Evaluation of the role of the glutathione redox cycle in Cu(II) toxicity to green algae by a chiral perturbation approach

Toxicity, physiological response, and biosorption mechanism of Dunaliella salina to copper, lead, and cadmium

Background

Heavy metal pollution has become a global problem, which urgently needed to be solved owing to its severe threat to water ecosystems and human health. Thus, the exploration and development of a simple, cost-effective and environmental-friendly technique to remove metal elements from contaminated water is of great importance. Algae are a kind of photosynthetic autotroph and exhibit excellent bioadsorption capacities, making them suitable for wastewater treatment.

Methods

The effects of heavy metals (copper, lead and cadmium) on the growth, biomolecules accumulation, metabolic responses and antioxidant response of Dunaliella salina were investigated. Moreover, the Box-Behnken design (BBD) in response surface methodology (RSM) was used to optimize the biosorption capacity, and FT-IR was performed to explore the biosorption mechanism of D. salina on multiple heavy metals.

Results

The growth of D. salina cells was significantly inhibited and the contents of intracellular photosynthetic pigments, polysaccharides and proteins were obviously reduced under different concentrations of Cu2+, Pb2+ and Cd2+, and the EC50 values were 18.14 mg/L, 160.37 mg/L and 3.32 mg/L at 72 h, respectively. Besides, the activities of antioxidant enzyme SOD and CAT in D. salina first increased, and then descended with increasing concentration of three metal ions, while MDA contents elevated continuously. Moreover, D. salina exhibited an excellent removal efficacy on three heavy metals. BBD assay revealed that the maximal removal rates for Cu2+, Pb2+, and Cd2+ were 88.9%, 87.2% and 72.9%, respectively under optimal adsorption conditions of pH 5-6, temperature 20-30°C, and adsorption time 6 h. Both surface biosorption and intracellular bioaccumulation mechanisms are involved in metal ions removal of D. salina. FT-IR spectrum exhibited the main functional groups including carboxyl (-COOH), hydroxyl (-OH), amino (-NH2), phosphate (-P=O) and sulfate (-S=O) are closely associated with the biosorption or removal of heavy metalsions.

Discussion

Attributing to the brilliant biosorption capacity, Dunaliella salina may be developed to be an excellent adsorbent for heavy metals.

Nitrogen forms and concentration influence the impact of titanium dioxide nanoparticles on the biomass and antioxidant enzyme activities of Microcystis aeruginosa

Nanoparticles (NPs) are becoming more widely produced, used, and released into the aquatic environment. In aquatic ecosystems, these NPs affect different populations of photosynthesizing organisms, such as cyanobacteria. This study aimed to evaluate the effects of titanium dioxide (TiO₂) NPs (48 mg l^-1) combined with low (0.04 mM) and high (9 mM) concentrations of urea and nitrate on Microcystis aeruginosa. Microcystins (MCs) production and release were monitored in the cyanobacterium. The results showed that high urea concentration (9 mM) combined with TiO₂ NPs inhibited growth, pigment, and malondialdehyde (MDA) content by 82%, 63%, and 47%, respectively. The treatment also increased the reactive oxygen species (ROS) and glutathione S-transferase (GST) activity by 40.7% and 67.7%, respectively. Similarly, low nitrate (0.04 mM) combined with TiO₂ NPs inhibited growth by 40.3% and GST activity by 36.3% but stimulated pigment production and ROS concentration in M. aeruginosa. These responses suggest that high urea combined with TiO₂.NPs and high nitrate combined with TiO₂ NPs induced oxidative stress in cyanobacteria. The peroxidase (POD) activity of M. aeruginosa decreased by 17.7% with increasing urea concentrations. Our findings suggest that TiO₂ NPs combined with changing nutrient (urea and nitrate) concentrations may adversely affect cyanobacterial development and antioxidant defense enzymes.

