Mechanistic understanding toward the toxicity of graphene-family materials to freshwater algae

Do the joint effects of size, shape and ecocorona influence the attachment and physical eco(cyto)toxicity of nanoparticles to algae?

We systematically investigated how the combinations of size, shape and the natural organic matter (NOM)-ecocorona of gold (Au) engineered nanoparticles (ENPs) influence the attachment of the particles to algae and physical toxicity to the cells. Spherical (10, 60 and 100 nm), urchin-shaped (60 nm), rod-shaped (10 × 45, 40 × 60 and 50 × 100 nm), and wire-shaped (75 × 500, 75 × 3000 and 75 × 6000 nm) citrate-coated and NOM-coated Au-ENPs were used. Among the spherical particles only the spherical 10 nm Au-ENPs caused membrane damage to algae. Only the rod-shaped 10 × 45 nm induced membrane damage among the rod-shaped Au-ENPs. Wire-shaped Au-ENPs caused no membrane damage to the algae. NOM ecocorona decreased the membrane damage effects of spherical 10 nm and rod-shaped 10 × 45 nm ENPs. The spherical Au-ENPs were mostly loosely attached to the cells compared to other shapes, whereas the wire-shaped Au-ENPs were mostly strongly attached compared to particles with other shapes. NOM ecocorona determined the strength of Au-ENPs attachment to the cell wall, leading to the formation of loose rather than strong attachment of Au-ENPs to the cells. After removal of the loosely and strongly attached Au-ENPs, some particles remained anchored to the surface of the algae. The highest concentration was detected for spherical 10 nm Au-ENPs followed by rod-shaped 10 × 45 nm Au-ENPs, while the lowest concentration was observed for the wire-shaped Au-ENPs. The combined effect of shape, size, and ecocorona controls the Au-ENPs attachment and physical toxicity to cells.

Inhibitory Effects of Cu2O/SiO2 on the Growth of Microcystis aeruginosa and Its Mechanism

In this study, a novel nanomaterial Cu₂O/SiO₂ was synthesized based on nano-SiO₂, and the inhibitory effects of different concentrations of Cu₂O/SiO₂ on the growth of Microcystis aeruginosa (M. aeruginosa) were studied. At the same time, the mechanism of Cu₂O/SiO₂ inhibiting the growth of M. aeruginosa was discussed from the aspects of Cu²⁺ release, chlorophyll a destruction, oxidative damage, total protein, and the phycobiliprotein of algae cells. The results showed that low doses of Cu₂O/SiO₂ could promote the growth of M. aeruginosa. When the concentration of Cu₂O/SiO₂ reached 10 mg/L, it exhibited the best inhibitory effect on M. aeruginosa, and the relative inhibition rate reached 294% at 120 h. In terms of the algae inhibition mechanism, Cu₂O/SiO₂ will release Cu²⁺ in the solution and induce metal toxicity to algae cells. At the same time, M. aeruginosa might suffer oxidative damage by the free radicals, such as hydroxyl radicals released from Cu₂O/SiO₂, affecting the physiological characteristics of algae cells. Moreover, after the addition of Cu₂O/SiO₂, a decrease in the content of chlorophyll a, total soluble protein, and phycobiliprotein was found, which eventually led to the death of M. aeruginosa. Therefore, Cu₂O/SiO₂ can be used as an algaecide inhibitor for controlling harmful cyanobacteria blooms.

The effect of water hardness on the toxicity of graphene oxide to bacteria in synthetic surface waters

Graphene oxide (GO), used in a wide variety of applications, is increasingly being introduced into aquatic environments; this situation calls for research on GO toxicity to assess its environmental risks. In this study, the toxic effect of GO to E.coli was studied before and after its aggregation equilibrium in the synthetic surface waters (the soft water, moderately hard water, and hard water) to reveal the effects of GO aggregation and solution hardness. The cytotoxicity of GO increased with increasing solution hardness while decreased after GO aggregation. The 3 h 50% inhibitory concentration (IC₅₀) values of dispersed GO in the soft water, moderately hard water, and hard water were 12.2 ± 2.2, 8.5 ± 1.5, and 4.0 ± 1.0 mg/L, respectively. After 24 h shaking (aggregation equilibrium) in the synthetic surface waters, the dispersed GO aggregated and the 3 h IC₅₀ values of GO aggregates in the three synthetic waters were 40.3 ± 6.9, 15.9 ±  2.2, and 7.5 ± 1.5 mg/L, respectively. The dispersed GO sheets wrapped E. coli cells and cut the cell membrane, resulting in the disruption of cell membrane and the cell inactivation. With increasing water hardness, the heteroaggregation between GO sheets/aggregates and E. coli cells was enhanced, resulting in the increase of toxic effect. The GO aggregates could also entrap E.coli cells while exhibited limited effect on cell membrane disruption without sharp edges, thereby causing the lower toxic effect compared with the dispersed GO sheets. These outcomes shed new light on the assessment of ecological effects of GO.

