Chronic exposure of the freshwater alga Pseudokirchneriella subcapitata to five oxide nanoparticles: Hazard assessment and cytotoxicity mechanisms

Activation of stress reactions in the dinophyte microalga Prorocentrum cordatum as a consequence of the toxic effect of ZnO nanoparticles and zinc sulfate

According to the results of the experimental study, the main regularities of changes in morphological, structural-functional and fluorescent indices of P. cordatum were established when zinc oxide nanoparticles ZnO NPs (0.3-6.4 mg L-1) and Zn in form of salt (0.09-0.4 mg L-1) were added to the medium. The studied pollutants have cytotoxic (growth inhibition, development of oxidative stress, destruction of cytoplasmic organelles, disorganization of mitochondria) and genotoxic (changes in the morphology of nuclei, chromatin condensation) effects on microalgae, affecting almost all aspects of cell functioning. Despite the similar mechanism of action of zinc sulfate and ZnO NPs on P. cordatum cells, the negative effect of ZnO NPs is also due to the inhibition of photosynthetic activity of cells (significant decrease in the maximum quantum yield of photosynthesis and electron transport rate), reduction of chlorophyll concentration from 3.5 to 1.8 pg cell-1, as well as mechanical effect on cells: deformation and damage of cell membranes, aggregation of NPs on the cell surface. Apoptosis-like signs of cell death upon exposure to zinc sulfate and ZnO NPs were identified by flow cytometry and laser scanning confocal microscopy methods: changes in cell morphology, cytoplasm retraction, development of oxidative stress, deformation of nuclei, and disorganization of mitochondria. It was shown that the first signs of cell apoptosis appear at 0.02 mg L-1 Zn and 0.6 mg L-1 ZnO NPs after 72 h of exposure. At higher concentrations of pollutants, a dose-dependent decrease in algal enzymatic activity (up to 5 times relative to control) and mitochondrial membrane potential (up to 4 times relative to control), and an increase in the production of reactive oxygen species (up to 4-5 times relative to control) were observed. The results of the presented study contribute to the disclosure of fundamental mechanisms of toxic effects of pollutants and prediction of ways of phototrophic microorganisms reaction to this impact.

Single and combined toxic effects of nCu and nSiO2 on Dunaliella salina

There are many studies on the toxic effects of single nanoparticles on microalgae; however, many types of nanoparticles are present in the ocean, and more studies on the combined toxic effects of multiple nanoparticles on microalgae are needed. The single and combined toxic effects of nCu and nSiO2 on Dunaliella salina were investigated through changes in instantaneous fluorescence rate (Ft) and antioxidant parameters during 96-h growth inhibition tests. It was found that the toxic effect of nCu on D. salina was greater than that of nSiO2, and both showed time and were dose-dependent with the greatest growth inhibition at 96 h. A total of 0.5 mg/L nCu somewhat promoted the growth of microalgae, but 4.5 and 5.5 mg/L nCu showed negative growth effects on microalgae. The Ft of D. salina was also inhibited by increasing concentrations of nanoparticles and exposure time. nCu suppressed the synthesis of TP and elevated the MDA content of D. salina, which indicated the lipid peroxidation of algal cells. The activities of SOD and CAT showed a trend of increasing and then decreasing with the increase of nCu concentration, suggesting that the enzyme activity first increased and then decreased. The toxic effect of a high concentration of nCu was reduced after the addition of nSiO2. SEM and EDS images showed that nSiO2 could adsorb nCu in seawater. nSiO2 also adsorbed Cu2+ in the cultures, thus reducing the toxic effect of nCu on D. salina to a certain extent. TEM image was used to observe the morphology of algal cells exposed to nCu.

