Influence of Alpha and Gamma-Iron Oxide Nanoparticles on Marine Microalgae Species

Toxicity of ZnFe-SO4 layered double hydroxide in Tetradesmus obliquus and evaluation of some physiological responses of the microalgae for stress management

Layered double hydroxides (LDHs), regarding their physical and structural properties, have different and wide applications industry and their increasing use may raise ecological and human health concerns. However, the potential toxicity mechanisms of LDHs in different organisms are still unclear. In the present work, after synthesizing of ZnFe-SO4 LDH and studying of its characterization by XRD, FT-IR, SEM, EDX-mapping, TEM and Raman, its toxicity in Tetradesmus obliquus was evaluated. According to experimental results, the growth of the algae and content of photosynthetic pigments were significantly decreased after treatment with 100 mg/L of ZnFe-SO4 LDH. The high dose exposure to the LDH also inhibited the activity of SOD and POD enzymes, possibly due to the LDH- catalyzed reactive oxygen species production. In addition, lipid peroxidation and the content of phenolic compounds, as no-enzymatic antioxidants were increased by enhancement of the LDH concentration. The rise of phenol, flavonoids and MDA contents could be regarded as some manifestations and responses to the toxic effects of the contaminant in the algae cells. The results provided a better understanding of the undesirable effects and toxicity of LDHs in aquatic organisms.

Elevated nano-α-Fe2O3 enhances arsenic metabolism and dissolved organic carbon release of Microcystis aeruginosa under a phytate environment

Little information is available on the effects of nano-α-Fe2O3 on arsenic (As) metabolism of algae and potential associated carbon (C) storage in As-contaminated water with dissolved organic phosphorus (DOP) as a phosphorus (P) source. In this study, Microcystis aeruginosa (M. aeruginosa) was used to investigate impacts of nano-α-Fe2O3 on cell growth and As metabolism of algae under a phytate (PA) environment as well as potential associated C storage. Results showed that nano-α-Fe2O3 had a subtle influence on algal cell growth in a PA environment. Herein, algal cell density (OD680) and chlorophyll a (Chla) were inhibited at elevated nano-α-Fe2O3 levels, which simultaneously limited the decrease of Yield. As suggested, the complexation of PA with nano-α-Fe2O3 could alleviate the negative influence on algal cell growth. Furthermore, the elevated nano-α-Fe2O3 increased As methylation in the PA environment due to higher monomethylarsenic (MMA) and dimethylarsenic (DMA) concentrations in the test media. Additionally, microcystins (MCs) in the media changed consistently with UV254, both of which were relatively lower at 10.0 mg·L-1 nano-α-Fe2O3. Enhanced As(V) methylation of algal cells was found to simultaneously reduce the release risk of As(III) and MC while increasing dissolved organic carbon (DOC) content in media, suggesting unfavorable C storage. Three-dimensional fluorescence analysis revealed that the main DOC constituent was the tryptophan-like component in aromatic proteins. Correlation analysis showed that decreases in pH and the zeta potential and an increase in Chla may lead to metabolic As improvements in M. aeruginosa. The obtained findings highlight the need for greater focus on the potential risks of DOP combined with nano-α-Fe2O3 on algal blooms as well as the biogeochemical cycling processes of As and C storage in As-contaminated water with DOP as the P source.

Enhancing arsenate metabolism in Microcystis aeruginosa and relieving risks of arsenite and microcystins by nano-Fe2O3 under dissolved organic phosphorus conditions