Dysbiosis of gut microbiota by dietary exposure of three graphene-family materials in zebrafish (Danio rerio)

The increasing application and subsequent mass production of graphene-family materials (GFMs) will lead to greater possibilities for their release into the environment. Although GFMs exhibit toxicity toward various aquatic organisms, little information is available on their influence on gut microbiota of aquatic organism. In this study, zebrafish were fed diets containing three GFMs, namely, monolayer graphene powder (GR), graphene oxide nanosheet (GO) and reduced graphene oxide powder (rGO), or appropriate control for 21 days. The gut bacterial communities were then characterized for comparison of the exposure effects of each GFM. Alterations of the intestinal morphology and oxidative stress indicators were also examined. The results showed GFMs led to different inflammatory responses and significantly altered the relative composition of the gut bacterial species by increasing the relative abundance of Fusobacteria and the genus Cetobacterium and Lactobacillus and decreasing the abundance of Firmicutes and the genus Pseudomonas; GR caused marked shifts in the diversity of the gut microbiota. The GFMs also altered the intestinal morphology and antioxidant enzyme activities by inducing more vacuolation and generating more goblet cells. Our findings demonstrate that GFM exposure poses potential health risks to aquatic organisms through alteration of the gut microbiota.

Interaction of graphene oxide with co-existing arsenite and arsenate: Adsorption, transformation and combined toxicity

The outstanding commercial application potential of graphene oxide (GO) will inevitably lead to its increasing release into the environment, and then affect the environmental behavior and toxicity of conventional pollutants. Interactions between arsenite [As (III)]/arsenate [As (V)] with GO and their combined toxicity to Chlorella pyrenoidosa were investigated. Under abiotic conditions, approximately 42% of the adsorbed As (III) was oxidized by GO with simulated sunlight illumination, which was induced by electron-hole pairs on the surface of GO. Co-exposure with GO greatly enhanced the toxicity of As (III, V) to alga. When adding 10 mg/L GO, the 72 h median effect concentration of As (III) and As (V) to C. pyrendoidosa decreased to 12.7 and 9.4 mg/L from 30.1 and 16.3 mg/L in the As alone treatment, respectively. One possible mechanism by which GO enhanced As toxicity could be that GO decreased the phosphate concentration in the algal medium, and then increased the accumulation of As (V) in algae. In addition, transmission electron microscope (TEM) images demonstrated that GO acted as a carrier for As (III) and As (V) transport into the algal cells. Also, GO induced severe oxidative stress, which could have subsequently compromised important detoxification pathways (e.g., As complexation with glutathione, As methylation, and intracellular As efflux) in the algal cells. Our findings highlight the significant impact of GO on the fate and toxicity of As in the aquatic environment.

Graphene-based materials production and application in textile wastewater treatment: color removal and phytotoxicity using Lactuca sativa as bioindicator

The dyes used in textile industries are usually difficult to degrade in aquatic environments, being highly toxic to micro fauna and flora. Thus, textile wastewater treatments have been developed, among them, one that stands out is adsorption process. With the rise of nanomaterials applied to adsorption, graphene oxide (GO) shows promise in the removal of dyes. This work aimed to produce a more economical and environmentally friendly GO by reducing H2SO4 concentration during the synthesis. Adsorption tests were performed with methylene blue (MB) and brilliant blue (BB), adsorbent regeneration tests, as well as a kinetic study using real wastewater, and toxicological assays with lettuce seeds. Results showed that the sample produced with less H2SO4 (GO-21) performed better for MB (99% removal) and BB (29% removal); and recycling test showed that despite the decrease in removal efficiency, it remained high in the first cycles. Kinetics showed that equilibrium was reached in 30 min, removing 67.43% of color and 90.23% of the effluent's turbidity. Phytotoxicity assays indicated that the wastewater treated with GO-21 was the least toxic, compared to other wastewater samples analyzed. Therefore, GO has demonstrated its potential to be an effective and less toxic option to treat textile effluents.[Formula: see text].

Graphene oxide mediated reduction of silver ions to silver nanoparticles under environmentally relevant conditions: Kinetics and mechanisms

We systematically investigated the reduction mechanisms and reduction kinetics of silver ions (Ag ions) by graphene oxide (GO) under ambient condition. UV-vis spectroscopy, transmission electron microscopy, and electron diffraction results revealed that silver nanoparticles (Ag NPs) could be formed from aqueous Ag ions in the presence of GO at pH 8 under light. Formation of Ag NPs increased with increasing pH (7.4, 8, and 9) and temperature (from 30 to 90); however, the increasing ionic strength and dissolved oxygen reduced the Ag NPs yield. The Ag ions reduction by GO followed pseudo-first-order kinetics under both dark and light, and light irradiation significantly accelerated the Ag NPs formation induced by GO. The phenolic-OH on GO was the dominating electron donator for Ag ion reduction in dark. Exposure to light increased the concentration of phenolic-OH on the GO surface, thereby stimulating the reduction rate of Ag ions by GO. In addition, the light induced electron-hole pairs on GO surface and light activated oxygen-centered radicals on GO surface promoted the reduction of adsorbed Ag ions by GO. Our findings provide important information for the role of GO in reducing Ag ions to Ag NPs in aquatic environments, and shed light on understanding the environmental fate and risk of both Ag ions and GO materials.