Selection and characterization of plant-derived alkaloids with strong antialgal inhibition: growth inhibition selectivity and inhibitory mechanism

In recent years, researches on microalgae inhibition with plant-derived active substances have attracted much attention. In this study, the inhibition of six plant-derived alkaloids (neferine, isoliensinine, linensinine, nuciferine, capsaicin, and hordenine) on bloom-forming cyanobacteria Microcystis aeruginosa were investigated. The results showed that neferine and nuciferine had stronger inhibition on the growth of M. aeruginosa compared with the other four alkaloids, and the relative inhibition rate reached 91.27% and 88.70% at the concentration of 4.5 mg/L after 7 d of exposure, respectively. Different from neferine, nuciferine has no inhibition on Chlorella sp. and Tetradesmus obliquus. It also increased the diversity and species homogeneity of phytoplankton in the environmental water samples. Nuciferine decreased the contents of chlorophyll a and β-carotene in M. aeruginosa with the extension of treatment time, which was 59.40% and 31.90% of the control at the concentration of 1.04 mg/L after 48 h, respectively. After 48 h of nuciferine exposure, the values of fluorescence parameters including maximum quantum yield (Fv/Fm), actual quantum yield of PSII (Yield), non-photochemical quenching (qN and NPQ), and electron transport rates (ETR) of M. aeruginosa cells were significantly decreased and photosynthetic capacity was weakened. The superoxide dismutase (SOD), catalase (CAT), ascorbic acid (ASA), and glutathione (GSH) in the cells were significantly reduced, and the hydrogen peroxide (H₂O₂) and malonaldehyde (MDA) contents continued to accumulate, causing severe oxidative damage. Therefore, the good biological safety and strong specific inhibition of nuciferine makes it have great application prospects in the inhibition of cyanobacteria blooms.

Cadmium Caused Different Toxicity to Photosystem I and Photosystem II of Freshwater Unicellular Algae Chlorella pyrenoidosa (Chlorophyta)

Heavy metals such as Cd pose environmental problems and threats to a variety of organisms. The effects of cadmium (Cd) on the growth and activities of photosystem I (PSI) and photosystem II (PSII) of Chlorella pyrenoidosa were studied. The growth rate of cells treated with 25 and 100 µM of Cd for longer than 48 h were significantly lower than the control, accompanying with the inhibition of photosynthesis. The result of quantum yields and electron transport rates (ETRs) in PSI and PSII showed that Cd had a more serious inhibition on PSII than on PSI. Cd decreased the efficiency of PSII to use the energy under high light with increasing Cd concentration. In contrast, the quantum yield of PSI did not show a significant difference among different Cd treatments. The activation of cyclic electron flow (CEF) and the inhibition of linear electron flow (LEF) due to Cd treatment were observed. The photochemical quantum yield of PSI and the tolerance of ETR of PSI to Cd treatments were due to the activation of CEF around PSI. The activation of CEF also played an important role in induction of non-photochemical quenching (NPQ). The binding features of Cd ions and photosystem particles showed that Cd was easier to combine with PSII than PSI, which may explain the different toxicity of Cd on PSII and PSI.

Potential of three local marine microalgae from Tunisian coasts for cadmium, lead and chromium removals