Little information is available on how nano-Fe₂O₃ substituted iron ions as a possible iron source impacting on algal growth and arsenate (As(V)) metabolism under dissolved organic phosphorus (DOP) (D-glucose-6-phosphate (GP)) conditions. We investigated the growth of Microcystis aeruginosa and As(V) metabolism together with their metabolites in As(V) aquatic environments with nano-Fe₂O₃ and GP as the sole iron and P sources, respectively. Results showed that nano-Fe₂O₃ showed inhibitory effects on M. aeruginosa growth and microcystin (MCs) release under GP conditions in As(V) polluted water. There was little influence on As species changes in GP media under different nano-Fe₂O₃ concentrations except for obvious total As (TAs) removal in 100 mg L^-1 nano-Fe₂O₃ levels. As(V) metabolism dominated with As(V) biotransformation in algal cells was facilitated and arsenite (As(III)) releasing risk was relieved clearly by nano-Fe₂O₃ under GP conditions. The dissolved organic matter (DOM) in media exhibited more fatty acid analogs containing -CO, -CH₂ =CH₂, and -CH functional groups with increasing nano-Fe₂O₃ concentrations, but the fluorescent analogs were relatively reduced especially for the fluorescent DOM dominated by aromatic protein-like tryptophan which was significantly inhibited by nano-Fe₂O_3. Thus, As methylation that was facilitated in M. aeruginosa by nano-Fe₂O₃ in GP environments also caused more organic substances to release that absorb infrared spectra while reducing the release risks of As(III) and MCs as well as protein-containing tryptophan fractions. From ¹H-NMR analysis, this might be caused by the increased metabolites of aromatic compounds, organic acid/amino acid, and carbohydrates/glucose in algal cells. The findings are vital for a better understanding of nano-Fe₂O₃ role-playing in As bioremediation by microalgae and the subsequent potential aquatic ecological risks.

Nano-ecotoxicology in a changing ocean

The ocean faces an era of change, driven in large by the release of anthropogenic CO2, and the unprecedented entry of pollutants into the water column. Nanomaterials, those particles < 100 nm, represent an emerging contaminant of environmental concern. Research on the ecotoxicology and fate of nanomaterials in the natural environment has increased substantially in recent years. However, commonly such research does not consider the wider environmental changes that are occurring in the ocean, i.e., ocean warming and acidification, and occurrence of co-contaminants. In this review, the current literature available on the combined impacts of nanomaterial exposure and (i) ocean warming, (ii) ocean acidification, (iii) co-contaminant stress, upon marine biota is explored. Here, it is identified that largely co-stressors influence nanomaterial ecotoxicity by altering their fate and behaviour in the water column, thus altering their bioavailability to marine organisms. By acting in this way, such stressors, are able to mitigate or elevate toxic effects of nanomaterials in a material-specific manner. However, current evidence is limited to a relatively small set of test materials and model organisms. Indeed, data is biased towards effects upon marine bivalve species. In future, expanding studies to involve other ecologically significant taxonomic groups, primarily marine phytoplankton will be highly beneficial. Although limited in number, the available evidence highlights the importance of considering co-occurring environmental changes in ecotoxicological research, as it is likely in the natural environment, the material of interest will not be the sole stressor encountered by biota. As such, research examining ecotoxicology alongside co-occurring environmental stressors is essential to effectively evaluating risk and develop effective long-term management strategies.

Decreasing arsenic accumulation but promoting arsenate biotransformation in Microcystis aeruginosa regulated by nano-Fe2O3

Iron oxide nanoparticles (nano-Fe₂O₃) widely distribute in waters with low toxicity to aquatic organisms. But it is unclear for nano-Fe₂O₃ to affect the fate of coexisting arsenic (As) with its bioaccumulation and biotransformation. In this study, we thus mainly investigated arsenate (As(V)) toxicity, uptake kinetics, biotransformation and subcellular distribution in Microcystis aeruginosa influenced by nano-Fe₂O₃. The results showed that M. aeruginosa was more sensitive to As(V) associated with nano-Fe₂O₃. Due to the exaggerated increase of efflux rate constants of As compared with the uptake rate constants in algal cells affected by different levels of nano-Fe₂O₃, the As(V) bioconcentration factor decreased with nano-Fe₂O₃ increasing correspondingly, indicating that As bioaccumulation was diminished by nano-Fe₂O_3. The decreased As accumulation in M. aeruginosa could be supported by the evidential As(V) sequestration through high adsorption of nano-Fe₂O₃, which resulted in decreasing free As level for algae uptake in media. Meanwhile, As subcellular distribution was adjusted by nano-Fe₂O₃ with decreasing in cell walls and rising in cytoplasmic organelles compared with nano-Fe₂O₃ free. As(V) reduction and methylation were enhanced with increasing nano-Fe₂O₃, stimulating by its sensitivity to the interaction of nano-Fe₂O₃ and As(V) as well as the rising level of As in cytoplasmic organelles of this algae. It is confirmed by the higher relative gene expression levels of arsC and arsM in elevated nano-Fe₂O₃. Accordingly, it is highlighted to be deserved more attention that the changing behavior of As(V) by nano-Fe₂O₃ that reduce As bioaccumulation and accelerate its biotransformation in algae in As contaminated water.