Biotoxicity of water-soluble species in PM2.5 using Chlorella

China has been faced with severe haze pollution, which is hazardous to human health. Among the air pollutants, PM_2.5 (particles with an aerodynamic diameter ≤ 2.5 μm) is the most dangerous because of its toxicity and impact on human health and ecosystems. However, there has been limited research on PM_2.5 particle toxicity. In the present study, we collected daily PM_2.5 samples from January 1 to March 31, 2018 and selected samples to extract water-soluble species, including SO₄^2-, NO₃^-, WSOC, and NH₄⁺. These samples represented clean, good, slight, moderate, and heavy pollution days. After extraction using an ultrasonic method, PM_2.5 solutions were obtained. We used Chlorella as the test algae and studied the content of chlorophyll a, as well as the variation in fluorescence when they were placed into the PM_2.5 extraction solution, and their submicroscopic structure was analyzed using transmission electron microscopy (TEM). The results showed that when the air quality was relatively clean and good (PM_2.5 concentration ≤ 75 μg m^-3), the PM_2.5 extraction solutions had no inhibiting effects on Chlorella, whereas when the air quality was polluted (PM_2.5 concentration > 75 μg m^-3) and heavily polluted (PM_2.5 concentration > 150 μg m^-3), with increasing PM_2.5 concentrations and exposure time, the chlorophyll a content in Chlorella decreased. Moreover, the maximum photochemical quantum yield (F_v/F_m) of Chlorella obviously decreased, indicating chlorophyll inhibition during polluted days with increasing PM_2.5 concentrations. The effects on the chlorophyll fluorescence parameters were also obvious, leading to an increase of energy dissipated per unit reaction center (DI_o/RC), suggesting that Chlorella could survive when exposed to PM_2.5 solutions, whereas the physiological activities were significantly inhibited. The TEM analysis showed that there were few effects on Chlorella cell microstructure during clean days, whereas plasmolysis occurred during light- and medium-polluted days. With increasing pollution levels, plasmolysis became more and more apparent, until the organelles inside the cells were thoroughly destroyed and most of the parts could not be recognized.

New insight into the aggregation of graphene oxide in synthetic surface water: Carbonate nanoparticle formation on graphene oxide

Graphene oxide (GO), used in a wide variety of applications, is increasingly being introduced into aquatic environments; this situation calls for research on GO aggregation and sedimentation to regulate the environmental behaviors and risks. Many studies have investigated the aggregation and the mechanism of GO in water with a single background salt (monosalt system); however, this may not reflect real water environments where multiple salts coexist (multisalt system). A typical synthetic surface water (soft water) with representative multisalts was therefore used to study the aggregation and sedimentation of GO. The GO concentration-dependent aggregation (low concentration aggregation, high concentration stability) was observed in the soft water, and this concentration-dependent aggregation is opposite to the aggregation in monosalt systems (NaCl or CaCl₂ solutions). The presence of GO sheets induced the formation of amorphous CaMg(CO₃)₂ nanoparticles on the GO surfaces in the soft water, and the formed nanoparticles promoted the aggregation and sedimentation of low concentrations of GO through bridging action. Neutral and alkaline conditions were favorable for the formation of CaMg(CO₃)₂ nanoparticles and the induced GO aggregation. These findings show a new mechanism of GO aggregation in environmentally relevant waters and help us to better evaluate the environmental fate of GO.

Effects of oxidation degree on photo-transformation and the resulting toxicity of graphene oxide in aqueous environment

Graphene oxide (GO) has been demonstrated to be key component for diverse applications. However, their potential environmental reactivity, fate and risk have not been fully evaluated to date. In this study, we investigated the photochemical reactivity of four types of GO with different oxidation degrees in aqueous environment, and their related toxicity to two bacterial models Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was further compared. After UV-irradiation, a large amount of oxygen functional groups on GO were reduced and the electronic conjugations within GO were restored as indicated by UV-visible absorption spectra, X-ray photoelectron spectroscopy and Raman spectroscopy analysis. Moreover, the higher the oxidation degree of the pristine GO was, the more obvious of the photo-transformation changes were. In order to further reveal the photochemical reactivity mechanisms, the reactive oxygen species (ROS) generation of GO was monitored. The quantity of ROS including singlet oxygen (¹O₂), superoxide anions (O₂^·-), and hydroxyl radicals (·OH) increased with increasing oxidation degree of GO, which was in accordance with the previous characterization results. Scanning electron microscopy and cell growth analyses of E. coli and S. aureus showed that the photochemical transformation enhanced the toxicity of GO, which might be due to an increase in functional group density. The higher conductivity of the reduced graphene oxide (RGO) was responsible for its stronger toxicity than GO through membrane damage and oxidative stress to bacteria. This study revealed that the oxidation degrees play important roles in photochemical transformation and the resulting toxicity of GO, which is helpful for understanding the environmental behaviors and risks of GO in aquatic environments.

Phytotoxicity of Graphene Family Nanomaterials and Its Mechanisms: A Review

Graphene family nanomaterials (GFNs) have experienced significant development in recent years and have been used in many fields. Despite the benefits, they bring to society and the economy, their potential for posing environmental and health risks should also be considered. The increasing release of GFNs into the ecosystem is one of the key environmental problems that humanity is facing. Although most of these nanoparticles are present at low concentrations, many of them raise considerable toxicological concerns, particularly regarding their accumulation in plants and the consequent toxicity introduced at the bottom of the food chain. Here, we review the recent progress in the study of toxicity caused by GFNs to plants, as well as its influencing factors. The phytotoxicity of GFNs is mainly manifested as a delay in seed germination and a severe loss of morphology of the plant seedling. The potential mechanisms of phytotoxicity were summarized. Key mechanisms include physical effects (shading effect, mechanical injury, and physical blockage) and physiological and biochemical effects (enhancement of reactive oxygen species (ROS), generation and inhibition of antioxidant enzyme activities, metabolic disturbances, and inhibition of photosynthesis by reducing the biosynthesis of chlorophyll). In the future, it is necessary to establish a widely accepted phytotoxicity evaluation system for safe manufacture and use of GFNs.

