate in samples was extracted by pure water, purified using a liquid-liquid approach combined with a dispersive solid-phase extraction, and detected by liquid chromatography tandem mass spectrometry (LC-MS/MS). The Box-Behnken design (BBD) study was used to find the optimal solutions for the time of liquid-liquid purification, the amount of extraction solvent, and the amounts of cleanup sorbents for the analytical method. The average recovery of the method was in the range of 80.0% to 101.1%, the average relative standard deviation (RSD) was 5.3-9.9%, and the detection limit was lower than 0.05 mg/kg. The BBD study not only improved the extraction rate of the method, but also saved time and was operated easily. The final residues of cuaminosulfate in watermelon at different sampling intervals were all lower than 0.05 mg/kg under field conditions. The cuaminosulfate in soils dissipated following exponential kinetics, with half-life values in the range of 9.39 to 12.58 days, which varied by different locations. Based on the validated method, food safety residues and soil residues can be determined rapidly and accurately.

Further Limitations of Synthetic Fungicide Use and Expansion of Organic Agriculture in Europe Will Increase the Environmental and Health Risks of Chemical Crop Protection Caused by Copper-Containing Fungicides

Copper-containing fungicides have been used in agriculture since 1885. The divalent copper ion is a nonbiodegradable multisite inhibitor that has a strictly protective, nonsystemic effect on plants. Copper-containing plant protection products currently approved in Germany contain copper oxychloride, copper hydroxide, and tribasic copper sulfate. Copper is primarily used to control oomycete pathogens in grapevine, hop, potato, and fungal diseases in fruit production. In the environment, copper is highly persistent and toxic to nontarget organisms. The latter applies for terrestric and aquatic organisms such as earthworms, insects, birds, fish, Daphnia, and algae. Hence, copper fungicides are currently classified in the European Union as candidates for substitution. Pertinently, copper also exhibits significant mammalian toxicity (median lethal dose oral = 300-2500 mg/kg body wt in rats). To date, organic production still profoundly relies on the use of copper fungicides. Attempts to reduce doses of copper applications and the search for copper substitutes have not been successful. Copper compounds compared with modern synthetic fungicides with similar areas of use display significantly higher risks for honey bees (3- to 20-fold), beneficial insects (6- to 2000-fold), birds (2- to 13-fold), and mammals (up to 17-fold). These data contradict current views that crop protection in organic farming is associated with lower environmental or health risks. Further limitations in the range and use of modern single-site fungicides may force conventional production to fill the gaps with copper fungicides to counteract fungicide resistance. In contrast to the European Union Green Deal goals, the intended expansion of organic farming in Europe would further enhance the use of copper fungicides and hence increase the overall risks of chemical crop protection in Europe. Environ Toxicol Chem 2024;43:19-30. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Molecular changes in phenolic compounds in Euglena gracilis cells grown under metal stress

Metal presence in the aquatic ecosystem has increased and diversified over the last decades due to anthropogenic sources. These contaminants cause abiotic stress on living organisms that lead to the production of oxidizing molecules. Phenolic compounds are part of the defense mechanisms countering metal toxicity. In this study, the production of phenolic compounds by Euglena gracilis under three different metal stressors (i.e. cadmium, copper, or cobalt) at sub-lethal concentration was assessed using an untargeted metabolomic approach by mass spectrometry combined with neuronal network analysis (i.e. Cytoscape). The metal stress had a greater impact on molecular diversity than on the number of phenolic compounds. The prevalence of sulfur- and nitrogen-rich phenolic compounds were found in Cd- and Cu-amended cultures. Together these results confirm the impact of metallic stress on phenolic compounds production, which could be utilized to assess the metal contamination in natural waters.

Cyanobacteria control using Cu-based metal organic frameworks derived from waste PET bottles

Copper sulfate (CuSO₄) is actively used to control the proliferation of harmful algal blooms because of its fast and effective killing mechanism. However, its use unintentionally harms innocuous aquatic organisms. Therefore, there is a need to find non-toxic solutions for controlling algal blooms. In this study, Cu-based metal-organic framework (Cu-BDC MOF) chips (ca. 2 × 2 cm) were synthesized using waste polyethylene terephthalate (PET) bottles. The as-synthesized Cu-BDC MOF chips efficiently inhibited the cyanobacteria species Microcystis aeruginosa, which was comparable to the conventional dose of CuSO₄ algaecide (1.00 mg L^-1). Moreover, unlike the CuSO₄ algaecide, Cu-BDC MOF chips did not cause any acute toxicity (48 h) to the water flea Daphnia magna. Both Cu-BDC MOF and Cu₂O seemed to be responsible for the generation of reactive oxygen species, which resulted in the aggregation, photosynthesis disruption, and eventually growth inhibition of M. aeruginosa. This study suggests that the environmentally safe Cu-BDC MOF chip is a promising agent to sustainably control harmful algal blooms.

The role of antioxidant response and nonphotochemical quenching of chlorophyll fluorescence in long-term adaptation to Cu-induced stress in Chlamydomonas reinhardtii

Copper is an essential micronutrient, but at supraoptimal concentrations it is also highly toxic, inducing oxidative stress and disrupting photosynthesis. The aim of the present study was to analyze selected protective mechanisms in strains of Chlamydomonas reinhardtii adapted and not adapted for growth in the presence of elevated copper concentrations. Two algal lines (tolerant and non-tolerant to high Cu2+ concentrations) were used in experiments to study photosynthetic pigment content, peroxidase activity, and non-photochemical quenching. The content of prenyllipids was studied in four different algal lines (two of the same as above and two new ones). The copper-adapted strains contained about 2.6 times more α-tocopherol and plastoquinol and about 1.7 times more total plastoquinone than non-tolerant strains. Exposure to excess copper led to oxidation of the plastoquinone pool in non-tolerant strains, whereas this effect was less pronounced or did not occur in copper-tolerant strains. Peroxidase activity was approximately 1.75 times higher in the tolerant strain than in the non-tolerant one. The increase in peroxidase activity in the tolerant strain was less pronounced when the algae were grown in dim light. In the tolerant line nonphotochemical quenching was induced faster and was usually about 20-30% more efficient than in the non-tolerant line. The improvement of antioxidant defense and photoprotection may be important factors in the evolutionary processes leading to tolerance to heavy metals.

Two novel superoxide dismutase genes (CuZnSOD and MnSOD) in the toxic marine dinoflagellate Alexandrium pacificum and their differential responses to metal stressors

Superoxide dismutase (SOD) is an important antioxidant enzyme that is involved in the first line of defense against reactive oxygen species (ROS) within cells. Herein, we determined two novel CuZnSOD and MnSOD genes from the toxic marine dinoflagellate Alexandrium pacificum (designated as ApCuZnSOD and ApMnSOD) and characterized their structural features and phylogenetic affiliations. In addition, we examined the relative gene expression and ROS levels following exposure to heavy metals. ApCuZnSOD encoded 358 amino acids (aa) with two CuZnSOD-conserved domains. ApMnSOD encoded 203 aa that contained a mitochondrial-targeting signal and a MnSOD signature motif but missed an N-terminal domain. Phylogenetic trees showed that ApCuZnSOD clustered with other dinoflagellates, whereas ApMnSOD formed a clade with green algae and plants. Based on the 72-h median effective concentration (EC₅₀), A. pacificum showed toxic responses in the order of Cu, Ni, Cr, Zn, Cd, and Pb. SOD expression levels dramatically increased after 6 h of Pb (≥6.5 times) and 48 h of Cu treatment (≥3.9 times). These results are consistent with the significant increase in ROS production in the A. pacificum exposed to Pb and Cu. These suggest that the two ApSODs are involved in the antioxidant defense system but respond differentially to individual metals.