Bioavailability and effect of α-Fe2O3 nanoparticles on growth, fatty acid composition and morphological indices of Chlorella vulgaris

The wide application of α-Fe₂O₃ nanoparticles (NPs) in different fields has resulted in release and accumulation of these materials into the aquatic ecosystem. Therefore, it is important to understand the potential impact of these NPs on aquatic organisms especially primary producers i.e., microalgae. Present study aimed to investigate the bioavailability and the effect of α-Fe₂O₃ NPs on growth of iron deprived cells of Chlorella vulgaris. Results showed that α-Fe₂O₃ NPs are not available as iron source to support the growth of C. vulgaris. Moreover，α-Fe₂O₃ NPs induced stress condition to C. vulgaris, which were reflected in its growth rates, total lipid contents, fatty acid profile and cell morphology. Specifically, low concentrations of α-Fe₂O₃ NPs (0.1, 0.5, 2.5, 5, 10 mg/L) showed similar growth profile and total lipid contents at both exponential and stationary growth phases. At 50 and 100 mg/L α-Fe₂O₃ NPs concentrations biomass reduced by 41.2% and 83.7% whereas total lipid contents increased by 39.7% and 25.5% respectively at exponential growth phase along with reduction in fatty acids. The results illustrated novel insights into the microalgal interaction with nanoparticles, providing fundamental knowledge for the development of future microalgae ecology and cultivation technology.

Fecitrate converted from Fe2O3 particles in coal-fired flue gas promoted microalgal biomass and lipid productivities

In order to reutilize Fe₂O₃ particles in flue gas from coal-fired power plant as a ferrum nutrient for improving microalgae growth, Na-Citrate was proposed to chelate FeCl₃ derived from Fe₂O₃ and HCl reactions to promote biomass and lipid productivities of Chlorella PY-ZU1. Fe-Citrate gave much higher biomass and lipid productivities than FeCl₃, Fe-EDTA, Fe-DTPA and Fe-HEDTA, because organic chelator prevented Fe³⁺ from depositing, lower stability constant resulted in easier dissociation of ferric chelate, smaller chelate facilitated Fe²⁺ (reduced from Fe³⁺) transportation through cell membranes. The biomass growth and photosynthetic capacity of Chlorella PY-ZU1 cultivated with Fe-Citrate (converted from Fe₂O₃ particles) medium were similar to those with commercial ferric ammonium citrate medium. The biomass and lipid productivities of Chlorella PY-ZU1 cultivated with 5 mg L^-1 Fe-Citrate medium were 1.30 and 1.72 times, respectively, higher than those with FeCl₃ growth medium.

Investigating the Nanocomposite Thin Films of Hematite α-Fe2O3 and Nafion for Cholesterol Biosensing Applications

Nanocomposite materials are multi-phase materials, usually solids, which have two or more component materials having different chemical and physical properties. When blended together, a newer material is formed with distinctive properties which make them an eligible candidate for many important applications. In the present study, thin films of nafion (polymer) and hematite or α-Fe2O3 (nanoparticles) nanocomposite is fabricated on indium tin oxide (ITO) coated glass substrates, due to its enhanced ionic conductivity, for cholesterol biosensor applications. Scanning electron microscopy and Atomic force microscopy revealed the formation of nanorod structured α-Fe2O3 in the films. The cyclic voltammetry (CV) studies of nafion-α-Fe2O3/ITO revealed the redox properties of the nanocomposites. The sensing studies were performed on nafion-α-Fe2O3/CHOx/ITO bioelectrode using differential pulse voltammetry (DPV) at various concentrations of cholesterol. The enzyme immobilization leaded to the selective detection of cholesterol with a sensitivity of 64.93 × 10−2 μA (mg/dl)−1 cm−2. The enzyme substrate interaction (Michaelis–Menten) constant Km, was obtained to be 19 mg/dl.