Study of the Persistence of the Phytotoxicity Induced by Graphene Oxide Quantum Dots and of the Specific Molecular Mechanisms by Integrating Omics and Regular Analyses

Although increasing attention has been paid to the nanotoxicity of graphene oxide quantum dots (GOQDs) due to their broad range of applications, the persistence and recoverability associated with GOQDs had been widely ignored. Interestingly, stress-response hormesis for algal growth was observed for Chlorella vulgaris as a single-celled model organism. Few physiological parameters, such as algal density, plasmolysis, and levels of reactive oxygen species, exhibited facile recovery. In contrast, the effects on chlorophyll a levels, permeability, and starch grain accumulation exhibited persistent toxicity. In the exposure stage, the downregulation of genes related to unsaturated fatty acid biosynthesis, carotenoid biosynthesis, phenylpropanoid biosynthesis, and binding contributed to toxic effects on photosynthesis. In the recovery stage, downregulation of genes related to the cis-Golgi network, photosystem I, photosynthetic membrane, and thylakoid was linked to the persistence of toxic effects on photosynthesis. The upregulated galactose metabolism and downregulated aminoacyl-tRNA biosynthesis also indicated toxicity persistence in the recovery stage. The downregulation and upregulation of phenylalanine metabolism in the exposure and recovery stages, respectively, reflected the tolerance of the algae to GOQDs. The present study highlights the importance of studying nanotoxicity by elucidation of stress and recovery patterns with metabolomics and transcriptomics.

The effects of humic acid on the toxicity of graphene oxide to Scenedesmus obliquus and Daphnia magna

The wide production and application of graphene oxide (GO) has inevitably caused its release to the aquatic ecosystem. However, the influence of natural organic matter (NOM) on the toxicity of GO to aquatic organisms needs further investigation. In this study, we conducted several toxicity tests (i.e., acute toxicity and oxidative damage) with Scenedesmus obliquus (S. obliquus) and Daphnia magna (D. magna), as well as a chronic toxicity test with D. magna, to investigate the toxicity of GO with or without the presence of humic acid (HA). Our results showed that GO induced significant toxicity to S. obliquus and D. magna, and the median lethal concentrations (72 h-LC₅₀ and 48 h-LC₅₀) for acute toxicity were 20.6 and 84.2 mg L^-1, respectively, while the 21 d-LC₅₀ for chronic toxicity to D. magna was 3.3 mg L^-1. Additionally, HA mitigated the acute toxicity of GO to S. obliquus and D. magna by 28.6% and 32.3%, respectively, and mitigated the chronic toxicity of GO to D. magna. In the presence of HA, the decreased toxicity of GO was attributed to the alleviation of oxidative damage by HA to both S. obliquus and D. magna, the mitigation of surface envelopment to S. obliquus and the body accumulation in D. magna. Our study provides useful and basic biotoxicity data of GO with a consideration of its interaction with NOM which could aid in preventing an overestimation of the risks of GO to the natural aquatic environment.

Graphene-Based Nanomaterials Toxicity in Fish

Due to their unique physicochemical properties, graphene-based nanoparticles (GPNs) constitute one of the most promising types of nanomaterials used in biomedical research. GPNs have been used as polymeric conduits for nerve regeneration and carriers for targeted drug delivery and in the treatment of cancer via photothermal therapy. Moreover, they have been used as tracers to study the distribution of bioactive compounds used in healthcare. Due to their extensive use, GPN released into the environment would probably pose a threat to living organisms and ultimately to human health. Their accumulation in the aquatic environment creates problems to aquatic habitats as well as to food chains. Until now the potential toxic effects of GPN are not properly understood. Despite agglomeration and long persistence in the environment, GPNs are able to cross the cellular barriers successfully, entered into the cells, and are able to interact with almost all the cellular sites including the plasma membrane, cytoplasmic organelles, and nucleus. Their interaction with DNA creates more potential threats to both the genome and epigenome. In this brief review, we focused on fish, mainly zebrafish (Danio rerio), as a potential target animal of GPN toxicity in the aquatic ecosystem.

Graphene oxide and carbon dots as broad-spectrum antimicrobial agents - a minireview

Due to the increasing global population, growing contamination of water and air, and wide spread of infectious diseases, antibiotics are extensively used as a major antibacterial drug. However, many microbes have developed resistance to antibiotics through mutation over time. As an alternative to antibiotics, antimicrobial nanomaterials have attracted great attention due to their advantageous properties and unique mechanisms of action toward microbes. They inhibit bacterial growth and destroy cells through complex mechanisms, making it difficult for bacteria to develop drug resistance, though some health concerns related to biocompatibility remain for practical applications. Among various antibacterial nanomaterials, carbon-based materials, especially graphene oxide (GO) and carbon dots (C-Dots), are promising candidates due to the ease of production and functionalization, high dispersibility in aqueous media, and promising biocompatibility. The antibacterial properties of these nanomaterials can be easily adjusted by surface modification. They are promising materials for future applications against multidrug-resistant bacteria based on their strong capacity in disruption of microbial membranes. Though many studies have reported excellent antibacterial activity of carbon nanomaterials, their impact on the environment and living organisms is of concern due to the accumulatory and cytotoxic effects. In this review, we discuss antimicrobial applications of the functional carbon nanomaterials (GO and C-Dots), their antibacterial mechanisms, factors affecting antibacterial activity, and concerns regarding cytotoxicity.