The inhibitory effects of simulated light sources on the activity of algae cannot be ignored in photocatalytic inhibition

Harmful algal blooms (HABs) destroy the balance of the aquatic ecosystem, causing huge economic losses and even further endangers human health. In addition to traditional methods of algae removal, photocatalytic inhibition of algae is drawing more and more interests with rich application scenarios and considerable potential. Simulated visible light sources are used to excite photocatalytic materials and optimize their performance. However, most of the light irradiation intensities used in the study exceeded 50 mW/cm². And the effects of intense light irradiation conditions on algal growth have rarely been addressed in previous studies. So we focused on the effect of different intensity of light irradiation on the growth of algae. We explored the relationship between light irradiation intensity and algal inactivation rate, and investigated the changes in ROS levels in algal cells under different light irradiation and the resulting response of the antioxidant system. We have found that several major antioxidant enzyme activities, such as SOD and CAT, were significantly higher and lipid peroxidation products (MDA) were accumulating. Intense light irradiation had the most direct effect on the photosynthetic system of algal cells, with the photosynthetic rate and relative electron transfer rate decaying to almost 0 within 30 min, indicating that algal photosynthesis was inhibited in a fairly short period of time. We further observed the physiological and morphological changes of algal cells during this process using TEM and found that the progressive dissolution of the cell membrane system and the damage of organelles associated with photosynthesis play a major role in promoting cell death. We thus conclude that light irradiation has a significant effect on the physiological activity of algal cells and is a non-negligible factor in the study of photocatalytic removal of harmful algae. It will provide theoretical guidance for the future study of photocatalysis on algae inhibition.

Copper Effect on Microalgae: Toxicity and Bioremediation Strategies

Microalgae are increasingly recognised as suitable microorganisms for heavy metal (HM) removal, since they are able to adsorb them onto their cell wall and, in some cases, compartmentalise them inside organelles. However, at relatively high HM concentrations, they could also show signs of stress, such as organelle impairments and increased activities of antioxidant enzymes. The main aim of this review is to report on the mechanisms adopted by microalgae to counteract detrimental effects of high copper (Cu) concentrations, and on the microalgal potential for Cu bioremediation of aquatic environments. Studying the delicate balance between beneficial and detrimental effects of Cu on microalgae is of particular relevance as this metal is widely present in aquatic environments facing industrial discharges. This metal often induces chloroplast functioning impairment, generation of reactive oxygen species (ROS) and growth rate reduction in a dose-dependent manner. However, microalgae also possess proteins and small molecules with protective role against Cu and, in general, metal stress, which increase their resistance towards these pollutants. Our critical literature analysis reveals that microalgae can be suitable indicators of Cu pollution in aquatic environments, and could also be considered as components of eco-sustainable devices for HM bioremediation in association with other organisms.

Sub-chronic ecotoxicity of triphenyl phosphate to earthworms (Eisenia fetida) in artificial soil: Oxidative stress and DNA damage

As a flame retardant, triphenyl phosphate (TPHP) is commonly added to various daily products. Due to its easy diffusion, TPHP pollution has become a global concern. Despite the wide focus on environmental risk, the sub-chronic ecotoxicity of TPHP in soil organisms remains unclear. In this study, the artificial soil exposure method was used to analyze the oxidative stress and DNA damage in earthworms with 0, 20, 40, 60 and 80 mg/kg TPHP treatments through the response of reactive oxygen species (ROS), antioxidant and detoxifying enzymes, malondialdehyde (MDA) and olive tail moment (OTM) at 7, 14, 21 and 28 days. Throughout the experimental period, the results showed that the ROS content in earthworms treated with 20, 40, 60 and 80 mg/kg TPHP treatments increased by 9.43-18.37 %, 6.07-25.73 %, 7.71-42.61 % and 8.22-46.70 %, respectively, compared to the control treatment. Meanwhile, the activities of antioxidant and detoxification enzymes in earthworms with all TPHP treatments were significantly activated after exposure for 7 and 14 days, and then inhibited at 21 and 28 days. Despite the protection of antioxidant enzymes and detoxification enzymes, MDA content in earthworms with the 20 mg/kg treatment still significantly increased at 7 and 14 days of exposure, as well as in the other three treatments. Compared to the control treatment, the obviously higher OTM values in earthworms with TPHP treatments possibly indicated a genotoxicity of TPHP in earthworms. Furthermore, the integrated biomarker response index (IBRv2) revealed that earthworms showed an obvious biochemical response TPHP-contaminated soil, which was strongly correlated with TPHP concentrations and exposure time. This study provides insights into the TPHP hazard in the soil environment and offers a reference to assess its environmental risk to soil ecosystems.

Role of iron in gene expression and in the modulation of copper uptake in a freshwater alga: Insights on Cu and Fe assimilation pathways

Metal uptake and toxicity can generally be related to its aqueous speciation and to the presence of competitive ions as described by the biotic ligand model. Beyond these simple chemical interactions at the surface of aquatic organisms, several internal biological feedback mechanisms can also modulate metal uptake. This is particularly important for essential elements for which specific transport systems were developed over the course of evolution. Based on the results of short-term Cu²⁺ uptake experiments and on the analysis of the expression of certain genes involved in Cu and Fe homeostasis, we studied the effects of Fe³⁺ on Cu²⁺ uptake by the freshwater green alga Chlamydomonas reinhardtii. We observed a significant increase in Cu²⁺ uptake rate in algal cells acclimated to a low Fe³⁺ medium up to 4.7 times greater compared to non-acclimated algal cells. The overexpression of the ferroxidase FOX1 and permease FTR1 genes suggests an activation of the high affinity Fe³⁺ assimilation system, which could constitute a plausible explanation for the increase in Cu²⁺ uptake rate in acclimatized algae. We show that Fe availability can have a significant impact on Cu uptake. Our observations reinforce the importance of considering physiological factors to better predict metal bioavailability.

Toxicity of biochar influenced by aging time and environmental factors

The extensive application of biochar has drawn more attentions on its potential risk to aquatic organisms. However, the influence of environmental factors (i.e. pH, HA, SDBS and aging time) after they discharged into environment on their toxicity have not been clarified. Here, we synthesized biochar with local pine needles via pyrolysis, and then aged in different media. Followed, the toxicity of pristine and aged biochar was checked with Scenedesmus obliquus (S. obliquus). Our investigation showed that the toxicity of biochar was mitigated when aged in different pH levels or SDBS, while it was opposite in the presence of HA. The increment of pH decreased the toxicity of both the pristine and the aged biochar, while the presence of HA did same impact on the pristine biochar. The presence of SDBS decreased the toxicity of pristine biochar but increased that of aged biochar. Meanwhile, we showed these environmental factors (pH, HA, SDBS and aging time) influenced the biochar toxicity may be due to the adjustment of the aggregation and adhesion of biochar on cell surfaces or the intracellular oxidative stress. Further, the PFRs contained in biochar did influence the toxicity, along with the physicochemical properties of biochar (i.e. carbon structure, functional group or surface charge). Our results aimed to reflect the toxicity profile of biochar in the natural aquatic environment, without misunderstanding of potential ecological risk of biochar in the future application.