Exposure effects of iron oxide nanoparticles and iron salts in blackfish (Capoeta fusca): Acute toxicity, bioaccumulation, depuration, and tissue histopathology

We assessed the toxicity of iron oxide nanoparticles compared with iron salts in the blackfish (Capoeta fusca). After an acute toxicity assessment, we conducted a chronic exposure to a sub-lethal concentration of Fe₃O₄ NPs, and iron salts (ferric nitrate (Fe(NO₃)₃), ferric chloride (FeCl₃), ferrous sulfate (FeSO₄)) to measure iron uptake over a period of 28 days and then subsequent clearance of the iron uptake in the exposed fish that were transferred to clean water for 28 days. Fe(NO₃)₃ was the most acutely toxic compound followed by FeCl₃, FeSO₄, and Fe₃O₄ NPs. Exposure to Fe₃O₄ NPs and iron salts induced histopathology anomalies in both gills and intestine that included aneurism, hyperplasia, oedema, fusion of lamellae, lamellar synechiae, and clear signs of necrosis (in the gills) and increases in the number of goblet cells, blood cell counts, and higher numbers of lymphocyte (in the intestine). Fe₃O₄ NPs showed a higher level of uptake in the body tissues compared with iron salts (p < 0.05) with levels of Fe in the gill > intestine > liver > kidney. Fe was shown to be eliminated most efficiently from the gills, followed by the kidney, then liver and finally the intestine. The highest tissue bioconcentration factors (BCF) occurred in the liver for FeCl₃, Fe₃O₄ NPs, and FeSO₄ and in the gills for Fe(NO₃)₃. We thus show differences in the patterns of tissue accumulation, clearance and toxicological responses for exposures to Fe₃O₄ NPs and iron salts in blackfish with implications for different susceptibilities for biological effects.

Crystalline phase-dependent toxicity of aluminum oxide nanoparticles toward Daphnia magna and ecological risk assessment

Aluminum oxide nanoparticles (Al₂O₃ NPs) can be found in different crystalline phases, and with the emergence of nanotechnology there has been a rapid increase in the demand for Al₂O₃ NPs in different engineering areas and for consumer products. However, a careful evaluation of the potential environmental and human health risks is required to assess the implications of the release of Al₂O₃ NPs into the environment. Thus, the objective of this study was to investigate the toxicity of two crystalline phases of Al₂O₃ NPs, alpha (α-Al₂O₃ NPs) and eta (η-Al₂O₃ NPs), toward Daphnia magna and evaluate the risk to the aquatic ecology of Al₂O₃ NPs with different crystalline phases, based on a probabilistic approach. Different techniques were used for the characterization of the Al₂O₃ NPs. The toxicity toward Daphnia magna was assessed based on multiple toxicological endpoints, and the probabilistic species sensitivity distribution (PSSD) was used to estimate the risk of Al₂O₃ NPs to the aquatic ecology. The results obtained verify the toxic potential of the NPs toward D. magna even in sublethal concentrations, with a more pronounced effect being observed for η-Al₂O₃ NPs. The toxicity is associated with an increase in the reactive oxygen species (ROS) content and deregulation of antioxidant enzymatic/non-enzymatic enzymes (CAT, SOD and GSH). In addition, changes in MDA levels were observed, indicating that D. magna was under oxidative stress. The most prominent chronic toxic effects were observed in the organisms exposed to η-Al₂O₃ NPs, since the lowest LOEC was 3.12 mg/L for all parameters, while for α-Al₂O₃ NPs the lowest LOEC was 6.25 mg/L for longevity, growth and reproduction. However, the risk assessment results indicate that, based on a probabilistic approach, Al₂O₃ NPs (alpha, gamma, delta, eta and theta) only a very limited risk to organisms in surface waters.