Pristine graphene induces cardiovascular defects in zebrafish (Danio rerio) embryogenesis

The multiple effect of pristine graphene (pG) toxicity on cardiovascular developmental defects was assessed using zebrafish as a model. Recently, the nanotoxicity is emerging as a critical issue, and it is more significant in embryogenesis. Especially, graphene and its derivatives have attracted a lot of interest in biomedical applications. However, very little is known about the toxic effects of pG which has been widely used carbon nanomaterial according to concentration and its effects on biological and cardiovascular development. In the present study, we examined the development of zebrafish embryos by exposing to pG (5, 10, 15, 20 and 25 μg/L) under different developmental toxicity end-points such as cardiotoxicity, cardiovascular defect, retardation of cardiac looping, apoptosis and globin expression analysis. For this, the developmental cardiotoxicity of pG at different concentrations and the specific cardiovascular defects thereof were elucidated for the first time. As a result, the exposure to pG was found to be a potential risk factor to cardiovascular system of zebrafish embryos. However, a further study on the variations of physical, molecular properties and mechanisms of nanotoxicity which vary depending on production method and surface functionalization is required. In addition, the potential risks of pG flakes to aquatic organisms and human health should be considered or checked before releasing them to the environment.

Safety Assessment of Graphene-Based Materials: Focus on Human Health and the Environment

Graphene and its derivatives are heralded as "miracle" materials with manifold applications in different sectors of society from electronics to energy storage to medicine. The increasing exploitation of graphene-based materials (GBMs) necessitates a comprehensive evaluation of the potential impact of these materials on human health and the environment. Here, we discuss synthesis and characterization of GBMs as well as human and environmental hazard assessment of GBMs using in vitro and in vivo model systems with the aim to understand the properties that underlie the biological effects of these materials; not all GBMs are alike, and it is essential that we disentangle the structure-activity relationships for this class of materials.

Toxicity of GO to Freshwater Algae in the Presence of Al2O3 Particles with Different Morphologies: Importance of Heteroaggregation

The roles of Al₂O₃ particles with different morphologies in altering graphene oxide (GO) toxicity to Chlorella pyrenoidosa were investigated. Algal growth inhibition by GO with coexisting Al₂O₃ particles was much lower than the sum of inhibitions from the individual materials for all the three Al₂O₃, showing the toxicity mitigation by Al₂O₃. The lowest GO toxicity was observed at the concentrations of 300, 150, and 100 mg/L for Al₂O₃ nanoparticles (NPs, 8-10 nm), bulk particles (BPs, 100-300 nm), and fibers (diameter: 10 nm; length: 400 nm), respectively. GO-Al₂O₃ heteroaggregation was responsible for the observed toxicity reduction. GO-induced algal membrane damage was suppressed by the three types of Al₂O₃ due to GO-Al₂O₃ heteroaggregation, and the reduction in intracellular reactive oxygen species generation and physical contact were confirmed as two main mechanisms. Moreover, the exposure sequence of GO and Al₂O₃ could highly influence the toxicity, and the simultaneous exposure of individual GO and Al₂O₃ showed the lowest toxicity due to minimum direct contact with algal cells. Humic acid further decreased GO-Al₂O₃ toxicity due to enhanced steric hindrance through surface coating of GO-Al₂O₃ heteroaggregates. This work provides new insights into the role of natural mineral particles in altering the environmental risk of GO.

Interpreting the effects of natural organic matter on antimicrobial activity of Ag2S nanoparticles with soft particle theory

Natural organic matter (NOM) ubiquitously exists in natural waters and would adsorb onto the particle surface. Previous studies showed that NOM would alleviate the toxicity of nanomaterials, while the mechanism is seldom quantitatively interpreted. Herein, the effects of humic substances [Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA)] and biomacromolecules [alginate and bovine serum albumin (BSA)] on the aggregation and antimicrobial effects of silver sulfide nanoparticles (Ag₂S-NPs) were investigated. The aggregation kinetics of Ag₂S-NPs in electrolyte solutions were in agreement with the results based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The dynamic light scattering (DLS) results showed that the SRFA, SRHA, alginate and BSA molecules coated on the Ag₂S-NPs surfaces. The NOM coating layer prevented salt-induced coagulation of Ag₂S-NPs, and the effects of BSA and SRHA on Ag₂S-NPs stabilizing were more obvious than that of SRFA and alginate. Flow cytometry analysis results suggested that BSA and SRHA were more effective on alleviating the Ag₂S-NPs induced cell (Escherichia coli) membrane damage than SRFA and alginate. After interpreting the electrophoretic mobility (EPM) data of the NOM coated Ag₂S-NPs by Ohshima's soft particle theory, it was found that the thickness of the NOM coating layers followed the orders of BSA > SRHA > alginate > SRFA. The E.coli cell membrane damage level was negatively correlated with the thickness and softness of the coating layer. NOM coating may physically alleviate the contact between NPs and E. coli cells and thus attenuate the extent of cell membrane damage caused by the NP-cell interaction. This work provides a new perspective for quantitatively interpreting the influence of NOM on the environmental behaviors and risks of nanomaterials.

Effects of carbon and silicon nanotubes and carbon nanofibers on marine microalgae Heterosigma akashiwo

The effect of carbon and silicon nanotubes (CNTs and SiNTs) and carbon nanofibers (CNFs) to microscopic marine algae Heterosigma akashiwo was studied, using algal growth inhibition for 3 days (acute effect) and 7 days (chronic effect) as toxicity endpoints. The criterion of the toxic effect was the statistically significant reduction of the number of algal cells in the exposed samples compared to the control. Samples did not demonstrate toxic effects at doses 1 mg/l and 10 mg/l. CNTs and SiNTs samples at 100 mg/l exhibited both acute and chronic toxic effects. We assume that the main cause of cell death in these samples was related to the mechanical damage of cell integrity. CNFs at concentrations of 100 mg/l did not inhibit algal growth, but cells with irregular shapes were observed, which were not observed after exposure to CNTs and SiNTs. Nickel impurities present in CNFs samples are presumably the main cause of observed cell deformations.