Metal elements are widely used in various industrial activities and are considered as common water source contaminants. Thus, the development of cost-effective, simple design and efficient processes for trace metal elements removal from contaminated water sources is of great interest. The effects of cadmium, lead and chromium on growth, biomolecules accumulation and metabolic responses of Amphora coffaeiformis, Navicula salinicola and Dunaliella salina isolated from Tunisian coasts were tested. The bioremediation capacities of the three microalgae strains and the mechanisms involved in ions metal removal were also investigated. N. salinicola and D. salina seem to be better tolerating to Cr, while A. coffaeiformis and N. salinicola showed high resistance to Pb. The expression profile analyses by qRT-PCR of the antioxidant defense-related genes revealed that Cd, Pb and Cr treatments induce the up-regulation of catalase and superoxide dismutase coding genes for A. coffaeiformis and D. salina. Regarding N. salinicola, the catalase coding gene seems to be overexpressed after Cd, Pb and Cr exposure while only Cd and Cr induce superoxide dismutase gene overexpression. Moreover, the phytochelatin synthase (a metal chelator synthesis-related gene) was up-regulated in N. salinicola, A. coffaeiformis and D. salina after Cr exposure and also in A. coffaeiformis and D. salina after Cd exposure. While Pb treatments induce overexpression of phytochelatin synthase coding gene only for D. salina. Studied strains showed promising metal removal efficiencies for both Pb and Cr ions metals reached 95% for D. salina. Ion metal removal mechanisms study revealed that intracellular bioaccumulation process is used by D. salina for Cr up-taking. However, both intracellular and extracellular removal mechanisms are involved for Pb and Cr removal using A. coffaeiformis, N. salinicola and for Pb removal using D. salina. FTIR analysis demonstrated that several functional groups as carboxyl, hydroxyl, amino, phosphate and sulfate may participate in the bioadsorption process.

Ultrasonic seed treatment improved morpho-physiological and yield traits and reduced grain Cd concentrations in rice

Rice cultivation under cadmium (Cd) contaminated soil often results in reduced growth with excess grain Cd concentrations. A pot experiment was conducted to assess the potential of ultrasonic seed treatment to alleviate Cd stress in rice. Seeds of two aromatic rice cultivars i.e., Xiangyaxiangzhan and Meixiangzhan 2 and two non-aromatic rice cultivars i.e., Huahang 31 and Guangyan 1 were exposed to ultrasonic waves for 1.5 min in 20-40 KHz mixing frequency. The experimental treatments were comprised of untreated seeds (U0) and ultrasonic treated seeds (U1) transplanted in un-contaminated soil (H0) and Cd-contaminated soil (H1). Results revealed that Cd contents and Cd accumulation in grain in U1 were 33.33-42.31% and 12.86-57.58% lower than U0 for fragrant rice cultivars under H1. Meanwhile, biomass production was higher in U1 than U0 under H0 and better yield was assessed in U1 for all cultivars under H1. The activity of peroxidase (POD) in flag leaves was increased by 8.28-115.65% for all cultivars while malondialdehyde (MDA) contents were significantly decreased in U1 compared with U0 under H0. Conclusively, ultrasonic treatment modulated Cd distribution and accumulation in different parts while improved physiological performance as well as yield and grain quality of rice under Cd contaminated conditions.

Cadmium biosorption and biomass production by two freshwater microalgae Scenedesmus acutus and Chlorella pyrenoidosa: An integrated approach

Cadmium (Cd) contamination in different water bodies is a matter of serious concern, as it can cause biomagnification in our food chain up to several trophic levels. In this study, Cd toxicity was investigated in the micro-algae Chlorella pyrenoidosa and Scenedesmus acutus exposed to various concentrations of Cd for 96 h. The inhibitory and toxic effects of Cd²⁺ on growth and photosynthetic parameters of algae were demonstrated. The bioremediation potentials of these algae were investigated and bioremoval mechanisms were confirmed using qualitative electron microscopic assay such as scanning/transmission electron microscope (S/TEM). The photochemical quenching (Fv/Fm), quantum yield (YII), relative electron transfer rate (rETR) and non-photochemical quenching (NPQ) were inhibited significantly and reduced by ≥ 50% of the control at MIC 50 values. The C. pyrenoidosa and S. acutus biomass have shown 30% and 20% reduction in carbon content and 10% and 12% reduction in nitrogen content at MIC50 values of Cd²⁺ treatment, respectively. During bioremoval studies, C. pyrenoidosa and S. acutus have shown 45.45% and 57.14% Cd²⁺ removal of Cd²⁺ from initial concentration of 1.5 ppm. Out of total cadmium removal C. pyrenoidosa was reported 3% bioaccumulation and 97% biosorption. Whereas S. acutus showed 1.5% accumulation and 98.5% biosorption. The S/TEM images showed the surface accumulation and bioaccumulation of cadmium inside the cytoplasm, vacuoles, and chloroplast. Thus cultivating C. pyrenoidosa and S. acutus would be beneficial in Cd²⁺ contaminated water bodies as they serve the dual purpose by Cd remediation and algal biomass production.

