Algal Foods Reduce the Uptake of Hematite Nanoparticles by Downregulating Water Filtration in Daphnia magna

Changes in population fitness and gene co-expression networks reveal the boosted impact of toxic cyanobacteria on Daphnia magna through microplastic exposure

The concomitant prevalence of toxic cyanobacteria blooms and plastic pollution in aquatic ecosystems is emerging as a pressing global water pollution dilemma. While toxic cyanobacteria and microplastics (MPs) can each independently exert significant impacts on aquatic biota, the magnitude and trajectory of the combined interactions remains rudimentary. In this study, we evaluated how MPs influences cyanobacterial stress on keystone grazer Daphnia, focusing on population, individual, biochemical and toxicogenomic signatures. We found that toxic Microcystis (TM) adversely affected the fitness of Daphnia populations (intrinsic rate of population increase), and these adverse effects were amplified in the presence of MPs. Through detailed observation, it was ascertained that MPs promoted the ingestion of TM, culminating in enhanced microcystin bioaccumulation. Using the Eco-Evo model, we found that there was potential absence of correlation between the MPs toxicity and the effect size of MPs on the TM. Utilizing gene set enrichment analysis (GSEA), we further identified a marked suppression of molecular pathways and entities crucial to individual growth and development in the TM-MPs consortium compared to exposure to TM alone. The present study provides important insights about the influence of MPs on cyanobacteria toxicity and the prediction the risk of harmful algal blooms in aquatic ecosystems.

Silica Nanoparticle Size Determines the Mechanisms Underlying the Inhibition of Iron Oxide Nanoparticle Uptake by Daphnia magna

Aquatic environments are complicated systems that contain different types of nanoparticles (NPs). Nevertheless, recent studies of NP toxicity, and especially those that have focused on bioaccumulation have mostly investigated only a single type of NPs. Assessments of the environmental risks of NPs that do not consider co-exposure regimes may lead to inaccurate conclusions and ineffective environmental regulation. Thus, the present study examined the effects of differently sized silica NPs (SiO2 NPs) on the uptake of iron oxide NPs (Fe2O3 NPs) by the zooplankton Daphnia magna. Both SiO2 NPs and Fe2O3 NPs were well dispersed in the experimental medium without significant heteroaggregation. Although all three sizes of SiO2 NPs inhibited the uptake of Fe2O3 NPs, the underlying mechanisms differed. SiO2 NPs smaller than the average mesh size (∼200 nm) of the filtering apparatus of D. magna reduced the accumulation of Fe2O3 NPs through uptake competition, whereas larger SiO2 NPs inhibited the uptake of Fe2O3 NPs mainly by reducing the water filtration rate of the daphnids. Overall, in evaluations of the risks of NPs in the natural environment, the different mechanisms underlying the effects of NPs of different sizes on the uptake of dissimilar NPs should be considered.

Internalization of the Metal-Organic Framework MIL-101(Cr)-NH2 by a Freshwater Alga and Transfer to Zooplankton

The common metal-organic framework (MOF) MIL-101(Cr)-NH₂ has attracted considerable attention due to its great potential applications in the environmental field. Nevertheless, its behavior and fate in aquatic systems are unknown. This study quantified and visualized the interactions of MIL-101(Cr)-NH₂ with the freshwater phytoplanktonic alga Chlamydomonas reinhardtii and its potential trophic transfer to zooplankton. The unicellular alga absorbed and accumulated the MOF by surface attachment, forming agglomerates and eventually cosettling out from water. Bioimaging revealed that MIL-101(Cr)-NH₂ was internalized by the algal cells and mainly occurred in the pyrenoid. Without algae in a freshwater system, MIL-101(Cr)-NH₂ was ingested by Daphnia magna, showing steadily increasing concentrations approaching 1-9% of dry body weight. Addition of algae substantially suppressed D. magna uptake of MIL-101(Cr)-NH₂ by 63.8-97.9%. Such inhibition could be explained by the competitive uptake of MOF by the algae and the inductive effects of algal food on MOF elimination by D. magna. The MOF (≤1 mg/L) ingested by D. magna was centered in the gut regions, whereas large MOF or algae-MOF aggregates were adsorbed onto the carapace and appendages, including the antennae, at 10 mg/L. Overall, the algae were the major targets for MIL-101(Cr)-NH₂, with nearly all algal cells settling out at 10 mg/L within 24 h. The possibility of trophic transfer of MIL-101(Cr)-NH₂ to D. magna in aquatic systems with algae present was limited due to its low accumulation potential and short retention time in D. magna.

