Uptake and effects of cerium(III) and cerium oxide nanoparticles to Chlamydomonas reinhardtii

Settling selection of Chlamydomonas reinhardtii for samarium uptake

Samarium (Sm) is a rare-earth element recently included in the list of critical elements due to its vital role in emerging new technologies. With an increasing demand for Sm, microbial bioremediation may provide a cost-effective and a more ecologically responsible alternative to remove and recover Sm. We capitalized on a previously selected Chlamydomonas reinhardtii strain tolerant to Sm (1.33 × 10-4 M) and acidic pH and carried out settling selection to increase the Sm uptake performance. We observed a rapid response to selection in terms of cellular phenotype. Cellular size decreased and circularity increased in a stepwise manner with every cycle of selection. After four cycles of selection, the derived CSm4 strain was significantly smaller and was capable of sequestrating 41% more Sm per cell (1.7 × 10-05 ± 1.7 × 10-06 ng) and twice as much Sm in terms of wet biomass (4.0 ± 0.4 mg Sm · g-1) compared to the ancestral candidate strain. The majority (~70%) of the Sm was bioaccumulated intracellularly, near acidocalcisomes or autophagic vacuoles as per TEM-EDX microanalyses. However, Sm analyses suggest a stronger response toward bioabsorption resulting from settling selection. Despite working with Sm and pH-tolerant strains, we observed an effect on fitness and photosynthesis inhibition when the strains were grown with Sm. Our results clearly show that phenotypic selection, such as settling selection, can significantly enhance Sm uptake. Laboratory selection of microalgae for rare-earth metal bioaccumulation and sorption can be a promising biotechnological approach.

Humic acid-mediated reduction in toxicity of Co3O4 NPs towards freshwater and marine microalgae in surfactant mixed medium

The ever-increasing applications of Co3O4 nanoparticles (NPs) have posed a serious concern about their discharge in the aquatic environment and ecotoxic implications. Being toxic towards aquatic species, the impact of other aquatic components such as dissolved organic matter (DOM), salinity, and surfactants are not studied sufficiently for their effect on the stability and ecotoxicity of Co3O4 NPs. The present study aims at the influence of humic acid (HA) on the toxicity of Co3O4 NPs in freshwater (C. minutissima) and marine (T. suecica) microalgae under surfactants mixed medium. The measure of % reduction in biomass and photosynthetic pigment were used as toxicity endpoints. Among various tested concentrations of HA, 25 mg/L HA was found suitable to minimize the NP's toxicity with or without the presence of surfactants. Co3O4 NPs mediated reduction in biomass of C. minutissima was significantly minimized by the cumulative effect of HA with T80 (51.68 ± 4.55%) followed by CTAB (46.23 ± 5.62%) and SDS (42.60 ± 2.46%). Similarly, HA with T80 (26.93 ± 6.38%) followed by SDS (17.02 ± 6.64%) and CTAB (13.01 ± 3.81%) were found to minimize the growth inhibitory effect of Co3O4 NPs in T. suecica. The estimation of chlorophyll - a content also indicated that microalgae treated with HA could maintain their photosynthetic ability more than control even in the co-presence of surfactants. Also, the reduced toxicity of Co3O4 NPs were attributed to an increase in hydrodynamic sizes of HA-treated Co3O4 NPs in both marine media (f/2) and freshwater media (BG11) due to increased aggregation and faster sedimentation of Co3O4 NPs.

Nanoplastics increase algal absorption and toxicity of Cd through alterations in cell wall structure and composition

Nanoplastics (NPs) may act as carriers of heavy metals and cause complex toxicity to aquatic organisms, while the exact role of NPs in the joint toxicity remains unclear. Here, we investigated the joint toxicity of polystyrene NPs (PS-NPs) and Cd to freshwater algae (Chlorella vulgaris). It was found that PS-NPs (1 mg L-1) could hardly enter algal cells and slightly inhibit algal growth (p < 0.01). The effect of PS-NPs as carriers on the joint toxicity of PS-NPs and heavy metals could be neglected because of the limited adsorption of Cd by PS-NPs, while the PS-NPs altered the cell wall structure and composition, which resulted in the increased algal absorption and toxicity of Cd. Compared to the low dose Cd (0.4 mg L-1) treatment alone, the extracellular and intracellular Cd contents in the cotreatment were significantly increased by 27.3 % and 18.0 %, respectively, due to the increased contents of cell wall polysaccharides (pectin and hemicellulose in particular) by the PS-NPs. Furthermore, after the high dose Cd (2 mg L-1) exposure, the inhibited polysaccharide biosynthesis and the loosen cell wall structure weakened the tolerance of cell wall to abiotic stress, facilitating the entry of PS-NPs into the algal cells and inducing the higher toxicity. These results elucidate the mechanism by which NPs enhance heavy metal toxicity to algae, providing a novel insight into environmental risks of NPs.