Cadmium accumulation and physiological response of Amaranthus tricolor L. under soil and atmospheric stresses

In this study, cadmium (Cd) solution spraying and Cd-contaminated soil pot experiments were conducted to investigate the influence of Cd from atmospheric deposition and soil on the growth, cumulative distribution, chemical morphology, physiological, and biochemical responses of Amaranthus tricolor L. The results indicated that Cd in plants mainly came from soil (92-98%) and was stored in the roots in large quantities while the portion from atmospheric deposition could also effectively increase Cd content in stems and leaves (2-3%). Cd was mainly stored in plant cell walls and would transfer to the soluble part under high-concentration soil stress Cd from atmospheric deposition alone promoted the growth of plants, but high Cd concentrations from soil had the negative influence. The contents of H₂O₂ and MDA in plants increased under soil and atmospheric Cd stress, indicating that the plant cells were damaged by oxidative stress. The content of antioxidant enzymes such as POD, CAT, SOD, and antioxidants like AsA and GSH increased under low-concentration Cd stress but decreased under elevated stress, suggesting that high Cd-contaminated soil poses severe toxicity on the antioxidant system of the plants. Hence, the accumulation and physiological response of plants under multi-source Cd contamination were mainly affected by high soil Cd concentrations. Though the effect of atmospheric deposition is relatively less, it cannot be ignored.

Ultrasonic seed treatment improved cadmium (Cd) tolerance in Brassica napus L

Cadmium (Cd) affects crop growth and productivity by disrupting normal plant metabolism. To determinate whether ultrasonic (US) seed treatment can alleviate Cd stress in rape (Brassica napus L.), the seeds of two oilseed rape cultivars i.e., 'Youyanzao18' and 'Zaoshu104' were exposed to ultrasonic waves for 1 min at 20 KHz frequency. Seeds without US treatment were taken as control (CK). Results revealed that the germination rate of both cultivars was significantly (P < 0.05) higher in US treatment than CK only at 0 and 10 mg Cd L^-1. The shoot and root length of both cultivars were significantly higher in US treatment than CK at all Cd treatments except the root length of Youyanzao18 at 50 mg Cd L^-1. The fresh weight Youyanzao18 was significantly (P < 0.05) higher in US than CK except for Youyanzao18 at 25 mg Cd L^-1. Moreover, the superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) activities and the proline, glutathione (GSH), and soluble protein contents in Youyanzao18 were relatively higher in the US treatment than CK. The malondialdehyde (MDA) contents were prominently reduced in US treatment than CK. The pods per plant, seeds per pod and rapeseed yield were increased by 15.9, 11.4, and 16.4% in Youyanzao18 and 10.3, 9.5, and 11.5% in Zaoshu104, respectively in US treatment, compared to CK. Moreover, the contents of Cd in root, stem, leaf, rape pod shell, and rapeseeds were comparatively less in US treatment than CK whereas the Cd concentrations in different plant parts of both rape cultivars were recorded as: leaf ˃ root ˃ stem ˃ rape pod shell ˃ rapeseed. In sum, the US treatment improved the morphological growth and rapeseed yield whereas reduced the Cd accumulation in different plant parts of rapeseed under Cd contaminated soil.