Trace phenolic acids simultaneously enhance degradation of chlorophenol and biofuel production by Chlorella regularis

Coupling the cultivation of microalgae with wastewater treatment is a promising technology to recover bioresources from wastewater. However, toxic pollutants in wastewater seriously inhibit the growth of microalgae and the removal of pollutants. Phenolic acids are similar to phytohormones, could potentially relieve the toxicity to microalgae and simultaneously promote pollutant degradation and lipid accumulation. Chlorella and 4-chlorophenol (4-CP) were utilized to simulate the toxic wastewater treatment, and the roles of two typical phenolic acids, such as p-hydroxybenzoic acid (p-HBA) and caffeic acid (CA), were explored. The 0.2 μM concentration of p-HBA or CA improved the specific growth rate by 7.6% by enhancing photosynthesis and DNA replication. The oxidative damage caused by 4-CP was reduced by 30.3-49.7% via the synthesis of more antioxidant enzymes and the direct scavenging of free radicals by phenolic acids. Furthermore, the 4-CP removal rate increased by 27.0%, and toxic 4-CP was degraded into non-toxic compounds. The phenolic acids did not change the 4-CP degradation pathway but accelerated its removal and detoxification by enhancing the expression of 4-CP degradation enzymes. Simultaneously, lipid production increased by 20.5-23.1% due to the upregulation of enzymes related to fatty acid and triacylglycerol synthesis. Trace phenolic acids stimulated the mitogen-activated protein kinase signaling cascade and the calcium signaling pathway to regulate the physiology of the microalgae and protect cells from toxic stress. This study provides a promising new strategy for toxic wastewater treatment and bioresource recovery.

Hierarchical deep learning model to simulate phytoplankton at phylum/class and genus levels and zooplankton at the genus level

Harmful algal blooms (HABs) have become a global issue, affecting public health and water industries in numerous countries. Because funds for monitoring HABs are limited, model development may be an alternative approach for understanding and managing HABs. Continuous monitoring based on grab sampling is time-consuming, costly, and labor-intensive. However, improving simulation performance remains a major challenge in modeling, and current methods are limited to simulating phytoplankton (e.g., Microcystis sp., Anabaena sp., Aulacoseira sp., Cyclotella sp., Pediastrum sp., and Eudorina sp.) and zooplankton (e.g., Cyclotella sp., Pediastrum sp., and Eudorina sp.) at the genus level. The traditional modeling approach is limited for evaluating the interactions between phytoplankton and zooplankton. Recently, deep learning (DL) models have been proposed for solving modeling problems because of their large data handling capabilities and model structure flexibilities. In this study, we evaluated the applicability of DL for simulating phytoplankton at the phylum/class and genus levels and zooplankton at the genus level. Our work was an explicit representation of the taxonomic and ecological hierarchy of the DL model structure. The prerequisite for this model design was the data collection at two taxonomic and hierarchical levels. Our model consisted of hierarchical DL with classification transformer (TF) and regression TF models. These DL models were hierarchically connected; the output of the phylum/class level model was transferred to the genus level simulation model, and the output of the genus level model was fed into the zooplankton simulation model. The classification TF model determined the phytoplankton occurrence initiation date, whereas the regression TF model quantified the cell concentration of plankton. The hierarchical DL showed potential to simulate phytoplankton at the phylum/class and genus levels by producing average R^2, and root mean standard error values of 0.42 and 0.83 [log(cells mL^-1)], respectively. All simulated plankton results closely matched the measured concentrations. Particularly, the simulated cyanobacteria showed good agreement with the measured cell concentration, with an R² value of 0.72. In addition, our simulated result showed good agreement in peak concentration compared to observations. However, a limitation remained in following the temporal variation of Tintinnopsis sp. and Bosmia sp. Using an importance map from the TF model, water temperature, total phosphorus, and total nitrogen were identified as significant variables influencing phytoplankton and zooplankton blooms. Overall, our study demonstrated that DL can be used for modeling HABs at the phylum/class and genus levels.

Metabolomic analysis of Scenedesmus obliquus reveals new insights into the phytotoxicity of imidazolium nitrate ionic liquids

Due to the persistence of ionic liquids (ILs) in aquatic environments, it is necessary to reveal their ecological risk to aquatic organisms. Herein, the biotoxicity of two alkyl-methylimidazolium nitrate ILs ([C₁₀mim]NO₃ and [C₁₂mim]NO₃) against Scenedesmus obliquus were studied. Results showed that the growth inhibition of S. obliquus increased with increasing concentrations of ILs, maximum values of 94.61% at 4 mg/L of [C₁₀mim]NO₃ and 97.34% at 0.8 mg/L of [C₁₂min]NO₃ were observed. The fluorescence parameters of photosystem II, such as light quantum yield and electron transfer rate, showed a negative relationship with the exposure dose. [C₁₂mim]NO₃ had a more significant effect than [C₁₀mim]NO₃. Moreover, the redox homeostasis of algae was disrupted; the accumulation of reactive oxygen species, leading to obvious inhibition of superoxide dismutase and catalase activities was observed. A metabolomic analysis indicated that the contents of most metabolites were reduced significantly, and fructose and galactose decreased significantly by 42.3% and 88.6%, respectively, in the [C₁₀mim]NO₃ treatment compared to those in the control. The inhibition of amino acid biosynthesis and glyoxylate and dicarboxylate metabolism explained the more serious biotoxicity of [C₁₂mim]NO₃ than that of [C₁₀mim]NO₃. This study facilitates a better understanding of the environmental safety and ecological risks of ILs.

Oxidative stress of Microcystis aeruginosa induced by algicidal bacterium Stenotrophomonas sp. KT48

Cyanobacterial harmful algal blooms are a worldwide problem with substantial adverse effects on the aquatic environment as well as human health. Among the multiple physicochemical and biotic approaches, algicidal bacterium is one of the most promising and eco-friendly ways to control bloom expansion. In this study, Stenotrophomonas sp. KT48 isolated from the pond where cyanobacterial blooms occurred exhibited a strong inhibitory effect on Microcystis aeruginosa. However, the algicidal performance and mechanisms of Stenotrophomonas sp. remain under-documented. To explore the algicidal performance and physiological response againt M. aeruginosa, further works were implemented here. Our results indicated that the algicidal rate of strain KT48 cultured in 1/8 LB medium supplemented with 0.3% starch or glucose was about 30% higher than that in 1/8 LB medium. Strain KT48 culture, cell-free filtrate, and cells re-suspended were inoculated into the M. aeruginosa culture, and the Chl-a content was determined. Those results indicated that the algicidal activity of cells re-suspended was far higher than that of cell-free filtrate and culture. Thus, strain KT48 exhibited algicidal activity mainly through direct attacking M. aeruginosa rather than excretion of algicides. Furthermore, strain KT48 led to an increase in cellular reactive oxygen species (ROS) and caused lipid peroxidation as supported by the increase in malondialdehyde (MDA) levels. The ROS and MDA levels in algal cells treated with strain KT48 cells re-suspended were about 3.23-fold and 2.80-fold higher than those of untreated algal cells on day 11. And a further inhibition to the antioxidant system is suggested by a sharp decrease in the superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities. In addition, we also observed that the morphology of most algal cells changed from integrity to break. This study not only indicated strain KT48 with strong algicidal activity, but also explored the underlying algicidal mechanisms to provide a source of bacterial agent for the biocontrol of cyanobacterial blooms. KEY POINTS: • Strain KT48 exhibited strong algicidal activity mainly through direct attacking M. aeruginosa. • The addition of glucose could enhance the algicidal rate of strain KT48 by about 30%. • Strain KT48 led to an increase in cellular reactive oxygen species (ROS) level that causes membrane damage as supported by the increase in malondialdehyde (MDA) levels.