Features of the microalga Raphidocelis subcapitata: physiology and applications

The microalga Raphidocelis subcapitata was isolated from the Nitelva River (Norway) and subsequently deposited in the collection of the Norwegian Institute of Water Research as "Selenastrum capricornutum Printz". This freshwater microalga, also known as Pseudokirchneriella subcapitata, acquired much of its notoriety due to its high sensitivity to different chemical species, which makes it recommended by different international organizations for the assessment of ecotoxicity. However, outside this scope, R. subcapitata continues to be little explored. This review aims to shed light on a microalga that, despite its popularity, continues to be an "illustrious" unknown in many ways. Therefore, R. subcapitata taxonomy, phylogeny, shape, size/biovolume, cell ultra-structure, and reproduction are reviewed. The nutritional and cultural conditions, chronological aging, and maintenance and preservation of the alga are summarized and critically discussed. Applications of R. subcapitata, such as its use in aquatic toxicology (ecotoxicity assessment and elucidation of adverse toxic outcome pathways) are presented. Furthermore, the latest advances in the use of this alga in biotechnology, namely in the bioremediation of effluents and the production of value-added biomolecules and biofuels, are highlighted. To end, a perspective regarding the future exploitation of R. subcapitata potentialities, in a modern concept of biorefinery, is outlined. KEY POINTS: • An overview of alga phylogeny and physiology is critically reviewed. • Advances in alga nutrition, cultural conditions, and chronological aging are presented. • Its use in aquatic toxicology and biotechnology is highlighted.

Comparative toxicity assessment of individual, binary and ternary mixtures of SiO2, Fe3O4, and ZnO nanoparticles in freshwater microalgae, Scenedesmus obliquus: Exploring the role of dissolved ions

Metal oxide nanoparticles (NPs) are considered among the most prevalent engineered nanomaterials. To have a deeper understanding of the mode of action of multiple metal oxide nanoparticles in mixtures, we have used a unicellular freshwater microalga Scenedesmus obliquus as a model organism. The toxicity of silicon dioxide (SiO2), iron oxide (Fe3O4), and zinc oxide (ZnO) NPs was studied individually as well as in their binary (SiO2 + Fe3O4, Fe3O4 + ZnO, and ZnO + SiO2) and ternary (SiO2 + Fe3O4 + ZnO) combinations. The effects of metal ions from ZnO and Fe3O4 were investigated as well. The results observed from the study, showed that a significant amount of toxicity was contributed by the dissolved ions in the mixtures of the nanoparticles. Decreases in the cell viability, ROS generation, lipid peroxidation, antioxidant enzyme activity, and photosynthetic efficiency were analyzed. Among all the individual particles, ZnO NPs showed the maximum effects and increased the toxicities of the binary mixtures. The binary and ternary mixtures of the NPs clearly showed increased toxic effects in comparison with the individual entities. However, the ternary combination had lesser toxic effects than the binary combination of Fe3O4 + ZnO. The decline in cell viability and photosynthetic efficiency were strongly correlated with various oxidative stress biomarkers emphasizing the crucial role of reactive oxygen species in inducing the toxic effects. The findings from this study highlight the importance of evaluating the combinatorial effects of various metal oxide NPs as part of a comprehensive ecotoxicity assessment in freshwater microalgae.

Ecotoxicity of non- and PEG-modified lanthanide-doped nanoparticles in aquatic organisms