Differential sensitivity of marine algae Dunaliella salina and Chlorella sp. to P25 TiO2 NPs

The use of P25 TiO₂ NPs in consumer products, their release, and environmental accumulation will have harmful effects on the coastal ecosystems. The sensitivity to TiO₂ NPs may vary depending on the structural property and physiological mechanism of algal species. Therefore, the present study investigates the differences in sensitivity of two marine algae, Dunaliella salina and Chlorella sp., towards P25 TiO₂ NPs. Among the two species, Chlorella sp. was more sensitive to TiO₂ NPs than Dunaliella salina. The different working concentrations of TiO₂ NPs, 0.1, 1, and 10 mg L^-1, were selected based on the EC₅₀ value. The EC₅₀ value of TiO₂ NPs for Dunaliella salina was found to be 1.8 and 13.3 mg L^-1 under UV-A and dark conditions, respectively. The EC₅₀ value of TiO₂ NPs for Chlorella sp. was found to be 1.6 and 5.0 mg L^-1 under UV-A and dark conditions, respectively. The decrease in cell viability was significantly higher for Chlorella sp. compared to Dunaliella salina at all concentrations except 0.1 mg L^-1. The cellular viability data was in correlation with the oxidative stress markers such as total ROS and LPO. A concentration-dependent increase in ROS and lipid peroxidation was noted under UV-A exposure, which was higher in Chlorella sp. compared to Dunaliella salina. The decrease in the SOD activity with NP concentration was more in Dunaliella salina than Chlorella sp. under both conditions, whereas Chlorella sp. showed increased CAT activity with increasing concentration. The uptake of TiO₂ NPs was more in Chlorella sp. than Dunaliella salina.

Cytotoxic impacts of CuO nanoparticles on the marine microalga Nannochloropsis oculata

The toxic impacts of CuO nanoparticles (NPs) on the marine phytoplankton Nannochloropsis oculata were evaluated by measuring a number of biological parameters. Exposure to different concentrations of CuO-NPs (5-200 mg/L) significantly decreased the growth and content of chlorophyll a of N. oculata. The results showed that CuO-NPs were toxic to this microalga with a half maximal effective concentration (EC50) of 116.981 mg/L. Exposure to CuO-NPs increased the hydrogen peroxide (H₂O₂) content and induced the membrane damages. Moreover, the concentration of phenolic compounds was increased, while the levels of carotenoids were markedly decreased in comparison to the control sample. The activity of catalase (CAT), ascorbate peroxidase (APX), polyphenol oxidase (PPO) and lactate dehydrogenase (LDH) enzymes significantly was increased in response to CuO-NPs treatments. These results indicated that CuO-NPs stimulated the antioxidant defense system in N. oculata to protect the cells against the oxidative damages. The Fourier-transform infrared spectroscopy (FTIR) analyses showed that the main functional groups (C=O and C-O-C) interacted with CuO-NPs. The images of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the cell membrane damage and the change of cell wall structure which may be contributed to the nanotoxicity. These findings may provide additional insights into the mechanisms of cytotoxicity induced by CuO-NPs.

Metabolomics reveals the role of acetyl-l-carnitine metabolism in γ-Fe2O3 NP-induced embryonic development toxicity via mitochondria damage

Iron oxides nanoparticles (FeO_X NPs), including α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄, are employed in many technological applications. However, very few studies have investigated the embryonic developmental toxicity of FeO_X NPs. In this study, metabolomics analysis were used to uncover the potential mechanisms of FeO_X NPs developmental toxicity on embryo-larval zebrafish and mice. Our results indicated that γ-Fe₂O₃ NP treatment could cause increased mortality, dropped hatching rate, etc., while α-Fe₂O₃ and Fe₃O₄ NPs showed no obvious effect. Through metabolomics analysis, a total of 42 metabolites were found to be significantly changed between the γ-Fe₂O₃ NP-treated group and the control group (p < 0.05). Pathway enrichment analysis indicated the impairment of mitochondria function. γ-Fe₂O₃ NP treatment caused abnormal mitochondrion structure and a decrease in mitochondrial membrane potential in zebrafish embryos. Meanwhile, ATP synthesis was decreased while oxidative stress levels were affected. It is noteworthy that acetyl-l-carnitine (ALCAR) (p = 6.79E - 04) and l-carnitine (p = 1.43E - 03) were identified with minimal p values, the relationship between the two counter-balance was regulated by acetyltransferase (crata). Subsequently, we performed rescue experiments with ALCAR on zebrafish embryos, and found that the mortality rates reduced and hatching rates raised significantly in the γ-Fe₂O₃ NP-treated group. Additionally, γ-Fe₂O₃ exposure could lead to increased absorbed fetus rate, decreased placental weight, lower expression of acetyltransferase (Crat), reduced ATP synthesis as well as increased oxidative stress (p < 0.05). Our findings demonstrated that γ-Fe₂O₃ NP might affect the mitochondrial membrane potential and ATP synthesis by affecting the metabolism of ALCAR, thereby stimulating oxidative stress, cell apoptosis, and causing embryonic development toxicity.