Nanoecotoxicity assessment of graphene oxide and its relationship with humic acid

The risk assessment of nanomaterials is essential for regulatory purposes and for sustainable nanotechnological development. Although the application of graphene oxide has been widely exploited, its environmental risk is not well understood because several environmental conditions can affect its behavior and toxicity. In the present study, the graphene oxide effect from aquatic ecosystems was assessed considering the interaction with humic acid on 9 organisms: Raphidocelis subcapitata (green algae), Lemna minor (aquatic plant), Lactuca sativa (lettuce), Daphnia magna (planktonic microcrustacean), Artemia salina (brine shrimp), Chironomus sancticaroli (Chironomidae), Hydra attenuata (freshwater polyp), and Caenorhabditis elegans and Panagrolaimus sp. (nematodes). The no-observed-effect concentration (NOEC) was calculated for each organism. The different criteria used to calculate NOEC values were transformed and plotted as a log-logistic function. The hypothetical 5 to 50% hazardous concentration values were, respectively, 0.023 (0.005-0.056) and 0.10 (0.031-0.31) mg L-1 for graphene oxide with and without humic acid, respectively. The safest scenario associated with the predicted no-effect concentration values for graphene oxide in the aquatic compartment were estimated as 20 to 100 μg L-1 (in the absence of humic acid) and 5 to 23 μg L-1 (in the presence of humic acid). Finally, the present approach contributed to the risk assessment of graphene oxide-based nanomaterials and the establishment of nano-regulations. Environ Toxicol Chem 2018;37:1998-2012. © 2018 SETAC.

Toxicity of mixtures of zinc oxide and graphene oxide nanoparticles to aquatic organisms of different trophic level: particles outperform dissolved ions

Concomitant releases of various engineered nanoparticles (NPs) into the environment have resulted in concerns regarding their combined toxicity to aquatic organisms. It is however, still elusive to distinguish the contribution to toxicity of components in NP mixtures. In the present study, we quantitatively evaluated the relative contribution of NPs in their particulate form (NP_(particle)) and of dissolved ions released from NPs (NP_(ion)) to the combined toxicity of binary mixtures of ZnO NPs and graphene oxide nanoplatelets (GO NPs) to three aquatic organisms of different trophic levels, including an alga species (Scenedesmus obliquus), a cladoceran species (Daphnia magna), and a freshwater fish larva (Danio rerio). Our results revealed that the effects of ZnO NPs and GO NPs were additive to S. obliquus and D. magna but antagonistic to D. rerio. The relative contribution to toxicity (RCT) of the mixture components to S. obliquus decreased in the order of RCT_{GO NP(particle)} > RCT_{ZnO NP(particle)} > RCT_{ZnO NP(ion)}, while the RCT of the mixture components to D. magna and D. rerio decreased in the order of RCT_{ZnO NP(particle)} > RCT_{GO NP(particle)} > RCT_{ZnO NP(ion)}. This finding also implies that the suspended particles rather than the dissolved Zn-ions dictated the combined toxicity of binary mixtures of ZnO NPs and GO NPs to the aquatic organisms of different trophic level. The alleviation of the contribution to toxicity of the ionic form of ZnO NPs was caused by the adsorption of the dissolved ions on GO NPs. Furthermore, the ZnO NP_(particle) and GO NP_(particle) displayed a different contribution to the observed mixture toxicity, dependent on the trophic level of the aquatic organisms tested. The difference of the contributions between the two particulate forms was mainly associated with differences in the intracellular accumulation of reactive oxygen species. Our findings highlight the important role of particles in the ecological impact of multi-nanomaterial systems.

Humic acid alleviates the ecotoxicity of graphene-family materials on the freshwater microalgae Scenedesmus obliquus

The extensive application of graphene-family materials (GFMs) has increased its potential risk to aquatic organisms. However, the influence of humic acid (HA) on the biotoxicity of GFMs has not clarified. Here, we conduct a study on the toxicity of four GFMs, i.e. graphene (G), graphene oxide (GO), carboxyl-modified graphene (G-COOH) and amine-modified graphene (G-NH₂), with or without HA, using Scenedesmus obliquus (S. obliquus) as model organism. Our results showed that the four GFMs induced significant inhibition on cell growth and Chlorophyll-a (Chl-a) synthesis, loss of cell viability and membrane integrity as well as mitochondrial membrane potential (MMP), where G exhibited the highest toxicity with median effect concentration (EC₅₀) of 8.2 mg L^-1, and G-NH₂ exhibited the lowest toxicity with EC₅₀ of 84.0 mg L^-1. Meanwhile, HA mitigated the toxicity of GFMs in the order of G-NH₂ > G-COOH > GO > G for the most of endpoints. Furthermore, three possible mechanisms of the HA alleviation on toxicity were speculated as: (1) reduce the contact of GFMs with algae cells through regulating the structures and surface negative charges of GFMs; (2) mitigate physical penetration and damage through decreasing the deposition of GFMs on cells by interacting with HA; (3) react as an antioxidant with intracellular reactive oxygen species (ROS) and extracellular hydroxyl radical (OH). This work provides useful information for the environmental toxicity of GFMs and the possible antidotal mechanisms in the presence of HA, which could aid to avoiding the overestimation of potential risk of GFMs in natural aquatic environment.