Various types of nanoparticles (NPs) have been widely investigated recently and applied in areas such as industry, the energy sector, and medicine, presenting the risk of their release into the environment. The ecotoxicity of NPs depends on several factors such as their shape and surface chemistry. Polyethylene glycol (PEG) is one of the most often used compounds for functionalisation of NP surfaces, and its presence on the surfaces of NPs may affect their ecotoxicity. Therefore, the present study aimed to assess the influence of PEG modification on the toxicity of NPs. As biological model, we chose freshwater microalgae, a macrophyte and invertebrates, which to a considerable extent enable the assessment of the harmfulness of NPs to freshwater biota. SrF₂:Yb³⁺,Er³⁺ NPs were used to represent the broad group of up-converting NPs, which have been intensively investigated for medical applications. We quantified the effects of the NPs on five freshwater species representing three trophic levels: the green microalgae Raphidocelis subcapitata and Chlorella vulgaris, the macrophyte Lemna minor, the cladoceran Daphnia magna and the cnidarian Hydra viridissima. Overall, H. viridissima was the most sensitive species to NPs, which affected its survival and feeding rate. In this case, PEG-modified NPs were slightly more toxic than bare ones (non-significant results). No effects were observed on the other species exposed to the two NPs at the tested concentrations. The tested NPs were successfully imaged in the body of D. magna using confocal microscopy; both NPs were detected in the D. magna gut. The results obtained reveal that SrF₂:Yb³⁺,Er³⁺ NPs can be toxic to some aquatic species; however, the structures have low toxicity effects for most of the tested species.

Is hydrodynamic diameter the decisive factor? - Comparison of the toxic mechanism of nSiO2 and mPS on marine microalgae Heterosigma akashiwo

To investigate the toxic mechanism of SiO₂ nanoparticles (nSiO₂) and polystyrene microplastics (mPS) on microalgae Heterosigma akashiwo, growth inhibition tests were carried out. The growth and biological responses of the algae exposed to nSiO₂ (0.5, 1, 1.5, 2, 5, 10 and 30 mg L^-1) and mPS (1, 2, 5, 10, 30 and 75 mg L^-1) were explored in f/2 media for 96 h. It was found that the hydrodynamic diameter of the particles seems to be one of the more important factors to influence the algae. nSiO₂ and mPS with similar hydrodynamic diameters have the similar toxic mechanism on H. akashiwo, and the effects were dose- and time-dependent. High concentrations of micro-/nano-particles (MNPs) could inhibit the growth of algal cells, however, low concentrations of MNPs did not restrict or even promoted the growth of algae, known as "Hormesis" phenomenon. The 96 h-EC20 values of nSiO₂ and mPS on H. akashiwo were 2.69 and 10.07 mg L^-1, respectively, and chlorophyll fluorescence parameters indicated that the microalgal photosynthetic system were inhibited. The hydrophilic surface of nSiO₂ increased the likelihood of nSiO₂ binding to the hydrophilic functional group of microalgae, which may account for the slightly stronger toxic effect of nSiO₂ than mPS. The algae continued to produce reactive oxygen species (ROS) under stress conditions. Total protein (TP) levels reduced, and superoxide dismutase (SOD) and catalase (CAT) levels increased to maintain ROS levels in the cells. The decrease in adenosine triphosphate (ATPase) indicated an impact on cellular energy metabolism. Cell membrane damage, cytoplasm and organelle efflux under stress were confirmed by scanning and transmission electron microscopy (SEM and TEM) images. This study contributes to the understanding of the size effect of MNPs on the growth of marine microalgae.

Construction of luminescent Escherichia coli via expressing lux operons and their application on toxicity test

Recombinant luminescent Escherichia coli strains could be used to detect the toxicity of pure or mixed contaminants as a light-off sensor. In this work, the lux operon of Photobacterium phosphoreum T3 was identified for the first time. Recombinant luminescent E. coli strains were constructed via expressing the lux operons of P. phosphoreum T3 and Vibrio qinghaiensis Q67 in E. coli MG1655, and the optimal protectant containing 10% (w/v) trehalose and 4% sucrose was used to prepare the freeze-dried recombinant luminescent E. coli cells. Then, these freeze-dried E. coli cells were subjected to acute toxicity detection. The results showed that luminescent E. coli strains displayed sensitive toxic responses to BPA, nFe₂O₃, Cd, Pb, As, and Hg, for example, the EC₅₀ values of BPA and nFe₂O₃ to luminescent E. coli strains ranged from 1.54 to 50.19 mg/l and 17.50 to 21.52 mg/l, respectively. Indeed, luminescent E. coli strains exhibited more sensitive responses to Cd, Pb, and Hg than the natural strain Q67. The results suggested that recombinant luminescent E. coli strains could be used for the detection of acute toxicity. Furthermore, the combined toxicities of BPA and nFe₂O₃, Hg, and Pb were measured, and the joint effects of these mixtures were evaluated with luminescent E. coli. The results indicated that the joint effects of BPA and nFe₂O₃ suggested to be synergistic or additive to luminescent E. coli, while the joint effects of heavy metals and nFe₂O₃ exhibited additivities. The cellular endocytosis for Fe₂O₃ nanoparticles was not observed, which could explain the additive instead of synergistic effects between heavy metals and nFe₂O₃. KEY POINTS: • Sequence of the lux operon from P. phosphoreum T3 was reported for the first time. • Recombinant luminescent E. coli was more sensitive to Cd, Pb, and Hg than Q67. • Joint effects of BPA and nFe₂O₃ were synergistic or additive to luminescent E. coli.