Predictability of thermal fluctuations influences functional traits of a cosmopolitan marine diatom

Evolutionary theory predicts that organismal plasticity should evolve in environments that fluctuate regularly. However, in environments that fluctuate less predictably, plasticity may be constrained because environmental cues become less reliable for expressing the optimum phenotype. Here, we examine how the predictability of +5°C temperature fluctuations impacts the phenotype of the marine diatom Thalassiosira pseudonana. Thermal regimes were informed by temperatures experienced by microbes in an ocean simulation and featured regular or irregular temporal sequences of fluctuations that induced mild physiological stress. Physiological traits (growth, cell size, complexity and pigmentation) were quantified at the individual cell level using flow cytometry. Changes in cellular complexity emerged as the first impact of predictability after only 8–11 days, followed by deleterious impacts on growth on days 13–16. Specifically, cells with a history of irregular fluctuation exposure exhibited a 50% reduction in growth compared with the stable reference environment, while growth was 3–18 times higher when fluctuations were regular. We observed no evidence of heat hardening (increasingly positive growth) with recurrent fluctuations. This study demonstrates that unpredictable temperature fluctuations impact this cosmopolitan diatom under ecologically relevant time frames, suggesting shifts in environmental stochasticity under a changing climate could have widespread consequences among ocean primary producers.

The Beauty is a beast: Does leachate from the invasive terrestrial plant Impatiens glandulifera affect aquatic food webs?

Invasive alien species are a major threat to ecosystems. Invasive terrestrial plants can produce allelochemicals which suppress native terrestrial biodiversity. However, it is not known if leached allelochemicals from invasive plants growing in riparian zones, such as Impatiens glandulifera, also affect freshwater ecosystems. We used mesocosms and laboratory experiments to test the impact of I. glandulifera on a simplified freshwater food web. Our mesocosm experiments show that leachate from I. glandulifera significantly reduced population growth rate of the water flea Daphnia magna and the green alga Acutodesmus obliquus, both keystone species of lakes and ponds. Laboratory experiments using the main allelochemical released by I. glandulifera, 2-methoxy-1,4-naphthoquinone, revealed negative fitness effects in D. magna and A. obliquus. Our findings show that allelochemicals from I. glandulifera not only reduce biodiversity in terrestrial habitats but also pose a threat to freshwater ecosystems, highlighting the necessity to incorporate cross-ecosystem effects in the risk assessment of invasive species.

Simultaneous electrochemical removal of Microcystis aeruginosa and sulfamethoxazole and its ecologic impacts on Vallisneria spiralis

In this study, the simultaneous removal effects of electrochemical oxidation with boron-doped diamond anodes at different current densities were tested on Microcystis aeruginosa and sulfamethoxazole. Flow cytometry and non-invasive micro-test technology were applied to study the physiological states of M. aeruginosa and Vallisneria spiralis leaf cells. As the current density increased, the degradation effect of electrochemical oxidation on sulfamethoxazole and microcystin-LR increased and exceeded 60% within 6 h. In addition, population density of M. aeruginosa, fluorescence response of chlorophyll a, and cytoplasmic membrane integrity decreased, whereas the proportion of cells with excessive accumulation of intracellular reactive oxygen species (ROS) increased. The effect of electrochemical oxidation on the cell population of M. aeruginosa continued after the power was turned off. The physiological state of V. spiralis leaf cells was not severely affected at 10 mA/cm² for 24 h. Higher current intensity and longer electrolysis time would induce apoptosis or necrosis. In order to achieve a higher target pollutant removal effect and simultaneously avoid damage to the lake ecosystem, the current intensity of the electrochemical oxidation device should not exceed 10 mA/cm², and a single electrolysis treatment should range from 6 h to 24 h.

Physiological homeostasis alteration and cellular structure damage of Chlorella vulgaris exposed to silver nanoparticles with various microstructural morphologies

The toxicity of silver nanoparticles (AgNPs) with a single morphology to aquatic organisms has been well demonstrated in the past decade, but few studies have been carried out to evaluate the differences in toxicity among AgNPs with various microstructural morphologies. In this work, C. vulgaris was used as the tested organism to examine the differences in toxic effects among AgNSs, AgNCs, and AgPLs at concentrations of 0.5, 1.0, 2.0, and 5.0 mg/L. The results showed that the cell density and chlorophyll a content of C. vulgaris decreased when the dose of AgNPs was increased, while the inhibiting effects that were caused by AgPLs were stronger than those that were caused by AgNCs and AgNSs. Under short-term exposure to AgPLs, the ROS content was significantly higher than those under exposure to AgNCs and AgNSs, while the MDA content fluctuated without obvious regularity. The dose of AgPLs affected the antioxidative enzyme activity and lipid peroxidation more obviously than those of AgNSs and AgNCs. The superoxide dismutase and catalase contents in the former case were distinctly higher than those in the latter cases. Consequently, the cell apoptosis rate under exposure to AgPLs reached 83%, which was higher than those under exposure to AgNSs (50%) and AgNCs (71%). This work shows that the level of toxicity to C. vulgaris was in the order of AgPLs > AgNCs > AgNSs. The obtained results demonstrate that the microstructural morphologies of AgNPs determined their potential toxicity.

Toxicity of BPNSs against Chlorella vulgaris: Oxidative damage, physical damage and self-protection mechanism

Black phosphorus nanosheets (BPNSs) has extensive application prospect in the fields of optoelectronics and biomedicine, due to its unique physicochemical properties. Therefore, a systematic toxic study is necessary to assess its environmental safety. Herein, BPNSs was prepared by liquid exfoliation procedure, the primary producer Chlorella vulgaris (C. vulgaris) was used as a test subject. After the exposure for 120 h at 15, 45 and 75 mg/L BPNSs, the cell viabilities were 45.05%, 18.86% and 4.60% for each treatment group, respectively. The extent of lipid peroxidation and peroxidative damage in C. vulgaris was confirmed by measuring reactive oxygen species (ROS) levels, superoxide dismutase (SOD) and catalase (CAT) activities, followed by determination of malondialdehyde (MDA) content. Morphological analysis results (i.e., SEM and TEM) showed that BPNSs adhered to the cell surface and enter the cell to severely damage cell structure. Furthermore, BPNSs were shown to accelerate apoptosis in C. vulgaris by flow cytometry analysis. Finally, GC-MS was used to explore the metabolic regulatory mechanism of C. vulgaris in response to BPNSs stress. The results of this study can provide theoretical support for subsequent studies on the potential enrichment risk of BPNSs in the water environmental food chain.

Prediction of the impact induced by Cd in binary interactions with other divalent metals on wild-type and Cd-resistant strains of Dictyosphaerium chlorelloides

The metals present in freshwater have a toxic profile with bioaccumulation and are biomagnified along the aquatic food chain. The metals induce high sensitivity in most aquatic organisms, while others, such as some microalgae species, evolve towards resistance. Therefore, this research predicted through the Combination Index method the binary interaction exposed to divalent metals by inhibiting population growth in a Cd-resistant strain (Dc^RCd100) compared to the wild-type strain (Dc1M^wt) of Dictyosphaerium chlorelloides and evaluate the specific resistance level obtained by Dc^RCd100 to Cd relative to other divalent metals.The results showed that Dc^RCd100 presents resistance compared to Dc1M^wt in individual exposure in the order of Fe²⁺  > Ni²⁺  > Cd²⁺  > Co²⁺  > Zn²⁺  > Cu²⁺  > Hg²⁺ with 50% inhibitory concentration at 72 h of exposure (IC₅₀₍₇₂₎) values 1253, 644.4, 423, 162.7, 141.3, 35.1, and 9.9 µM, respectively. It induces cross-resistance with high antagonistic rates (Combination Index (CI); CI >  > 1) in the Cd/Zn and Cd/Cu. Cd/Ni, its initial response, is antagonistic, and it ends in an additive (CI = 1). Dc^RCd100 showed a lower resistance in Co, and Cd/Fe resistance was reduced individually. The interaction with Hg increased its resistance ten times more than individually.This research highlights the use of the CI as a highly efficient prediction method of the binary metal interactions in wild-type and Cd-resistant strains of D. chlorelloides. It may have the potential for metal accumulation, allowing the development of new methods of bioremediation of metals in effluents, and to monitor the concentration of metals in wastewater, its relative availability, transport, and mechanisms on resistant strains of microalgae.