Ecotoxicological effects of carbon based nanomaterials in aquatic organisms

An increasing amount of carbon-based nanomaterials (CNM) (mostly fullerenes, carbon nanotubes and graphene) has been observed in aquatic systems over the last years. However, the potential toxicity of these CNM on aquatic ecosystems remains unclear. This paper reviews the existing literature on the toxic effects of CNM in aquatic organisms as well as the toxic effects of CNM through influencing the toxicity of other micro-pollutants, and outlines a series of research needs to reduce the uncertainty associated with CNMs toxic effects. The results show that environmental concentrations of CNM do not pose a threat on aquatic organisms on their own. The observed concentrations of CNM in aquatic environments are in the order of ngL^-1 or even lower, much below than the lowest observed effect concentrations (LOEC) on different aquatic organisms (in the order of mgL^-1). Toxic effects have been mainly observed in short-term experiments at high concentrations, and toxicity principally depends on the type of organisms, exposition time and CNM preparation methods. Moreover, we observed that CNM interact (establishing synergistic and/or antagonistic effects) with other micro-pollutants. Apparently, the resulting interaction is highly dependent on the chemical properties of each micro-pollutant, CNM acting either as carriers or as sorbents, thereby modifying the original toxicity of the contaminants. Results stress the need of studying the interactive effects of CNM with other micro-pollutants at environmental relevant concentrations, as well as their effects on biological communities in the long-term.

Green Algae as Carriers Enhance the Bioavailability of 14C-Labeled Few-Layer Graphene to Freshwater Snails

The waterborne exposure of graphene to ecological receptors has received much attention; however, little is known about the contribution of food to the bioaccumulation potential of graphene. We investigated the effect of algal food on the uptake and distribution of ¹⁴C-labeled few-layer graphene (FLG) in freshwater snails, a favorite food for Asian people. In a water-only system, FLG (∼158 μg/L) was ingested by and accumulated in the snails. Adding algae to the water significantly enhanced FLG accumulation in the snails, with a bioaccumulation factor of 2.7 (48 h exposure). Approximately 92.5% of the accumulated FLG was retained in the intestine; in particular, the accumulated FLG in the intestine was able to pass through the intestinal wall and enter the intestinal epithelial cells. Of them, 1.3% was subsequently transferred/internalized to the liver/hepatocytes, a process that was not observed in the absence of the algae. Characterizations data further suggested that both of the extra- and intracellular FLG in the algae (the algae-bound fraction was 30.2%) significantly contributed to the bioaccumulation. Our results provide the first evidence that algae as carriers enhanced FLG bioavailability to the snails, as well as the potential of FLG exposure to human beings through consuming the contaminated snails.

Effect of graphene oxide on copper stress in Lemna minor L.: evaluating growth, biochemical responses, and nutrient uptake

The wide application and unique properties of graphene oxide (GO) make it to interact with other pollutants and subsequently alter their behavior and toxicity. We evaluated the influences of GO at different concentrations (1 and 5mg/L) on copper (Cu) stress in duckweed (Lemna minor L.) GO below a concentration of 5mg/L showed no adverse effects on L. minor. The addition of Cu above 10μM represented a stress condition, which was evidenced by various parameters such as frond number, percent inhibition of growth rate (I_r), total chlorophyll content, dry weight, superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). When L. minor was simultaneously exposed to GO and Cu, especially at a GO concentration of 5mg/L and a Cu level above 10μM, the increase of I_r and decrease of chlorophyll content were inhibited, suggesting that the Cu stress was diminished in the presence of GO. The addition of Cu alone, ranging between 5 and 20μM, increased Cu, B, Mn, Fe, Co, and Zn uptake, but decreased P uptake. Our results suggest that GO can lessen Cu stress in L. minor via Cu adsorption, thereby protecting the plants from the damaging effects of high Cu concentrations.

Influence of reduced graphene oxide on the growth, structure and decomposition activity of white-rot fungus Phanerochaete chrysosporium

Graphene materials have attracted great interest nowadays due to their large-scale production and wide applications. It is urgent to evaluate the ecological and environmental risk of graphene materials for the healthy development of the graphene industry. Herein, we evaluated the influence of reduced graphene oxide (RGO) on the growth, structure and decomposition activity of white-rot fungus, whose decomposition function is vital for carbon cycle. RGO slightly stimulated the fresh weight and dry weight gains of Phanerochaete chrysosporium. A larger number of fibrous structures were observed at low RGO concentrations in P. chrysosporium, which was consistent with the elongation of cells observed under a transmission electron microscope. RGO did not affect the chemical composition of P. chrysosporium. Moreover, the laccase production of P. chrysosporium was not influenced by RGO. The degradation activities of P. chrysosporium for dye and wood appeared to be promoted slightly, but the differences were insignificant compared to the control. Therefore, RGO had low toxicity to white-rot fungus and was relatively safe for the carbon cycle.

RGO stimulated the growth of white-rot fungus and did not influence its degradation activity.

Comparative toxicity of the plasticizer dibutyl phthalate to two freshwater algae

Phthalate esters (PAEs), a family of emerging environmental contaminants, have been frequently detected in soils and water. However, intensive studies on the toxicity of PAEs have focused on growth response of terrestrial and aquatic animals, while only limited attention has been paid to aquatic plants, especially phytoplankton, the primary producer in aquatic ecosystems. Therefore, the acute toxic effects and underlying mechanisms of dibutyl phthalate (DBP) at different concentrations (0-20mgL^-1) on two typical freshwater algae (Scenedesmus obliquus and Chlorella pyrenoidosa) were investigated. The growth of S. obliquus and C. pyrenoidosa was conspicuously inhibited by DBP exposure at 2-20mgL^-1. The 96-h median effective concentration values (96h-EC₅₀) were 15.3mgL^-1 and 3.14mgL^-1 for S. obliquus and C. pyrenoidosa, respectively, implying that the spherical C. pyrenoidosa is more sensitive to DBP than the spindle-shaped S. obliquus. As expected from the damage done to cell organelles (i.e. cell membranes, chloroplasts, and protein rings), cell densities and chlorophyll content conspicuously decreased under DBP treatments. Moreover, the algal growth inhibition was closely linked to the increased production of intracellular reactive oxygen species and malondialdehyde content, indicating oxidative stress and lipid peroxidation in both algae. This was proved by the increased activity of antioxidant enzymes such as superoxide dismutase and catalase. Our findings will contribute to the understanding of toxic mechanisms in PAEs and the evaluation of environmental risks for primary producers in aquatic ecosystems.