Electrostatics Control Nanoparticle Interactions with Model and Native Cell Walls of Plants and Algae

A lack of mechanistic understanding of nanomaterial interactions with plants and algae cell walls limits the advancement of nanotechnology-based tools for sustainable agriculture. We systematically investigated the influence of nanoparticle charge on the interactions with model cell wall surfaces built with cellulose or pectin and performed a comparative analysis with native cell walls of Arabidopsis plants and green algae (Choleochaete). The high affinity of positively charged carbon dots (CDs) (46.0 ± 3.3 mV, 4.3 ± 1.5 nm) to both model and native cell walls was dominated by the strong ionic bonding between the surface amine groups of CDs and the carboxyl groups of pectin. In contrast, these CDs formed weaker hydrogen bonding with the hydroxyl groups of cellulose model surfaces. The CDs of similar size with negative (-46.2 ± 1.1 mV, 6.6 ± 3.8 nm) or neutral (-8.6 ± 1.3 mV, 4.3 ± 1.9 nm) ζ-potentials exhibited negligible interactions with cell walls. Real-time monitoring of CD interactions with model pectin cell walls indicated higher absorption efficiency (3.4 ± 1.3 10-9) and acoustic mass density (313.3 ± 63.3 ng cm-2) for the positively charged CDs than negative and neutral counterparts (p < 0.001 and p < 0.01, respectively). The surface charge density of the positively charged CDs significantly enhanced these electrostatic interactions with cell walls, pointing to approaches to control nanoparticle binding to plant biosurfaces. Ca2+-induced cross-linking of pectin affected the initial absorption efficiency of the positively charged CD on cell wall surfaces (∼3.75 times lower) but not the accumulation of the nanoparticles on cell wall surfaces. This study developed model biosurfaces for elucidating fundamental interactions of nanomaterials with cell walls, a main barrier for nanomaterial translocation in plants and algae in the environment, and for the advancement of nanoenabled agriculture with a reduced environmental impact.

Mineralization of Few-Layer Graphene Made It Bioavailable in Chlamydomonas reinhardtii

Numerous studies have emphasized the toxicity of graphene-based nanomaterials to algae, however, the fundamental behavior and processes of graphene in biological hosts, including its transportation, metabolization, and bioavailability, are still not well understood. As photosynthetic organisms, algae are key contributors to carbon fixation and may play an important role in the fate of graphene. This study investigated the biological fate of 14C-labeled few-layer graphene (14C-FLG) in Chlamydomonas reinhardtii (C. reinhardtii). The results showed that 14C-FLG was taken up by C. reinhardtii and then translocated into its chloroplast. Metabolomic analysis revealed that 14C-FLG altered the metabolic profiles (including sugar metabolism, fatty acid, and tricarboxylic acid cycle) of C. reinhardtii, which promoted the photosynthesis of C. reinhardtii and then enhanced their growth. More importantly, the internalized 14C-FLG was metabolized into 14CO2, which was then used to participate in the metabolic processes required for life. Approximately 61.63%, 25.31%, and 13.06% of the total radioactivity (from 14CO2) was detected in carbohydrates, lipids, and proteins of algae, respectively. Overall, these results reveal the role of algae in the fate of graphene and highlight the potential of available graphene in bringing biological effects to algae, which helps to better assess the environmental risks of graphene.