Trophic transfer and environmental safety of carbon dots from microalgae to Daphnia

The application of carbon dots (CDs), a novel carbon nanomaterial, is extensive, leading to inevitable CD pollution. However, studies on their environmental fate and related risks to aquatic ecosystems are limited. Here, the trophic transfer of CDs from Chlorella pyrenoidosa to Daphnia magna and their toxic effects on the two organisms were analyzed. ¹⁴C-labelling was used to quantify and evaluate the fate of CDs. The results showed that the radioactivity of CDs in water was >80 % of the initial radioactivity, and that water extractable residues were dominant in organisms, with only 3 % or less recovered from the mineralization product ¹⁴CO₂. The distribution of radioactivity illustrated how the exposure routes changed the fate of CDs in aquatic environments. CD aggregates were found in algal cells and Daphnia intestinal tract, indicating the cellular uptake of CDs in these aquatic organisms. Wall-membrane detachment, cell collapse, and rupture were observed in the ultrastructural investigations of microalgae, whereas pneumatosis cystoides intestinalis was observed in the ultrastructural investigations of D. magna. CD exposure affected the growth and chlorophyll content of C. pyrenoidosa as well as the feeding behavior, oxidative stress system, digestive system, and symbiotic bacteria of D. magna. The toxicity of CDs is also affected by the route of exposure. These findings suggest that dietary exposure to CDs was more likely to cause environmental risk and adverse effects than aqueous exposure, and the environmental risks associated with CDs should not be underestimated.

ZnO nanoparticles interfere with top-down effect of the protozoan paramecium on removing microcystis

Under intensive human activity, sewage discharge causes eutrophication-driven cyanobacteria blooms as well as nanomaterial pollution. In biological control of harmful cyanobacteria, top-down effect of protozoan has great potentials for removing cyanobacterial populations, degrading cyanotoxins, and improving phytoplankton community. ZnO nanoparticles as a kind of emerging contaminants have attracted increasing attention because of wide application and their high bio-toxicity effects on reducing the ingestion of aquatic animals including Paramecium, thereby possibly disturbing top-down control of cyanobacteria. Therefore, this study investigated the effects of ZnO nanoparticles at environmental-relevant concentrations on the protozoan Paramecium removing toxic Microcystis. Results showed Paramecium effectively eliminated all the Microcystis, despite exposure to ZnO nanoparticles. However, their ingestion rate was significantly reduced at more than 0.1 mg L^-1 ZnO nanoparticles, thereby delaying Microcystis removal. Nevertheless, at 0.1 mg L^-1 ZnO nanoparticles, the time to Microcystis extinction decreased compared to the group without ZnO nanoparticles, because Microcystis populations were reduced under this circumstance, while ingestion rate of Paramecium was unaffected. Furthermore, ZnO nanoparticles obviously accumulated in food vacuoles of Paramecium, and the size of nanoparticles aggregates and zinc concentrations in Paramecium were increased with ZnO nanoparticles concentrations. At the end of experiment, these food vacuoles were not dissipated. Overall, these findings suggest that ZnO nanoparticles impair protozoan top-down effects through reducing Microcystis and ingestion rate as well as disturbing functions of their digestive organelles, and highlight the need to consider the interfering effects of environmental pollutants on cyanobacterial removal efficiency by protozoans in natural waters.

Uptake Route Altered the Bioavailability of Graphene in Misgurnus anguillicaudatus: Comparing Waterborne and Sediment Exposures

Numerous studies on the bioavailability of graphene-based nanomaterials relate to the water-only exposure route. However, the sediment exposure route should be the most important pathway for benthic organisms to ingest graphene, while to date little work on the bioavailability of graphene in benthic organisms has been explored. In this study, with the help of carbon-14-labeled few-layer graphene (¹⁴C-FLG), we quantificationally compared the bioaccumulation, biodistribution, and elimination kinetics of ¹⁴C-FLG in loaches via waterborne and sediment exposures. After 72 h of exposure, the accumulated ¹⁴C-FLG in loaches exposed via waterborne was 14.28 μg/g (dry mass), which was 3.18 times higher than that (4.49 μg/g) exposed via sediment. The biodistribution results showed that, compared to waterborne exposure, sediment exposure remarkably facilitated the transport of ¹⁴C-FLG from the gut into the liver, which made it difficult to be excreted. Although ¹⁴C-FLG did not cause significant hepatotoxicity, the disruption of intestinal microbiota homeostasis, immune response, and several key metabolic pathways in the gut were observed, which may be due to the majority of ¹⁴C-FLG being accumulated in the gut. Overall, this study reveals the different bioavailabilities of graphene in loaches via waterborne and sediment exposures, which is helpful in predicting its bioaccumulation capability and trophic transfer ability.