Toxicity evaluation of TiO2/MWCNT-CNF hybrid nanocomposites with enhanced photocatalytic activity toward freshwater microalgae: Pseudokirchneriella subcapitata

A wide range of semiconductor-assisted photocatalytic nanomaterials (NMs) are currently being considered and investigated as potential photocatalysts in water treatment. The applications of nanocomposites composed of nano-structured titania (nano-TiO₂) and multi-walled carbon nanotubes (MWCNTs) nanocomposites is growing markedly on account of enhanced photocatalytic efficiency. However, concurrent with the increasing production and application comes a serious concern of these emerging nanosystems about their potential risks in aquatic systems, and thereby potentially threatening aquatic organisms via toxic mechanisms that are, at present, poorly understood. In the present study, the lethal toxic effect and oxidative stress induced by TiO₂/MWCNT-CNF nanocomposite in freshwater Pseudokirchneriella subcapitata were assessed. The growth inhibition and sublethal oxidative stress produced by the nanocomposites were evaluated on green microalgae P. subcapitata after 3 days of exposure at 24 h intervals. Moreover, the nanocomposites were physicochemically characterized using a combination of analytical techniques (XRD, SEM/EDS, HRTEM, TGA, UV-Visible spectroscopy). Evaluation of the hybrid for the photocatalytic degradation of Acid Violet 7 dye indicated an enhanced dye removal performance for TiO₂/MWCNT-CNF (96.2%) compared to TiO₂ (75.2%) after 2 h of visible light irradiation. While the nanocomposite showed good potential for the degradation of the azo dye, overall, the findings herein indicated that acute exposure of P. subcapitata to various concentrations of TiO₂/MWCNT-CNF nanocomposite may cause algal growth inhibition including undesirable sublethal oxidative stress effects. The findings of this study contribute to a better understanding of the potential hazards of the developing nanocomposites materials towards the nano-bioremediation materials to treat wastewaters.

Ecotoxicity of silica nanoparticles in aquatic organisms: An updated review

This review aims to (i) provide a current overview of the main characteristics of SiNP (physical and chemical properties, applications, and emissions), (ii) evaluate the scientific production up to date concerning SiNP, with focus on their toxic effects, through a bibliometric analysis, (iii) describe the main toxic mechanisms of SiNP, (iv) assess the current knowledge about ecotoxicity of SiNP on aquatic organisms (marine and freshwater), and (v) identify the main gaps in the knowledge of SiNP toxicity from an environmentally point of view. The scientific production of SiNP concerning their chemical and physical characteristics has increased exponentially. However, little information is available regarding their ecotoxicity. Particle functionalization is a key factor that reduces SiNP toxicity. Most of the studies employed standard species as test organisms, being the local/native ones poorly represented. Further studies employing long-term exposures and environmentally relevant concentrations are needed to deepen the knowledge about this emergent pollutant.