Quantitative assessment of photosynthetic activity of Chlorella (Class Trebouxiophyceae) adsorbed onto soil by using fluorescence imaging

In the present study, we evaluate our previously developed non-destructive soil algal toxicity method using species from a different class of algae; Class Trebouxiophyceae (Chlorella vulgaris and Chlorella sorokiniana), and directly measure the photosynthetic activity of these species adsorbed onto the soil as a new toxicity endpoint. This study shows that non-destructive soil algal toxicity method is applicable to non-specific test species, including those of Class Trebouxiophyceae as well as Class Chlorophyceae (Chlorococcum infusionum and Chlamydomonas reinhardtii). Furthermore, by performing photosynthesis image analysis, we verify that it is possible to measure the photosynthetic activity of soil algae Chlorella vulgaris adsorbed onto soils without the need to extract algal cells from the soil. We propose that the non-destructive soil algal toxicity method represents a novel technique for 1) evaluating pollutants in soil using non-specific algae and 2) conveniently and rapidly assessing the photosynthetic activity of soil algae Chlorella vulgaris adsorbed onto soil as a new toxicity endpoint.

miR-489-3p Regulates the Oxidative Stress Response in the Liver and Gill Tissues of Hybrid Yellow Catfish (Pelteobagrus fulvidraco♀ × P. vachelli♂) Under Cu2+ Exposure by Targeting Cu/Zn-SOD

Copper/zinc superoxide dismutase (Cu/Zn-SOD) plays critical roles in protecting cells and tissues against oxidative damage. Excessive copper ions (Cu2+) in water can damage the cells of aquatic organisms, leading to impaired growth and development and reduced antioxidant defenses. Many regulatory factors control the response to excess Cu2+. Among them, microRNAs (miRNAs) are important small RNAs that regulate the expression of their target genes and participate in the oxidative stress response. In the present study, we used bioinformatics and dual luciferase reporter gene analyses to demonstrate that the miR-489-3p of hybrid yellow catfish (Pelteobagrus fulvidraco♀ × P. vachelli♂) binds to the 3'-untranslated region (UTR) of its target gene, which encodes a Cu/Zn-SOD. The regulatory relationship between this miRNA and its target gene Cu/Zn-SOD was analyzed using qRT-PCR and luciferase activity assays. We also investigated the effect of the loss of miR-489-3p expression on the oxidative stress response of hybrid yellow catfish exposed to Cu2+. The Cu/Zn-SOD 3'UTR region was found to be fully complementary to positions 2-9 of the 5'-end seed region of miR-489-3p. The miR-489-3p expression levels were negatively related to Cu/Zn-SOD expression. Silencing of miR-489-3p up-regulated Cu/Zn-SOD expression in the liver and gill tissues, increased activities of SOD and catalase, and reduced the malondialdehyde content. This study is the first to demonstrate that miR-489-3p targets Cu/Zn-SOD to mediate the oxidative response to metal stress. These findings provide a theoretical basis for further studies on the response to oxidative stress caused by metals in cultured fish, and provide an experimental basis for the management of the culture environment.

Probing Contaminant-Induced Alterations in Chlorophyll Fluorescence by AC-Dielectrophoresis-Based 2D-Algal Array

The investigation of contaminant impact on algae requires rapid and reliable cell collection and optical detection. The capability of alternative current (AC) dielectrophoresis (DEP) collection of whole cell arrays with combined fluorescence microscopy detection to follow the alterations of chlorophyll fluorescence during environmental contaminant exposure was explored. The application of an AC-field of 100 V cm^-1, 100 Hz for 30 min to capture and immobilize the cells of green alga Chlamydomonas reinhardtii in two-dimensional (2D) arrays does not induce changes in chlorophyll fluorescence. The results demonstrate that DEP-based 2D-arrays allow non-invasive detection of chlorophyll fluorescence change upon exposure to high concentrations of copper oxide nanoparticles and ionic copper. These results were in agreement with data obtained by flow cytometry used as a comparative method. The tool was also applied to follow the effect of a number of ubiquitous contaminants such as inorganic mercury, methylmercury, and diuron. However, a statistically significant short-term effect was observed only for mercury. Overall, DEP-based 2D-arrays of algal cells with fluorescence detection appear to be suitable for stain-free probing the effects on the photosynthetic microorganisms in highly polluted environment.