Effect of Exposure Concentration and Growth Conditions on the Association of Cerium Oxide Nanoparticles with Green Algae

The increasing release of engineered nanoparticles (NPs) into aquatic ecosystems makes it crucial to understand the interactions of NPs with aquatic organisms, such as algae. In this study, the association of CeO2 NPs with unicellular algae (Raphidocelis subcapitata) and changes to the cellular elemental profile were investigated using three exposure concentrations (1, 50, and 1000 µg CeO2/L) at two different algal growth conditions-exponential and inhibited growth (1% glutaraldehyde). After a 24 h-exposure, algal suspensions were settled by gravity and CeO2-NP/algae association was analyzed by single-cell inductively coupled plasma quadrupole mass spectrometry (sc-ICP-QMS) and ICP time-of-flight MS (sc-ICP-TOFMS). Concurrent detection of the cellular fingerprint with cerium indicated NP association with algae (adsorption/uptake) and changes in the cellular elemental profiles. Less than 5% of cells were associated with NPs when exposed to 1 µg/L. For 50 µg/L exposures in growing and inhibited cell treatments, 4% and 16% of cells were associated with CeO2 NPs, respectively. ICP-TOFMS analysis made it possible to exclude cellular exudates associated with CeO2 NPs due to the cellular fingerprint. Growing and inhibited cells had different elemental profile changes following exposure to CeO2 NPs-e.g., growing cells had higher Mg and lower P contents independent of CeO2 concentration compared to inhibited cells.

Comparing the toxicity of tungsten and vanadium oxide nanoparticles on Spirulina platensis

The production and release of nanoparticles and their impacts on living organisms are among the most important concerns in the world. Spirulina platensis was chosen because of its ability to absorb more elements than other algae. Therefore, an experiment was conducted to improve the product quality of spirulina exposed to new type of nanoparticles. In this experiment, vanadium oxide nanoparticles (VNPs) and tungsten oxide nanoparticles (WNPs) were used at concentrations of 0, 0.001, 0.017, and 0.05 g/l. The measured indices such as protein percentage and concentrations of phycobiliproteins and carbohydrates were the most important parameters of spirulina. Results showed that the concentration of 0.001 g/l of VNPs significantly affected the amounts of protein and phycocyanin. It has also been observed that 0.001 g/l of WNPs significantly influenced the amounts of protein (5.3%) and phycocyanin (90%); however, WNPs at all concentrations increased the concentrations of protein and phycocyanin. A concentration of 0.05 g/l of WNPs increased phycocyanin content by 83% over the control. The examination of nanoparticles by spirulina showed that VNPs were more adsorbed by spirulina than WNPs. In general, VNPs were toxic to algae at concentrations of 0.017 and 0.05 g/l, but WNPs did not show any fatal toxicity.

A comprehensive study of selenium and cerium oxide nanoparticles on mung bean: Individual and synergistic effect on photosynthesis pigments, antioxidants, and dry matter accumulation

In this study, the interaction effects of CeO₂ NPs (250, 500 and 1000 mg L^-1) and Se NPs (25, 50 and 75 mg L^-1) were evaluated in mung bean (Vigna radiata). Single NPs and their combinations were foliar applied to 45-day old mung bean plants under greenhouse conditions. In each pot, a total volume of 100 mL of NPs suspension was sprayed on the plants shoot in two steps and one-week interval. After 94 days of growth, membrane degradation, antioxidant activity, photosynthetic pigments, and dry matter accumulation were assessed. At 250 and 500 mg CeO₂-NPs L^-1, there was partial increase of dry matter, stimulated activity of antioxidant enzymes (p ≤ 0.05), and reactive oxygen species (ROS). However, at 1000 mg L^-1, CeO₂-NPs caused strong accumulation of ROS (p ≤ 0.05), enlargement of starch granules and swelling of chloroplasts. In addition, at such concentration, there was accumulation of starch granules, reduction of photosynthetic pigments, biological nitrogen fixation, chlorosis, and a significant retardation in plant growth, compared with control, (p ≤ 0.05). Combination of Se-NPs (25 and 50 mg L^-1) with 250 mg L^-1 of CeO₂ NPs decreased hydrogen peroxide, improved CAT, Chla, Chlb, and increased dry matter (p ≤ 0.05). At 1000 mg CeO₂ NPs L^-1, foliar spray of Se-NPs led to Ce accumulation in the cell wall and increased levels of SOD and proline (p ≤ 0.05). Results showed that 25 and 50 mg Se NPs L^-1 ameliorate the stress of CeO₂ NPs by upregulating photosynthesis pigments, antioxidants, and dry matter accumulation. Therefore, depending on the CeO₂ NPs concentration, the mechanisms of Se NPs in modulating CeO₂ NPs stress varied; low concentrations of Se NPs may strengthen the metabolism of legumes, and protect them against foliar toxicity of CeO₂ NPs in semi-arid ecosystems.