Assessment and prediction of joint algal toxicity of binary mixtures of graphene and ionic liquids

Graphene and ionic liquids (ILs) released into the environment will interact with each other. So far however, the risks associated with the concurrent exposure of biota to graphene and ILs in the environment have received little attention. The research reported here focused on observing and predicting the joint toxicity effects in the green alga Scenedesmus obliquus exposed to binary mixtures of intrinsic graphene (iG)/graphene oxide (GO) and five ILs of varying anionic and cationic types. The isolated ILs in the binary mixtures were the main contributors to toxicity. The binary GO-IL mixtures resulted in more severe joint toxicity than the binary iG-IL mixtures, irrespective of mixture ratios. The mechanism of the joint toxicity may be associated with the adsorption capability of the graphenes for the ILs, the dispersion stability of the graphenes in aquatic media, and modulation of the binary mixtures-induced oxidative stress. A toxic unit assessment showed that the graphene and IL toxicities were additive at low concentration of the mixtures but antagonistic at high concentration of the mixtures. Predictions made using the concentration addition and independent action models were close to the observed joint toxicities regardless of mixture types and mixture ratios. These findings provide new insights that are of use in the risk assessment of mixtures of engineered nanoparticles and other environmentally relevant contaminants.

Molecular Mechanisms of Developmental Toxicity Induced by Graphene Oxide at Predicted Environmental Concentrations

Developmental toxicity is a critical issue in nanotoxicity. However, very little is known about the effects of graphene oxide (GO, a widely used carbon material) at predicted environmental concentrations on biological development or the specific molecular mechanisms. The present study established that the development of zebrafish embryos exposed to trace concentrations (1-100 μg/L) of GO was impaired because of DNA modification, protein carbonylation and excessive generation of reactive oxygen species (ROS), especially the superoxide radical. Noticeably, there was a nonmonotonic response of zebrafish developmental toxicity to GO at μg/L to mg/L levels. Transcriptomics analysis revealed that disturbing collagen- and matrix metalloproteinase (MMP)-related genes affected the skeletal and cardiac development of zebrafish. Moreover, metabolomics analysis showed that the inhibition of amino acid metabolism and the ratios of unsaturated fatty acids (UFAs) to saturated fatty acids (SFAs) contributed to the above developmental toxicity. The present work verifies the developmental toxicity of GO at trace concentrations and illustrates for the first time the specific molecular mechanisms thereof. Because of the potential developmental toxicity of GO at trace concentrations, government administrators and nanomaterial producers should consider its potential risks prior to the widespread environmental exposure to GO.

Influence of Speciation of Thorium on Toxic Effects 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 nuclear fuel. Relatively little is known about the influence and toxicity of Th in the environment. In the present study, the toxicity of Th to the green algae Chlorella pyrenoidosa (C. pyrenoidosa) was evaluated by algal growth inhibition, biochemical assays and morphologic observations. In the cultural medium (OECD TG 201), Th(NO₃)₄ was transformed to amorphous precipitation of Th(OH)₄ due to hydrolysis. Th was toxic to C. pyrenoidosa, with a 96 h half maximum effective concentration (EC₅₀) of 10.4 μM. Scanning electron microscopy shows that Th-containing aggregates were attached onto the surface of the algal cells, and transmission electron microscopy indicates the internalization of nano-sized Th precipitates and ultrastructural alterations of the algal cells. The heteroagglomeration between Th(OH)₄ precipitation and alga cells and enhanced oxidative stress might play important roles in the toxicity of Th. To our knowledge, this is the first report of the toxicity of Th to algae with its chemical species in the exposure medium. This finding provides useful information on understanding the fate and toxicity of Th in the aquatic environment.

Reviews of the toxicity behavior of five potential engineered nanomaterials (ENMs) into the aquatic ecosystem

Presently, engineered nanomaterials (ENMs) are used in a wide variety of commercial applications, resulting in an uncontrolled introduction into the aquatic environment. The purpose of this review is to summarize the pathways and factors that controlling the transport and toxicity of five extensively used ENMs. These toxicological pathways are of great importance and need to be addressed for sustainable implications of ENMs without environmental liabilities. Here we discuss five potentially utilized ENMs with their possible toxicological risk factors to aquatic plants, vertebrates model and microbes. Moreover, the key effect of ENMs surface transformations by significant reaction with environmental objects such as dissolved natural organic matter (DOM) and the effect of ENMs surface coating and surface charge will also be debated. The transformations of ENMs are subsequently facing a major ecological transition that is expected to create a substantial toxicological effect towards the ecosystem. These transformations largely involve chemical and physical processes, which depend on the properties of both ENMs and the receiving medium. In this review article, the critical issues that controlling the transport and toxicity of ENMs are reviewed by exploiting the latest reports and future directions and targets are keenly discussed to minimize the pessimistic effects of ENMs.