A Novel Cyclophilin B Gene in the Red Tide Dinoflagellate Cochlodinium polykrikoides: Molecular Characterizations and Transcriptional Responses to Environmental Stresses

The marine dinoflagellate Cochlodinium polykrikoides is one of the most common ichthyotoxic species that causes harmful algal blooms (HABs), which leads to ecological damage and huge economic loss in aquaculture industries. Cyclophilins (CYPs) belong to the immunophilin superfamily, and they may play a role in the survival mechanisms of the dinoflagellate in stress environments. In the present study, we identified a novel cyclophilin gene from C. polykrikoides and examined physiological and gene transcriptional responses to biocides copper sulphate (CuSO₄) and sodium hypochlorite (NaOCl). The full length of CpCYP was 903 bp, ranging from the dinoflagellate splice leader (DinoSL) sequence to the polyA tail, comprising a 639 bp ORF, a 117 bp 5′-UTR, and a 147 bp 3′-UTR. Motif and phylogenetic comparisons showed that CpCYP was affiliated to group B of CYP. In biocide stressors, cell counts, chlorophyll a, and photosynthetic efficiency (Fv/Fm) of C. polykrikoides were considerably decreased in both exposure time- and dose-dependent manners. In addition, CpCYP gene expression was significantly induced after 24 h exposure to the biocide-treated stress conditions. These results indicate an effect of the biocides on the cell physiology and expression profile of CpCYP, suggesting that the gene may play a role in environmental stress responses.

Evaluation of new Cu(II) complexes as a novel class of inhibitors against plant carbonic anhydrase, glutathione reductase, and photosynthetic activity in photosystem II

Increasing inefficiency of production of important agricultural plants raises one of the biggest problems in the modern world. Herbicide application is still the best method of weed management. Traditional herbicides blocking only one of the plant metabolic pathways is ineffective due to the rapid growth of herbicide-resistant weeds. The synthesis of novel compounds effectively suppressing several metabolic processes, and therefore achieving the synergism effect would serve as the alternative approach to weed problem. For this reason, recently, we synthesized a series of nine novel Cu(II) complexes and four ligands, characterized them with different analyses techniques, and carried out their primary evaluation as inhibitors of photosynthetic electron transfer in spinach thylakoids (design, synthesis, and evaluation of a series of Cu(II) based metal-organic complexes as possible inhibitors of photosynthesis, J Photochem Photobiol B, submitted). Here, we evaluated in vitro inhibitory potency of these agents against: photochemistry and carbonic anhydrase activity of photosystem II (PSII); α-carbonic anhydrase from bovine erythrocytes; as well as glutathione reductase from chloroplast and baker's yeast. Our results show that all Cu(II) complexes excellently inhibit glutathione reductase and PSII carbonic anhydrase activity. Some of them also decently inhibit PSII photosynthetic activity.

The effect of cadmium on the growth and antioxidant response for freshwater algae Chlorella vulgaris