Toxicity mechanism of cerium oxide nanoparticles on cyanobacteria Microcystis aeruginosa and their ecological risks

The extensive application of cerium oxide nanoparticles (CeO₂ NPs), a type of rare earth nanomaterial, led to pollution into aquatic environments. Cyanobacteria, a significant component of freshwater ecosystems, can interact with CeO₂ NPs. However, little attention has been paid as to whether CeO₂ NPs will have adverse effects on cyanobacteria. In the present study, Microcystis aeruginosa (FACHB-942) was exposed to different concentrations (0, 1, 10, and 50 mg/L) of CeO₂ NPs. Results showed 50 mg/L CeO₂ NPs inhibited algal growth (11.48% ± 5.76%), suppressed photosynthesis and induced the generation of reactive oxygen species (ROS) after 72 h exposure. The toxicity mechanism is the adsorption of CeO₂ NPs on cell surface, the ROS formation and the intracellular Ce. Additionally, the intracellular microcystins (MCs) content was significantly induced (11.84% ± 1.47%) by 50 mg/L CeO₂ NPs, while no significance was found in 1 and 10 mg/L CeO₂ NP treatments. Results indicated high concentrations of CeO₂ NPs could be toxic to algae through the adverse effects on algal growth and photosynthesis. Moreover, the promoted MCs production could also pose a threat to freshwater ecosystems due to the possible release into the environment.

Effects of humic substances on the aqueous stability of cerium dioxide nanoparticles and their toxicity to aquatic organisms

The impacts of humic substances (HS) on the aquatic stability and toxicity of nano‑cerium dioxide (nCeO₂) to three organisms with different exposure characteristics were investigated. Addition of HS to suspensions of nCeO₂ lowered the surface zeta potential of the particles, reduced their hydrodynamic size, and increased the energy barrier as indicated by the total potential energy profile. This resulted in a more stable suspension compared to suspensions without HS added. Moreover, a higher concentration of HS further stabilized nCeO₂ in the suspension. Acute toxicity of the suspensions to the unicellular green alga Raphidocelis subcapitata and to the crustacean Chydorus sphaericus was lower as compared to exposure without HS added. The acute toxicity of nCeO₂ suspensions to the zebrafish (Danio rerio) eleutheroembryo was on the other hand significantly enhanced (additive and synergistic) upon increasing HS concentration. Our findings emphasize that HS is important to stabilize the nano-suspensions and that its impact on nCeO₂ toxicity differs across different aquatic organisms. Emphasizing the exposure characteristics of each of the organisms selected from the trophic levels can explain how particle stability impacts particle toxicity.

Influence of extracellular polymeric substance on the interaction between titanium dioxide nanoparticles and Chlorella pyrenoidosa cells

The presence of extracellular polymeric substance (EPS) plays a vital role in the accumulation and toxicity of nanoparticles to microorganisms, in which the involved processes and mechanisms are still waiting to be revealed. Herein, we specifically investigated the interfacial interaction between titanium dioxide nanoparticles (nTiO₂) and algae (Chlorella pyrenoidosa) with/without EPS and the effect of EPS on algal cell internalization of nTiO₂. Results showed that the presence of EPS on cell surface promoted heteroaggregation between nTiO₂ and algal cells, and induced more nTiO₂ accumulation on algal surface; however, algal cell internalization of nTiO₂ was limited by the presence of EPS. Pearson correlation analysis further proved that the presence of EPS had a positive effect on the surface accumulation of nTiO₂ and a negative effect on the internalization of nTiO₂. More than 60% of cell internalized nTiO₂ entered algal cells through the energy dependent endocytosis pathway. It is interesting to find that anatase nTiO₂ (nTiO₂-A) entered algal cells mainly through the clathrin dependent endocytosis, while rutile nTiO₂ (nTiO₂-R) mainly through the dynamin dependent endocytosis. This difference could be due to the different affinities of nTiO₂-A and nTiO₂-R to the mediating receptors referring to different endocytic pathways. The removal of EPS activated the associated mediating pathways, allowing more nTiO₂ to be internalized. These findings address the role of EPS on the interaction between nTiO₂ and algae and promote a deeper understanding of the ecological effect of nTiO₂.