Surface modification of Fe2O3 and MgO nanoparticles with agrowastes for the treatment of chlorosis in Glycine max

Surface modification of nanoparticles for biological applications is receiving enormous interest among the research community due to the ability to alchemy the toxic nanoparticles into biocompatible compounds. In this study, the agrowastes of Moringa oleifera and Coriandrum sativum were used to surface modify the magnesium oxide nanoparticles and ferric oxide nanoparticles respectively. The agrowaste amended magnesium oxide nano particles (AMNP) and agrowaste amended ferric oxide nanoparticles (AFNP) were characterized using scanning electron microscope, X-ray diffractometer, Fourier transformed-infra red spectroscope to justify the formation and surface modification of nanoparticles with the organic functional groups from the agro wastes. The surface modified nano particles were tested for their biocompatibility and ability to treat the chlorosis in Glycine max. On comparison between the two metal based nanoparticles, AMNP exhibited better chlorosis treating ability than the AFNP. Both the nano particles showed increased potency at minimal amount, 30 μg and the higher concentrations till 125 μg exhibited down run of the potency which was again enhanced from 250 μg of nanoparticle treatment to plants. Further the surface modified nanoparticles were assessed for biocompatibility on human embryonic kidney (HEK-293) cell line which proved that the cell lines are non-toxic to normal human cells. The size of the particles and the concentration is suggested to be responsible for the effective chlorosis treatment and the organic functional groups responsible for the reduction of toxicity of the particles to the plants.

Influence of Interaction Between α-Fe2O3 Nanoparticles and Dissolved Fulvic Acid on the Physiological Responses in Synechococcus sp. PCC7942

The ecotoxicity of α-Fe₂O₃ nanoparticles (NPs) and its interaction with a typical natural organic matter (NOM), fulvic acid (FA) on the physiological responses of Synechococcus sp. PCC7942 was studied. α-Fe₂O₃ NPs inhibited the algae growth at concentration higher than 10 mg L^-1 and induced oxidative stress, indicated by enhanced antioxidant enzymes activities, elevated protein and sugar content. FA could efficiently recover cell growth and reduce antioxidant enzyme activities which induced by α-Fe₂O₃ NPs, indicating the toxicity of NPs was alleviated in the presence of FA. α-Fe₂O₃ NPs could form large aggregates coating on cell surface and inhibit cell growth. FTIR spectra verified FA interacted with α-Fe₂O₃ NPs through carboxyl groups, partly replaced the binding sites of α-Fe₂O₃ NPs on algal cell walls, thus reduced NPs aggregates coating on cell surface. This favors reducing the oxidative stress caused by direct contact and increasing light availability, thus mitigate NPs toxicity.