The objective of the present work was to evaluate the effect of exogenously applied cadmium on the physiological response of green algae Chlorella vulgaris. The study investigated the long-term effect (18 days) of cadmium on the levels of algae biomass, assimilation pigment composition, soluble protein, oxidative status (production of hydrogen peroxide and superoxide anion), antioxidant enzymes (such as superoxide dismutase, peroxidase, catalase and glutathione reductase enzyme) in C. vulgaris. The results showed that growth, the amount of chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoids gradually decreased with increasing cadmium over 18 days exposure. Cadmium at concentration of 7 mg L(-1) inhibited algal growth expressed as the number of cells. Our research found that C. vulgaris has a high tolerance to cadmium. Contents of chlorophylls (Chl a and Chl b) and carotenoids (Car) of C. vulgaris was significantly decline with rising concentration of cadmium (p < 0.05). The decrease of 54.04 and 93.37 % in Chl a, 60.65 and 74.32 % in Chl b, 50.00 and 71.88 % in total carotenoids was noticed following the treatment with 3 and 7 mg L(-1) cadmium doses compared with control treatment, respectively. Cadmium treatments caused a significant change in the physiological competence (calculated as chlorophyll a/b) which increased with increasing Cd(II) doses up to 1 mg L(-1) but decreased at 3 mg L(-1). While accumulation of soluble protein was enhanced by presence of cadmium, the treatment with cadmium at 3 and 7 mg L(-1) increased the concentration of soluble proteins by 88, 95.8 % in C. vulgaris, respectively. Moreover, low doses of cadmium stimulated enzymatic (superoxide dismutase, catalase and glutathione reductase) in C. vulgaris, The content of peroxidase increased with the increasing cadmium concentration, and had slightly decreased at the concentration of 7 mg L(-1), but was still higher than control group, which showed that cadmium stress at high concentration mainly peroxidase works in C. vulgaris. And therefore, suppressed reactive oxygen species (hydrogen peroxide and superoxide) accumulated. The present study also showed that cadmium increased oxidative stress and induced antioxidant defense systems against reactive oxygen species. The observation in here analyzed C. vulgaris after exposure to cadmium indicate that hydrogen peroxide, superoxide and peroxidase in the alga with exposure to Cd(II) seemed to be parameters as biomarkers for metal-induced oxidative stress.

Synergistic effects of nano-sized titanium dioxide and zinc on the photosynthetic capacity and survival of Anabaena sp

Anabaena sp. was used to examine the toxicity of exposure to a nano-TiO2 suspension, Zn2+ solution, and mixtures of nano-TiO2 and Zn2+ suspensions. Typical chlorophyll fluorescence parameters, including effective quantum yield, photosynthetic efficiency and maximal electron transport rate, were measured by a pulse-amplitude modulated fluorometer. Nano-TiO2 particles exhibited no significant toxicity at concentrations lower than 10.0 mg/L. The 96 h concentration for the 50% maximal effect (EC50) of Zn2+ alone to Anabaena sp. was 0.38 ± 0.004 mg/L. The presence of nano-TiO2 at low concentrations (<1.0 mg/L) significantly enhanced the toxicity of Zn2+ and consequently reduced the EC50 value to 0.29 ± 0.003 mg/L. However, the toxicity of the Zn2+/TiO2 system decreased with increasing nano-TiO2 concentration because of the substantial adsorption of Zn2+ by nano-TiO2. The toxicity curve of the Zn2+/TiO2 system as a function of incremental nano-TiO2 concentrations was parabolic. The toxicity significantly increased at the initial stage, reached its maximum, and then decreased with increasing nano-TiO2 concentration. Hydrodynamic sizes, concentration of nano-TiO2 and Zn2+ loaded nano-TiO2 were the main parameters for synergistic toxicity.

Management of oxidative stress by microalgae

The aim of this review is to provide an overview of the current research on oxidative stress in eukaryotic microalgae and the antioxidant compounds microalgae utilize to control oxidative stress. With the potential to exploit microalgae for the large-scale production of antioxidants, interest in how microalgae manage oxidative stress is growing. Microalgae can experience increased levels of oxidative stress and toxicity as a result of environmental conditions, metals, and chemicals. The defence mechanisms for microalgae include antioxidant enzymes such as superoxide dismutase, catalase, peroxidases, and glutathione reductase, as well as non-enzymatic antioxidant molecules such as phytochelatins, pigments, polysaccharides, and polyphenols. Discussed herein are the 3 areas the literature has focused on, including how conditions stress microalgae and how microalgae respond to oxidative stress by managing reactive oxygen species. The third area is how beneficial microalgae antioxidants are when administered to cancerous mammalian cells or to rodents experiencing oxidative stress.