Fluctuation analysis to select for Samarium bio-uptaking microalgae clones the repurposing of a classical evolution experiment

Rare Earth Elements (REE) increasing demand prompts the research of biotechnological approaches to exploit secondary resources. We made use of the adapted Fluctuation analyses experiment to obtain Chlamydomonas reinhardtii ChlA strains resistant to Samarium (Sm) as the reference REE. The starting hypothesis was that adaptation to metal-containing media leads to an enhanced metal uptake. ChlA was able to adapt to 1.33·10^-4 Sm M and pH~3 by pre-existing genetic variability, allowing the evolutionary rescue of 13 of the 99 populations studied. The rescuing resistant genotypes presented a mutation rate of 8.65·10^-7 resistant cells per division. The resulting resistant population contradicted the expected fitness cost associated with the adaptation to Sm, selection resulted in larger and faster-growing resistant cells. Among the three isolated strains studied for Sm uptake, only one presented uplifted performance compared to the control population (46.64 μg Sm g-¹ of wet biomass and 3.26·10^-7 ng Sm per cell, mainly bioaccumulated within the cells). The selection of microalgae strains with improved tolerance to REEs by this methodology could be a promising solution for REES sequestration. However, increased tolerance can be independent or have negative effects on uptake performance and cellular features studied are not directly correlated with the metal uptake. SUMMARY SENTENCE: Repurposing a classic laboratory evolution experiment to select for microalgae Samarium adapted strains for metals recovery and biotechnology approaches. DATA AVAILABILITY STATEMENT: All data generated or analyzed during this study are included in this published article (and its raw files).

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.

Accumulation and cellular toxicity of engineered metallic nanoparticle in freshwater microalgae: Current status and future challenges

Metal nanoparticles (MNPs) are employed in a variety of medical and non-medical applications. Over the past two decades, there has been substantial research on the impact of metallic nanoparticles on algae and cyanobacteria, which are at the base of aquatic food webs. In this review, the current status of our understanding of mechanisms of uptake and toxicity of MNPs and metal ions released from MNPs after dissolution in the surrounding environment were discussed. Also, the trophic transfer of MNPs in aquatic food webs was analyzed in this review. Approximately all metallic nanoparticles cause toxicity in algae. Predominantly, MNPs are less toxic compared to their corresponding metal ions. There is a sufficient evidence for the trophic transfer of MNPs in aquatic food webs. Internalization of MNPs is indisputable in algae, however, mechanisms of their transmembrane transport are inadequately known. Most of the toxicity studies are carried out with solitary species of MNPs under laboratory conditions rarely found in natural ecosystems. Oxidative stress is the primary toxicity mechanism of MNPs, however, oxidative stress seems a general response predictable to other abiotic stresses. MNP-specific toxicity in an algal cell is yet unknown. Lastly, the mechanism of MNP internalization, toxicity, and excretion in algae needs to be understood carefully for the risk assessment of MNPs to aquatic biota.

Microalgal ecotoxicity of nanoparticles: An updated review

Nowadays, nanotechnology and its related industries are becoming a rapidly explosive industry that offers many benefits to human life. However, along with the increased production and use of nanoparticles (NPs), their presence in the environment creates a high risk of increasing toxic effects on aquatic organisms. Therefore, a large number of studies focusing on the toxicity of these NPs to the aquatic organisms are carried out which used algal species as a common biological model. In this review, the influences of the physio-chemical properties of NPs and the response mechanisms of the algae on the toxicity of the NPs were discussed focusing on the "assay" studies. Besides, the specific algal toxicities of each type of NPs along with the NP-induced changes in algal cells of these NPs are also assessed. Almost all commonly-used NPs exhibit algal toxicity. Although the algae have similarities in the symptoms under NP exposure, the sensitivity and variability of each algae species to the inherent properties of each NPs are quite different. They depend strongly on the concentration, size, characteristics of NPs, and biochemical nature of algae. Through the assessment, the review identifies several gaps that need to be further studied to make an explicit understanding. The findings in the majority of studies are mostly in laboratory conditions and there are still uncertainties and contradictory/inconsistent results about the behavioral effects of NPs under field conditions. Besides, there remains unsureness about NP-uptake pathways of microalgae. Finally, the toxicity mechanisms of NPs need to be thoughtfully understood which is essential in risk assessment.