Heavy metal-induced stress in eukaryotic algae-mechanisms of heavy metal toxicity and tolerance with particular emphasis on oxidative stress in exposed cells and the role of antioxidant response

Heavy metals is a collective term describing metals and metalloids with a density higher than 5 g/cm3. Some of them are essential micronutrients; others do not play a positive role in living organisms. Increased anthropogenic emissions of heavy metal ions pose a serious threat to water and land ecosystems. The mechanism of heavy metal toxicity predominantly depends on (1) their high affinity to thiol groups, (2) spatial similarity to biochemical functional groups, (3) competition with essential metal cations, (4) and induction of oxidative stress. The antioxidant response is therefore crucial for providing tolerance to heavy metal-induced stress. This review aims to summarize the knowledge of heavy metal toxicity, oxidative stress and antioxidant response in eukaryotic algae. Types of ROS, their formation sites in photosynthetic cells, and the damage they cause to the cellular components are described at the beginning. Furthermore, heavy metals are characterized in more detail, including their chemical properties, roles they play in living cells, sources of contamination, biochemical mechanisms of toxicity, and stress symptoms. The following subchapters contain the description of low-molecular-weight antioxidants and ROS-detoxifying enzymes, their properties, cellular localization, and the occurrence in algae belonging to different clades, as well as the summary of the results of the experiments concerning antioxidant response in heavy metal-treated eukaryotic algae. Other mechanisms providing tolerance to metal ions are briefly outlined at the end.

Synergistic toxicity to the toxigenic Microcystis and enhanced microcystin release exposed to polycyclic aromatic hydrocarbon mixtures

With the continued influx and accumulation of polycyclic aromatic hydrocarbons (PAHs) in eutrophic waters, the effects of PAHs on cyanobacteria bloom need to be clarified. PAHs usually existed as mixtures in aquatic environments, but the combined toxicity of PAH mixtures to toxigenic cyanobacteria remained unknown. This study investigated the effects of phenanthrene (Phe) and benzo [a]pyrene (BaP), alone or in combination, on the growth and physiology of Microcystis aeruginosa. The results showed that a hormesis effect on growth at low doses of the single Phe (≤1 mg/L) or PAH mixtures (≤0.279 mg/L) was observed, whereas the single BaP induced significant growth inhibitions at all concentrations (≥0.2 mg/L). The median effective concentrations (96 h) for Phe, BaP and their mixtures were 4.29, 1.29 and 1.07 mg/L, respectively. Mixture toxicity models showed that Phe and BaP elicited a synergistic interaction on M. aeruginosa. The synergy may be ascribed to the excessive oxidative stress induced by PAH mixtures, which further led to membrane structure damages, photosynthesis inhibitions and decreased metabolic activity. Moreover, the microcystins (MCs) release significantly increased by 25.3% and 31.9% upon exposure to 0.558 and 1.116 mg/L of PAH mixtures. In all, this study suggested that the enhanced release of MCs by PAH mixtures might exacerbate potential risks to the aquatic environment.

The combined toxicity and mechanism of multi-walled carbon nanotubes and nano copper oxide toward freshwater algae: Tetradesmus obliquus

Nanoparticles (NPs) are widely used for their special physical properties and released into the natural environment. When two types of NPs exist in the same environment, the presence of one type of NP may affect the properties of the other type of NP. This study investigated the toxic effects of multi-walled carbon nanotubes (MWCNTs) and copper oxide nanoparticles (CuO NPs) on Tetradesmus obliquus. Both NPs had toxic effects on algae, and the toxic effects of MWCNTs were significantly stronger than CuO NPs which the 96-hr median effective concentration to algae were 33.8 and 169.2 mg/L, respectively. Oxidative stress and cell membrane damage were the main reasons for the toxicity of NPs to algae, and they were concentration-dependent, and the existence of CuO NPs in some groups reduced cell membrane damage caused by MWCNTs which may because that CuO NPs formed heteroaggregation with MWCNTs, reducing the contact of nanoparticles with cell membranes, then reducing physical damage. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) results indicated cell damage, the heteroaggregation of MWCNTs-CuO NPs and obvious nanoparticles internalization. In some groups, the presence of CuO NPs significantly reduced reactive oxygen species (ROS) level induced by MWCNTs. However, for the highest concentration group, the ROS level was much higher than that of the two NPs alone treatment groups, which might be related to the high concentration of MWCNTs promoting the internalization of CuO NPs. MWCNTs and CuO NPs affected and interacted with each other, causing more complex toxic effects on aquatic organisms.

The combined toxicity of binary mixtures of antibiotics against the cyanobacterium Microcystis is dose-dependent: insight from a theoretical nonlinear combined toxicity assessment method

The entry of antibiotics into aquatic ecosystems has a serious impact. Antibiotics usually exist as mixtures in natural water bodies. Therefore, it is particularly important to evaluate the mixed toxicity of antibiotic mixtures. The study of the combined toxicity of binary mixtures of antibiotics is the basis for exploring the mixed toxicity of multiple antibiotics. In this investigation, Microcystis aeruginosa (M. aeruginosa) was used as the test organism, and a theoretical nonlinear combined toxicity assessment method was adopted to evaluate the effects of binary mixtures of antibiotics consisting of tetracycline (TC), sulfadiazine (SD), and sulfamethoxazole (SMX) on cell growth, enzymatic activity, and gene expression. The median lethal concentrations of TC, SD, and SMX to M. aeruginosa were 0.52 mg L^-1, 1.65 mg L^-1, and 0.71 mg L^-1, respectively. The results from the theoretical nonlinear combined toxicity assessment method showed that SD + TC was synergistic at low concentrations and antagonistic at high concentrations, while the combinations of SMX + SD and SMX + TC were synergistic. The determination of enzymatic activity and gene expression indicated that the antibiotics could inhibit the growth of M. aeruginosa by destroying the cell membrane structure, inhibiting photosynthesis, impeding the cell division process and the electron transfer process, and destroying the molecular structure of proteins and DNA. Different combinations of antibiotics have different degrees of damage to the antioxidant system and cell membrane self-repair function of M. aeruginosa, which are the reasons for the different combined toxicity effects.