Rapid In Situ Biomonitoring of Subsoil Contamination by Applying an Algae-Soaked Disc Seeding Assay

Various pollutants are pervasive in soil environments due to human activities, thereby damaging soil ecosystems. In this study, extension of a previously developed algae-soaked disc seeding assay for periodic evaluation of subsoil contamination over time was described. The assay can be used in different contamination configurations of silver nanoparticles in combination with examination of cell morphology, esterase activity, oxidative stress, and membrane permeability. In addition, we periodically attempted to repeat the algae-soaked disc seeding assay every three weeks. We evaluated applicability of this algae-soaked disc seeding assay using alga Chlamydomonas reinhardtii exposed to heterogeneous silver nanoparticle-contaminated soils. The results demonstrated that this assay is applicable for monitoring a change of subsoil contamination by periodic evaluation over time. The developed assay was identified as a periodically rapid in situ biomonitoring technique to measure subsoil contamination over time.

Influence of polystyrene microplastic and nanoplastic on copper toxicity in two freshwater microalgae

There has been increasing concern over the toxic effects of microplastics (MP), nanoplastics (NP), and copper (Cu) on microalgae. However, the combined toxicity of the metal in the presence of polystyrene (PS) MP/NP on microalgae has not been well studied, particularly after long-term exposure (i.e., longer than 4 days). The primary aim of the present study was to investigate the effect of PS MP and NP on Cu toxicity on two freshwater microalgae, namely Chlorella sp. TJ6-5 and Pseudokirchneriella subcapitata NIES-35 after acute exposure for 4 days and up to 16 days. The results showed that both microalgae were sensitive to Cu, but tolerant to MP/NP. However, MP/NP increased the toxicity of Cu at EC50 in both microalgae, which was only noticeable in chronic exposure. Single and combined treatment of MP/NP and Cu induced higher oxidative stress and caused morphological and ultrastructural changes in both microalgae. The adsorption of Cu to MP and NP was low (0.23-14.9%), with most of the Cu present in free ionic form (81.6-105.8%). The findings on different sensitivity of microalgae to Cu in the presence of MP/NP may have significant implication as microalgae are likely to be exposed to a mixture of both MP/NP and Cu in the environment. For example, in air-blasting technology, MP and NP are used as abrasive medium to remove Cu-containing antifouling paints on hulls of ship and submerged surfaces. Wastewater treatment plants receive household wastes containing MP and NP, as well as stormwater runoffs and industrial wastes contaminated with heavy metals.

Harmful effects of metal(loid) oxide nanoparticles

Abstract

The incorporation of nanomaterials (NMs), including metal(loid) oxide (MOx) nanoparticles (NPs), in the most diversified consumer products, has grown enormously in recent decades. Consequently, the contact between humans and these materials increased, as well as their presence in the environment. This fact has raised concerns and uncertainties about the possible risks of NMs to human health and the adverse effects on the environment. These concerns underline the need and importance of assessing its nanosecurity. The present review focuses on the main mechanisms underlying the MOx NPs toxicity, illustrated with different biological models: release of toxic ions, cellular uptake of NPs, oxidative stress, shading effect on photosynthetic microorganisms, physical restrain and damage of cell wall. Additionally, the biological models used to evaluate the potential hazardous of nanomaterials are briefly presented, with particular emphasis on the yeast Saccharomyces cerevisiae, as an alternative model in nanotoxicology. An overview containing recent scientific advances on cellular responses (toxic symptoms exhibited by yeasts) resulting from the interaction with MOx NPs (inhibition of cell proliferation, cell wall damage, alteration of function and morphology of organelles, presence of oxidative stress bio-indicators, gene expression changes, genotoxicity and cell dead) is critically presented. The elucidation of the toxic modes of action of MOx NPs in yeast cells can be very useful in providing additional clues about the impact of NPs on the physiology and metabolism of the eukaryotic cell. Current and future trends of MOx NPs toxicity, regarding their possible impacts on the environment and human health, are discussed.