Growth and Photosynthetic Inhibition of Cerium Oxide Nanoparticles on Soybean (Glycine max)

Cerium oxide nanoparticles (CeO2 NPs) are widely used in industries and have caused environmental problems. However, the phytotoxicity induced by CeO2 NPs lacks detailed information on phytotoxicity. In this research, the effect of CeO2 NPs on soybean plants (Glycine max) was studied. Scanning electron microscopy with the energy dispersion spectroscopy was used to characterize the NPs form in soybean. The growth of the root was increased, whereas the growth of shoot was inhibited. Besides, Chlorophyll Fluorescence Imager (CF Imager) showed that chlorophyll synthesis was inhibited: the maximum quantum yield of Photosystem II complex (PSII) (Fv/Fm) and photochemical quenching (qP) decreased. Moreover, transmission electron microscopy revealed that the chloroplast thylakoid structure was changed, and thus reduced the energy conversion in the Calvin cycle from C5 to C3. Our work suggests that CeO2 NPs will cause growth changes as well as irreversible damage to soybean plants. Our findings will provide evidence for estimation of plant toxicity induced by CeO2 NPs.

Potential Hazard of Lanthanides and Lanthanide-Based Nanoparticles to Aquatic Ecosystems: Data Gaps, Challenges and Future Research Needs Derived from Bibliometric Analysis

Lanthanides (Ln), applied mostly in the form of nanoparticles (NPs), are critical to emerging high-tech and green energy industries due to their distinct physicochemical properties. The resulting anthropogenic input of Ln and Ln-based NPs into aquatic environment might create a problem of emerging contaminants. Thus, information on the biological effects of Ln and Ln-based NPs is urgently needed for relevant environmental risk assessment. In this mini-review, we made a bibliometric survey on existing scientific literature with the main aim of identifying the most important data gaps on Ln and Ln-based nanoparticles' toxicity to aquatic biota. We report that the most studied Ln for ecotoxicity are Ce and Ln, whereas practically no information was found for Nd, Tb, Tm, and Yb. We also discuss the challenges of the research on Ln ecotoxicity, such as relevance of nominal versus bioavailable concentrations of Ln, and point out future research needs (long-term toxicity to aquatic biota and toxic effects of Ln to bottom-dwelling species).

Size-dependent toxicity of ThO2 nanoparticles to green algae Chlorella pyrenoidosa

Thorium (Th) is a natural radioactive element present in the environment and has the potential to be used as a clean nuclear fuel. Relatively little is known about the aquatic toxicity of Th, especially in nanoparticulate form, which may be the main chemical species of Th in the natural waters. In this study, impacts of ThO₂ nanoparticles (NPs) with two different sizes (52 ± 5 nm, s-ThO₂vs. 141 ± 6 nm, b-ThO₂) on a green alga Chlorella pyrenoidosa (C. pyrenoidosa) were evaluated. Results indicated that C. pyrenoidosa was more sensitive to s-ThO₂ (96-h EC₃₀ = 64.1 μM) than b-ThO₂ (96-h EC₃₀ = 100.2 μM). Exposure to 200 μM of ThO₂ NPs reduced the chlorophyll-a and chlorophyll-b contents of the algal cells. At 96 h, SEM and TEM showed that more agglomerates of s-ThO₂ than those of b-ThO₂ were attached onto the surface of algal cells. Reactive oxygen species (ROS) generation and membrane damage were induced after the attachment of high concentrations of ThO₂ NPs. The heteroagglomeration between ThO₂ NPs and algal cells and increased oxidative stress might play important roles in the toxicity of ThO₂ NPs. To the best of our knowledge, this is the first report on aquatic toxicity of ThO₂ NPs.

Effects of CeO2 nanoparticles on bacterial community and molecular ecological network in activated sludge system

The increasing use of cerium oxide nanoparticles (CeO₂ NPs) has caused concerns regarding their potential environmental risks. However, their effects on bacterial communities and network interactions in activated sludge process are still unclear. In this study, we carried out long-term exposure experiments (210 d) to investigate the influence of CeO₂ NPs on wastewater treatment performance, bacterial community structure and network interactions in activated sludge systems. The results showed that long-term exposure to 1 mg/L CeO₂ NPs induced the deterioration of denitrifying process, which was consistent with the inhibition of enzyme activities of nitrite oxidoreductase and nitrate reductase under CeO₂ NPs. CeO₂ NPs decreased the bacterial diversity and altered the overall bacterial community structure in activated sludge. Some dominant denitrifying bacteria, such as Flexibacter and Acinetobacter decreased significantly. Molecular ecological network analysis showed that CeO₂ NPs decreased the network complexity of bacterial community, and probably promoted the competition in bacterial communities of activated sludge. These changes of denitrifying bacteria and the bacterial network may be relevant to the deterioration of denitrifying process. This study provides insights into how the bacteria community and their molecular ecological network respond to CeO₂ NPs in activated sludge systems.