Effects of 4-epianhydrotetracycline on oxidative stress in zebrafish (Danio rerio) embryos

4-Epianhydrotetracycline (4-EATC) is a major intermediate product of tetracycline during its degradation progress in the natural environment, which is frequently detected in aquatic environments and poses a potential threat to aquatic organisms. In the present study, the ecotoxicity of 4-EATC have been studied from the perspective of oxidative stress by using zebrafish embryos. After 96 h exposure, the reactive oxygen species (ROS) levels, malondialdehyde (MDA) concentrations and protein carbonyl (PC) contents in zebrafish embryos in the lower-concentration 4-EATC treatment groups (1.25 mg/L and 2.50 mg/L) showed no significant differences compared with the negative control group. However, the total superoxide dismutase (SOD) activity was increased significantly. After exposed to the higher-concentration of 4-EATC (5.00, 10.0 and 20.0 mg/L) resulted in a significant increase in ROS levels, MDA concentrations and PC contents, in contrast, a significant decrease in SOD activities. The results indicate that exposure to high concentrations of 4-EATC (5.00, 10.0 and 20.0 mg/L) could disrupt the redox equilibrium in zebrafish embryos, leading to the occurrence of oxidative damage. Apoptosis of the embryonic cells could be induced by 4-EATC exposure at different concentration and the rate of apoptosis enhanced with the increase of 4-EATC concentration. The pericardium was the most frequent site of apoptosis. The present study points out that more attention should be paid to the potential ecological risks of antibiotic degradation products.

Multivariate trait analysis reveals diatom plasticity constrained to a reduced set of biological axes

Trait-based approaches to phytoplankton ecology have gained traction in recent decades as phenotypic traits are incorporated into ecological and biogeochemical models. Here, we use high-throughput phenotyping to explore both intra- and interspecific constraints on trait combinations that are expressed in the cosmopolitan marine diatom genus Thalassiosira. We demonstrate that within Thalassiosira, phenotypic diversity cannot be predicted from genotypic diversity, and moreover, plasticity can create highly divergent phenotypes that are incongruent with taxonomic grouping. Significantly, multivariate phenotypes can be represented in reduced dimensional space using principal component analysis with 77.7% of the variance captured by two orthogonal axes, here termed a 'trait-scape'. Furthermore, this trait-scape can be recovered with a reduced set of traits. Plastic responses to the new environments expanded phenotypic trait values and the trait-scape, however, the overall pattern of response to the new environments was similar between strains and many trait correlations remained constant. These findings demonstrate that trait-scapes can be used to reveal common constraints on multi-trait plasticity in phytoplankton with divergent underlying phenotypes. Understanding how to integrate trait correlational constraints and trade-offs into theoretical frameworks like biogeochemical models will be critical to predict how microbial responses to environmental change will impact elemental cycling now and into the future.

Algicidal mechanism of Raoultella ornithinolytica against Microcystis aeruginosa: Antioxidant response, photosynthetic system damage and microcystin degradation

Water eutrophication caused by harmful algal blooms (HABs) occurs worldwide. It causes huge economic losses and has serious and potentially life-threatening effects on human health. In this study, the bacterium Raoultella sp. S1 with high algicidal efficiency against the harmful algae Microcystis aeruginosa was isolated from eutrophic water. The results showed that Raoultella sp. S1 initially flocculated the algae, causing the cells to sediment within 180 min and then secreted soluble algicidal substances that killed the algal cells completely within 72 h. The algicidal activity was stable across the temperature range -85.0 to 85.0 °C and across the pH range 3.00-11.00. Scanning electron microscopy (SEM) revealed the crumpling and fragmentation of cells algal cells during the flocculation and lysis stages. The antioxidant system was activated under conditions of oxidative stress, causing the increased antioxidant enzymes activities. Meanwhile, the oxidative stress response triggered by the algicidal substances markedly increased the malondialdehyde (MDA) and glutathione (GSH) content. We investigated the content of Chl-a and the relative expression levels of genes related to photosynthesis, verifying that the algicidal compounds attack the photosynthetic system by degrading the photosynthetic pigment and inhibiting the expression of key genes. Also, the results of photosynthetic efficiency and relative electric transport rate confirmed that the photosynthetic system in algal cells was severely damaged within 24 h. The algicidal effect of Raoultella sp. S1 against Microcystis aeruginosa was evaluated by analyzing the physiological response and photosynthetic system impairment of the algal cells. The concentration of microcystin-LR (MC-LR) slightly increased during the process of algal cells ruptured, and then decreased below its initial level due to the biodegradation of Raoultella sp. S1. To further investigate the algicidal mechanism of Raoultella sp. S1, the main components in the cell-free supernatant was analyzed by UHPLC-TOF-MS. Several low-molecular-weight organic acids might be responsible for the algicidal activity of Raoultella sp. S1. It is concluded that Raoultella sp. S1 has the potential to control Microcystis aeruginosa blooms.

A High-Throughput Assay for Quantifying Phenotypic Traits of Microalgae

High-throughput methods for phenotyping microalgae are in demand across a variety of research and commercial purposes. Many microalgae can be readily cultivated in multi-well plates for experimental studies which can reduce overall costs, while measuring traits from low volume samples can reduce handling. Here we develop a high-throughput quantitative phenotypic assay (QPA) that can be used to phenotype microalgae grown in multi-well plates. The QPA integrates 10 low-volume, relatively high-throughput trait measurements (growth rate, cell size, granularity, chlorophyll a, neutral lipid content, silicification, reactive oxygen species accumulation, and photophysiology parameters: ETRmax, Ik, and alpha) into one workflow. We demonstrate the utility of the QPA on Thalassiosira spp., a cosmopolitan marine diatom, phenotyping six strains in a standard nutrient rich environment (f/2 media) using the full 10-trait assay. The multivariate phenotypes of strains can be simplified into two dimensions using principal component analysis, generating a trait-scape. We determine that traits show a consistent pattern when grown in small volume compared to more typical large volumes. The QPA can thus be used for quantifying traits across different growth environments without requiring exhaustive large-scale culturing experiments, which facilitates experiments on trait plasticity. We confirm that this assay can be used to phenotype newly isolated diatom strains within 4 weeks of isolation. The QPA described here is highly amenable to customisation for other traits or unicellular taxa and provides a framework for designing high-throughput experiments. This method will have applications in experimental evolution, modelling, and for commercial applications where screening of phytoplankton traits is of high importance.

Specific toxicity of azithromycin to the freshwater microalga Raphidocelis subcapitata

Pharmaceuticals are produced to inflict a specific physiological response in organisms. However, as only partially metabolized after administration, these types of compounds can also originate harmful side effects to non-target organisms. Additionally, there is still a lack of knowledge on the toxicological effects of legacy pharmaceuticals such as the antibiotic azithromycin. This macrolide occurs at high concentrations in the aquatic environment and can constitute a threat to aquatic organisms that are at the basis of the aquatic food chain, namely microalgae. This study established a high-throughput methodology to study the toxicity of azithromycin to the freshwater microalga Raphidocelis subcapitata. Flow cytometry and pulse amplitude modulated (PAM) fluorometry were used as screening tools. General toxicity was shown by effects in growth rate, cell size, cell complexity, cell viability and cell cycle. More specific outcomes were indicated by the analysis of mitochondrial and cytoplasmatic membrane potentials, DNA content, formation of ROS and LPO, natural pigments content and photosystem II performance. The specific mode of action (MoA) of azithromycin to crucial components of microalgae cells was revealed. Azithromycin had a negative impact on the regulation of energy dissipation at the PSII centers, along with an insufficient protection by the regulatory mechanisms leading to photodamage. The blockage of photosynthetic electrons led to ROS formation and consequent oxidative damage, affecting membranes and DNA. Overall, the used methodology exhibited its high potential for detecting the toxic MoA of compounds in microalgae and should be considered for future risk assessment of pharmaceuticals.