Key points

• The potential hazardous effects of MOx NPs are critically reviewed.

• An overview of the main mechanisms associated with MOx NPs toxicity is presented.

• Scientific advances about yeast cell responses to MOx NPs are updated and discussed.

Mathematical Modeling Approaches for Assessing the Joint Toxicity of Chemical Mixtures Based on Luminescent Bacteria: A Systematic Review

Developments in industrial applications inevitably accelerate the discharge of enormous substances into the environment, whereas multi-component mixtures commonly cause joint toxicity which is distinct from the simple sum of independent effect. Thus, ecotoxicological assessment, by luminescent bioassays has recently brought increasing attention to overcome the environmental risks. Based on the above viewpoint, this review included a brief introduction to the occurrence and characteristics of toxic bioassay based on the luminescent bacteria. In order to assess the environmental risk of mixtures, a series of models for the prediction of the joint effect of multi-component mixtures have been summarized and discussed in-depth. Among them, Quantitative Structure-Activity Relationship (QSAR) method which was widely applied in silico has been described in detail. Furthermore, the reported potential mechanisms of joint toxicity on the luminescent bacteria were also overviewed, including the Trojan-horse type mechanism, funnel hypothesis, and fishing hypothesis. The future perspectives toward the development and application of toxicity assessment based on luminescent bacteria were proposed.

Synthesis, Characterization, and Anti-Algal Activity of Molybdenum-Doped Metal Oxides

In this study, we attempted to synthesize visible light active nano-sized photocatalysts using metal oxides such as zinc oxide, zirconium oxide, tungsten oxide, and strontium titanium oxide with (MoCl5)2 as a dopant by the simple ball-milling method. Fourier-transform infrared spectroscopy data confirmed the presence of M-O-Mo linkage (M = Zn, Zr, W, and SrTi) in all the molybdenum-doped metal oxides (MoMOs), but only MoZnO inhibited the growth of the bloom-forming Microcystis aeruginosa under visible light in a concentration-dependent manner up to 10 mg/L. Further, structural characterization of MoZnO using FESEM and XRD exhibited the formation of typical hexagonal wurtzite nanocrystals of approximately 4 nm. Hydroxyl radical (·OH), reactive oxygen species (ROS), and lipid peroxidation assays revealed ·OH generated by MoZnO under the visible light seemed to cause peroxidation of the lipid membrane of M. aeruginosa, which led to an upsurge of intracellular ROS and consequently introduced the agglomeration of cyanobacteria. These results demonstrated that nano-sized MoZnO photocatalyst can be easily synthesized in a cost-effective ball-mill method and utilized for biological applications such as the reduction of harmful algal blooms. Further, our study implies that a simple ball-milling method can provide an easy, green, and scalable route for the synthesis of visible light active doped metal oxides.

In vivo toxicities of nine engineered nano metal oxides to the marine diatom Skeletonema costatum and rotifer Brachionus koreanus

This study compared in vivo acute toxicities of nine engineered nano metal oxides to the marine diatom Skeletonema costatum and rotifer Brachionus koreanus. The sequence of their toxicities to S. costatum, based on growth inhibition, was: nano zinc oxide (nZnO) > nTiO₂ (rutile) > nMgO > Annealed nMgO > nTiO₂ (anatase) > γ-nAl₂O₃ > nIn₂O₃ > α-nAl₂O₃ > nSnO₂. Similarly, nZnO was also the most toxic to B. koreanus, but the other nano metal oxides were non-lethal. nMgO and nZnO were confirmed to trigger reactive oxygen species (ROS) mediated toxicity to the two marine organisms, while nTiO₂ (both anatase and rutile forms) likely induced oxidative stress as shown by their acellular ROS production. nZnO may also cause damage in the endocrine system of B. koreanus, as indicated by the increased transcription of retinoid X receptor. Annealed nMgO reduces its toxicity via removal of O₂^- and impurities from its surface.