Improvement on lipid production by Scenedesmus obliquus triggered by low dose exposure to nanoparticles

Carbon nanotubes (CNTs), α-Fe₂O₃ nanoparticles (nano Fe₂O₃) and MgO nanoparticles (nano MgO) were evaluated for the effects on algae growth and lipid production. Nano Fe₂O₃ promoted cell growth in the range of 0-20 mg·L^-1. CNTs, nano Fe₂O₃ and nano MgO inhibited cell growth of Scenedesmus obliquus at 10, 40 and 0.8 mg·L^-1 respectively. Neutral lipid and total lipid content increased with the increasing concentration of all tested nanoparticles. The maximum lipid productivity of cultures exposed to CNTs, nano Fe₂O₃ and nano MgO was observed at 5 mg·L^-1, 5 mg·L^-1 and 40 mg·L^-1, with the improvement by 8.9%, 39.6% and 18.5%. High dose exposure to nanoparticles limited increase in lipid productivity, possibly due to the repression on cell growth caused by nanoparticles-catalyzed reactive oxygen species (ROS) generation, finally leading to reduction in biomass and lipid production. Reduced accumulation of fatty acids of C18:3n3, C18:3n6 and C20:2 was observed in cells exposed to nanoparticles.

Comparative impacts of iron oxide nanoparticles and ferric ions on the growth of Citrus maxima

The impacts of iron oxide nanoparticles (γ-Fe₂O₃ NPs) and ferric ions (Fe³⁺) on plant growth and molecular responses associated with the transformation and transport of Fe²⁺ were poorly understood. This study comprehensively compared and evaluated the physiological and molecular changes of Citrus maxima plants as affected by different levels of γ-Fe₂O₃ NPs and Fe³⁺. We found that γ-Fe₂O₃ NPs could enter plant roots but no translocation from roots to shoots was observed. 20 mg/L γ-Fe₂O₃ NPs had no impact on plant growth. 50 mg/L γ-Fe₂O₃ NPs significantly enhanced chlorophyll content by 23.2% and root activity by 23.8% as compared with control. However, 100 mg/L γ-Fe₂O₃ NPs notably increased MDA formation, decreased chlorophyll content and root activity. Although Fe³⁺ ions could be used by plants and promoted the synthesis of chlorophyll, they appeared to be more toxic than γ-Fe₂O₃ NPs, especially for 100 mg/L Fe³⁺. The impacts caused by γ-Fe₂O₃ NPs and Fe³⁺ were concentration-dependent. Physiological results showed that γ-Fe₂O₃ NPs at proper concentrations had the potential to be an effective iron nanofertilizer for plant growth. RT-PCR analysis showed that γ-Fe₂O₃ NPs had no impact on AHA gene expression. 50 mg/L γ-Fe₂O₃ NPs and Fe³⁺ significantly increased expression levels of FRO2 gene and correspondingly had a higher ferric reductase activity compared to both control and Fe(II)-EDTA exposure, thus promoting the iron transformation and enhancing the tolerance of plants to iron deficiency. Relative levels of Nramp3 gene expression exposed to γ-Fe₂O₃ NPs and Fe³⁺ were significantly lower than control, indicating that all γ-Fe₂O₃ NPs and Fe³⁺ treatments could supply iron to C. maxima seedlings. Overall, plants can modify the speciation and transport of γ-Fe₂O₃ NPs or Fe³⁺ for self-protection and development by activating many physiological and molecular processes.

Facile synthesis of (55)Fe-labeled well-dispersible hematite nanoparticles for bioaccumulation studies in nanotoxicology

Although water-dispersible engineered nanoparticles (ENPs) have a wide range of applications, the ENPs used in many nanotoxicological studies tend to form micron-sized aggregates in the exposure media and thus cannot reflect the toxicity of real nanoparticles. Here we described the synthesis of bare hematite nanoparticles (HNPs-0) and two poly(acrylic acid) (PAA)-coated forms (HNPs-1 and HNPs-2). All three HNPs were well dispersed in deionized water, but HNPs-0 quickly aggregated in the three culture media tested. By contrast, the suspensions of HNPs-1 and HNPs-2 remained stable, with negligible amounts of PAA and Fe(3+) liberated from either one under the investigated conditions. To better quantify the accumulation of the coated HNPs, a relatively innocuous (55)Fe-labeled form of HNPs-2 was synthesized as an example and its accumulation in three phytoplankton species was tested. Consistent with the uptake kinetics model for conventional pollutants, the cellular accumulation of HNPs-2 increased linearly with exposure time for two of the three phytoplankton species. These results demonstrate the utility of (55)Fe-labeled well-dispersible HNPs as a model material for nanoparticle bioaccumulation studies in nanotoxicology.