Response mechanisms of domoic acid in Pseudo-nitzschia multiseries under copper stress

A complex relationship exists between copper stress and the accumulation and release of domoic acid (DA) in toxin-producing Pseudo-nitzschia cells. To clarify the changes and role of DA in this process, we exposed the toxin-producing P. multiseries and the non-toxin-producing P. pungens to copper stress (5 and 9 μM) for 96 h. Results showed that P. multiseries grew better than P. pungens under the two aforementioned copper concentrations. DA content in the cells of P. multiseries increased with increased copper stress, and the dissolved DA in the medium under the 9 μM copper treatment increased. DA addition at a 9 μM copper concentration reduced the copper content in P. multiseries cells and cell walls, but did not change the free copper ion content in culture medium. Adding DA to the medium reduced the malondialdehyde (MDA) content in the cells of P. multiseries under copper stress, DA addition also reduced the activities of catalase (CAT) and superoxide dismutase (SOD) at 5 μM Cu, and the activity of peroxidase (POD) at 9 μM Cu. This suggests that DA may not alleviate copper stress by improving the antioxidant defense system of algal cells, nor can it be complexed with copper ions in the medium to alleviate copper stress. Furthermore, the reactive oxygen species (ROS) scavenger N-tert-butyl-α-phenylnitrone (BPN) was used to study the DA accumulated in cells. The BPN addition significantly reduced the accumulation of DA in the cells under copper stress, suggesting that DA content in cells was closely related to ROS. Moreover, further experiments demonstrated that DA addition can improve the growth of P. multiseries under hydrogen peroxide stress. Our results indicate that DA alleviates P. multiseries oxidative damage when expose to copper stress.

Effects of zinc and mercury on ROS-mediated oxidative stress-induced physiological impairments and antioxidant responses in the microalga Chlorella vulgaris

The rapid growth of industrialization and urbanization results in deterioration of freshwater systems around the world, rescinding the ecological balance. Among many factors that lead to adverse effects in aquatic ecology, metals are frequently discharged into aquatic ecosystems from natural and anthropogenic sources. Metals are highly persistent and toxic substances in trace amounts and can potentially induce severe oxidative stress in aquatic organisms. In this study, adverse effects of the two metal elements zinc (maximum concentration of 167.25 mg/L) and mercury (104.2 mg/L) were examined using Chlorella vulgaris under acute and chronic exposure period (48 h and 7 days, respectively). The metal-induced adverse effects have been analyzed through photosynthetic pigment content, total protein content, reactive oxygen species (ROS) generation, antioxidant enzymatic activities, namely catalase and superoxide dismutase (SOD) along with morphological changes in C. vulgaris. Photosynthetic pigments were gradually reduced (~32-100% reduction) in a dose-dependent manner. Protein content was initially increased during acute (~8-12%) and chronic (~57-80%) exposure and decreased (~44-56%) at higher concentration of the two metals (80%). Under the two metal exposures, 5- to 7-fold increase in ROS generation indicated the induction of oxidative stress and subsequent modulations in antioxidant activities. SOD activity was varied with an initial increase (58-129%) followed by a gradual reduction (~3.7-79%), while ~1- to 12-fold difference in CAT activity was observed in all experimental condition (~83 to 1605%). A significant difference was observed in combined toxic exposure (Zn+Hg), while comparing the toxic endpoint data of individual metal exposure (Zn and Hg alone). Through this work, lethal effects caused by single and combined toxicity of zinc and mercury were assessed, representing the significance of appropriate monitoring system to trim down the release of metal contaminants into the aquatic ecosystems.

The role of changes in environmental quality in multitrait plastic responses to environmental and social change in the model microalga Chlamydomonas reinhardtii

Intraspecific variation plays a key role in species' responses to environmental change; however, little is known about the role of changes in environmental quality (the population growth rate an environment supports) on intraspecific trait variation. Here, we hypothesize that intraspecific trait variation will be higher in ameliorated environments than in degraded ones. We first measure the range of multitrait phenotypes over a range of environmental qualities for three strains and two evolutionary histories of Chlamydomonas reinhardtii in laboratory conditions. We then explore how environmental quality and trait variation affect the predictability of lineage frequencies when lineage pairs are grown in indirect co-culture. Our results show that environmental quality has the potential to affect intraspecific variability both in terms of the variation in expressed trait values, and in terms of the genotype composition of rapidly growing populations. We found low phenotypic variability in degraded or same-quality environments and high phenotypic variability in ameliorated conditions. This variation can affect population composition, as monoculture growth rate is a less reliable predictor of lineage frequencies in ameliorated environments. Our study highlights that understanding whether populations experience environmental change as an increase or a decrease in quality relative to their recent history affects the changes in trait variation during plastic responses, including growth responses to the presence of conspecifics. This points toward a fundamental role for changes in overall environmental quality in driving phenotypic variation within closely related populations, with implications for microevolution.

Gamma-aminobutyric acid facilitates the simultaneous production of biomass, astaxanthin and lipids in Haematococcus pluvialis under salinity and high-light stress conditions

The effects of γ-aminobutyric acid (GABA) on the biomass and astaxanthin and lipids production in Haematococcus pluvialis under combined salinity stress and high-light stresses were investigated. The results showed that the highest biomass (1.65 g L^-1), astaxanthin production (3.86 mg L^-1 d^-1) and lipids content (55.11%) in H. pluvialis LUGU were observed under the 0.25 mM GABA treatment. Moreover, compared with salinity and high-light stress, GABA treatment also increased the transcript levels of biosynthesis genes, the contents of endogenous GABA and carbohydrates but decreased reactive oxygen species (ROS) levels. Further evidence revealed that intracellular GABA could regulate cell growth, astaxanthin production and lipids synthesis by mediating carotenogenesis, lipogenesis and ROS signalling. Collectively, this study provides a combined strategy for promoting the coproduction of astaxanthin and lipids and sheds light on the regulatory mechanism through which GABA affects cell growth, astaxanthin production and lipids biosynthesis in H. pluvialis under unfavourable conditions.

Effects of erythromycin and sulfamethoxazole on Microcystis aeruginosa: Cytotoxic endpoints, production and release of microcystin-LR

Antibiotics can cause severe ecological problems for aquatic ecosystems due to their wide use and incomplete removal. Microcystis aeruginosa was exposed to different levels of erythromycin (ERY) and sulfamethoxazole (SMX) separately to assess their cytotoxic effects on harmful cyanobacteria. The production and release of the toxin MC-LR was measured, and several endpoints were investigated using flow cytometry (FCM) for 7 d. ERY resulted in cell membrane hyperpolarization and a hormesis effect on growth rate and chlorophyll a fluorescence at environmentally relevant concentrations (0.5 and 5 μg/L). Microcystis exhibited elevated photosynthesis and hyperpolarization at 50 and 125 μg/L of SMX. An increase of metabolically non-active cells was observed in either ERY or SMX cultures while stimulation of esterase activity was also found at 7 d. ERY and SMX caused damage of membrane integrity due to the overproduction of ROS, which led to increased release of MC-LR. MC-LR production apparently was induced by ERY (0.5-500 μg/L) and SMX (50 and 125 μg/L). In conclusion, ERY and SMX can disrupt the physiological status of Microcystis cells and stimulate the production and release of MC-LR, which can exacerbate potential risks to water systems.

Azospirillum brasilense reduces oxidative stress in the green microalgae Chlorella sorokiniana under different stressors

In this study, we investigated oxidative stress in the green microalgae, Chlorella sorokiniana, in co-culture with the plant growth promoting bacteria (PGPB), Azospirillum brasilense. This relationship was studied in the absence of an exogenous stressor, under copper stress, and under nitrogen limitation stress. We confirmed that copper and nitrogen limitation induced algal oxidative stress and reductions in chlorophyll content. In all cases, the presence of A. brasilense lowered the accumulation of intracellular reactive oxygen species (ROS) while promoting chlorophyll content. This effect was driven, in part, by A. brasilense's secretion of the auxin hormone, indole-3-acetic acid, which is known to mitigate stress in higher plants. The findings of the present study show that stress mitigation by A. brasilense resulted in suppressed starch accumulation under nitrogen limitation stress and neutral lipid accumulation under copper stress. In fact, A. brasilense could almost completely mitigate oxidative stress in C. sorokiniana resulting from nitrogen limitation, with ROS accumulation rates comparable to the axenic control cultures. The biotechnological implication of these findings is that co-culture strategies with A. brasilense (and similar PGPB) are most effective for high growth applications. A second growth stage may be needed to induce accumulation of desired products.

Toxicity assessments of acrylamide in aquatic environment using two algae Nitzschia closterium and Scenedesmus quadricauda

The expanding production and widespread application of acrylamide caused inevitable release to aquatic ecosystems. Contrary to its extensive attention to human and animal health, the hazards of acrylamide to the aquatic primary productivity have been rarely studied. The potential effects of acrylamide on the marine algae (Nitzschia closterium) and the limnetic algae (Scenedesmus quadricauda) were investigated by monitoring cell abundance, total chlorophyll content, maximum photosystem II (PSII) quantum yield (Fv/Fm), and reactive oxygen species (ROS). The growth of two algae was significantly inhibited by acrylamide. The 96 h EC₅₀ of acrylamide on N. closterium and S. quadricauda were 5.50 mg L^-1 and 45.3 mg L^-1, and no observed effect concentration (NOEC) were 1.07 mg L^-1 and 6.97 mg L^-1, respectively. After 96 h exposure to 50 mg L^-1 of acrylamide, the total chlorophyll content declined to approximate 18% (N. closterium) and 48% (S. quadricauda), and Fv/Fm was observed to be 0.35 and 0.32 for N. closterium and S. quadricauda, respectively. ROS was significantly increased following higher exposure concentrations, and its levels increased around 2.1-fold and 1.4-fold following exposure to 5 mg L^-1 of acrylamide. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed the visible cell plasmolysis, rupture of the plasma membrane, cell vacuolization, and disintegration of chloroplasts of the algae caused acrylamide.

Oxidative stress response and proteomic analysis reveal the mechanisms of toxicity of imidazolium-based ionic liquids against Arabidopsis thaliana

Ionic liquids (ILs) are extensively used in various fields, posing a potential threat in the ecosystem because of their high stability, excellent solubility, and biological toxicity. In this study, the toxicity mechanism of three ILs, 1-octyl-3-methylimidazolium chloride ([C₈MIM]Cl), 1-decyl-3-methylimidazolium chloride ([C₁₀MIM]Cl), and 1-dodecyl-3-methylimidazolium chloride ([C₁₂MIM]Cl) on Arabidopsis thaliana were revealed. Reactive oxygen species (ROS) level increased with higher concentration and longer carbon chain length of ILs, which led to the increase of malondialdehyde (MDA) content and antioxidase activity, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX) and peroxidase (POD) activities. SOD, CAT, and GPX activities decreased in high ILs concentration due to the excessive ROS. Differentially expressed protein was analyzed based on Gene ontology (GO) and KEGG pathways analysis. 70, 45, 84 up-regulated proteins, and 72, 104, 79 down-regulated proteins were identified in [C₈MIM]Cl, [C₁₀MIM]Cl, and [C₁₂MIM]Cl treatment, respectively (fold change ≥ 1.5 with ≥95% confidence). Cellular aldehyde metabolic process, mitochondrial and mitochondrial respiratory chains, glutathione transferase and oxidoreductase activity were enriched as up-regulated proteins as the defense mechanism of A. thaliana to resist external stresses. Chloroplast, photosynthetic membrane and thylakoid, structural constituent of ribosome, and transmembrane transport were enriched as the down-regulated protein. Compared with the control, 8 and 14 KEGG pathways were identified forup-regulated and down-regulated proteins, respectively, in three IL treatments. Metabolic pathways, carbon metabolism, biosynthesis of amino acids, porphyrin and chlorophyll metabolism were significantly down-regulated. The GO terms annotation demonstrated the oxidative stress response and effects on photosynthesis of A. thaliana in ILs treatment from biological process, cellular component, and molecular function categories.

Ecotoxicological effects of alpha-cypermethrin on freshwater alga Chlorella sp.: Growth inhibition and oxidative stress studies

Alpha-cypermethrin (ACy) is a synthetic pyrethroid insecticide commonly used in agricultural practices for controlling a broad range of insect pests particularly belonging to the order Lepidoptera and Coleoptera. The present study aims to evaluate the toxic effect of ACy on microalgae by studying its influence on Chlorella sp. According to our knowledge, this is the first detailed study of ACy toxicity on microalgae. Significant growth inhibition of Chlorella sp. was observed at high ACy concentration (6-48 mg L^-1) during the entire 96 h bioassay. The 96 h median effective concentration (EC₅₀) of ACy was estimated to be 11.00 mg L^-1. Flow cytometry analysis showed an enhanced generation of reactive oxygen species (ROS) and intracellular lipid accumulation after 96 h exposure to 11.00 mg L^-1 of ACy. Further, the same ACy concentration showed a significant decrease in photosynthetic pigment content and an increase in antioxidant enzyme activity and malondialdehyde (MDA) content in Chlorella sp.

Impacts of Zn and Cu enrichment under ocean acidification scenario on a phytoplankton community from tropical upwelling system

Increasing dissolution of CO₂ in the surface ocean is rapidly decreasing its pH and changing carbon chemistry which is further affecting marine biota in several ways. Phytoplankton response studies under the combination of elevated CO₂ and trace metals are rare. We have conducted two consecutive onboard incubation experiments (R. V. Sindhu Sadhana; August 2017) in the eastern Arabian Sea (SW coast of India) during an upwelling event. A nutrient enriched diatom bloom was initiated onboard and grown under ambient (≈400 μatm, A-CO₂) and high CO₂ levels (≈1000 μatm; H-CO₂) with different zinc (Zn; 1 nM) and copper (Cu) concentrations (1 nM, 2 nM and 8 nM). Phytoplankton community composition and the dominant genera were different during these two experiments. CO₂ enrichment alone did not show any significant growth stimulating impact on the experimental community except enhanced cell density in the first experiment. Addition of Zn at A-CO₂ level revealed no noticeable responses; whereas, the same treatment under H-CO₂ level significantly reduced cell number. Considerably high protein content under H-CO₂+Zn treatment was possibly counteracting Zn toxicity which also caused slower growth rate. Cu addition did not show any noticeable impact on growth and biomass production except increased protein content as well as decreased carbohydrate: protein ratio. This can be attributed to relatively higher protein synthesis than carbohydrate to alleviate oxidative stress generated by Cu. The centric diatom Chaetoceros and toxin producing pennate diatom Pseudo-nitzschia showed no significant response to either CO₂ or Zn enrichment. Large centric diatom Leptocylindrus and Skeletonema responded positively to Zn addition in both CO₂ levels. The former species showed the most sensitive response at the highest Cu and H-CO₂ treatment; whereas, the pennate diatoms Nitzschia and Pseudo-nitzschia (toxigenic diatom) showed higher resilience under elevated CO₂ and Cu levels. This observation indicated that in future ocean, increasing CO₂ concentrations and trace metal pollution may potentially alter phytoplankton community structure and may facilitate toxigenic diatom bloom in the coastal waters.