Water Chemistry, Exposure Routes, and Metal Forms Determine the Bioaccumulation Dynamics of Silver (Ionic and Nanoparticulate) in Daphnia magna

Treatment wetlands utilize various physical and biological processes to reduce levels of organic contaminants, metals, bacteria, and suspended solids. Silver nanoparticles (AgNPs) are one type of contaminant that can enter treatment wetlands and impact the overall treatment efficacy. Grazing by filter-feeding zooplankton, such as Daphnia magna, is critical to treatment wetland functioning; but the effects of AgNPs on zooplankton are not fully understood, especially at environmentally relevant concentrations. We characterized the bioaccumulation kinetics of dissolved and nanoparticulate (citrate-coated) ¹⁰⁹ Ag in D. magna exposed to environmentally relevant ¹⁰⁹ Ag concentrations (i.e., 0.2-23 nmol L^-1 Ag) using a stable isotope as a tracer of Ag. Both aqueous and nanoparticulate forms of ¹⁰⁹ Ag were bioavailable to D. magna after exposure. Water chemistry affected ¹⁰⁹ Ag influx from ¹⁰⁹ AgNP but not from ¹⁰⁹ AgNO₃ . Silver retention was greater for citrate-coated ¹⁰⁹ AgNP than dissolved ¹⁰⁹ Ag, indicating a greater potential for bioaccumulation from nanoparticulate Ag. Feeding inhibition was observed at higher dietary ¹⁰⁹ Ag concentrations, which could lead to reduced treatment wetland performance. Our results illustrate the importance of using environmentally relevant concentrations and media compositions when predicting Ag bioaccumulation and provide insight into potential effects on filter feeders critical to the function of treatment wetlands. Environ Toxicol Chem 2022;41:726-738. © 2021 SETAC.

The ZrO2 NPs enhanced the risk of arsenate by promoting its accumulation and reducing its detoxification during food chain transfer from Daphnia magna to zebrafish

Arsenic (As) can co-occur with zirconium dioxide nanoparticles (ZrO₂ NPs) in aquatic environments, but their combined influence along the aquatic food chain is barely explored. This study constructed water flea Daphnia magna - zebrafish Danio rerio to evaluate the impact of ZrO₂ NPs on the accumulation, trophic transfer, transformation, and detoxification of arsenate (As(V)). The zebrafish were fed D. magna exposed to As(V), ZrO₂ NPs, or As(V) + ZrO₂ NPs for 20 d. Results demonstrated that ZrO₂ NPs significantly facilitated total As and As(V) sorption in D. magna and in tissues of zebrafish. ZrO₂ NPs enhanced the transformation of inorganic arsenic (iAs) to monomethylated acid (MMA), while decreased synthesis of arsenobetaine (AsB) in tissues, leading to iAs increased. Co-exposed As(V) and ZrO₂ NPs facilitated upregulation of absorption-related genes (aqp7) and As biotransformation-related genes (gst, gss), and detoxification and oxidative stress-related genes (mt2, cat, sod1 and sod2). Therefore, genetic expression coupling with biotransformation for the first time demonstrated that As(V) combined with ZrO₂ NPs led to increased harm to D. magna and zebrafish and amplified the ecological risks of As(V) along the aquatic food chain. Attention should be paid to the combined toxicity of As(V) and ZrO₂ NPs in aquatic environment.

Bacteria compete with hematite nanoparticles during their uptake by the ciliate Tetrahymena thermophila

Bacterial accumulation of engineered nanoparticles (NPs) result in their transfer along the food chain. However, there are a lot of NPs not associated with bacteria. Whether bacteria, as representative biotic particles, influence the biological uptake of these non-associated NPs in aquatic ecosystems is unclear. In the present study, we examined the effects of four bacterial species on the uptake kinetics of polyacrylate-coated hematite nanoparticles (HemNPs) by the ciliate Tetrahymena thermophila. The HemNPs were well dispersed. Their adsorption on the bacteria was low with negligible uptake by T. thermophila through bacterial ingestion. This result demonstrated the feasibility of examining the effects of bacteria on the uptake of non-associated HemNPs. Our study further showed that all four bacterial species inhibited the uptake of HemNPs by T. thermophila; however, the effects of the bacterial cells on the physiological activities of the ciliate with respect to its uptake of HemNPs were negligible. In the absence of phagocytosis by T. thermophila, none of the bacteria inhibited HemNP uptake. This observation suggested that bacterial cells competed with the HemNPs for uptake via phagocytosis. Therefore, in evaluations of the environmental risks of NPs, their competition with biotic particles should be taken into account.

Waterborne and dietary accumulation of well-dispersible hematite nanoparticles by zebrafish at different life stages

The widespread use of nanoparticles (NPs) has drawn considerable attention because of their potential toxicity and the environmental consequences thereof. However, the effects of the exposure route and life stage of an organism on the bioaccumulation and toxicity of NPs are largely unknown. In the present study, we investigated the accumulation kinetics (uptake, assimilation, and efflux) and tissue distribution of waterborne and dietary hematite NPs (HemNPs) during three life stages (embryo, larva, and adult) of the zebrafish Danio rerio. For all zebrafish life stages, the waterborne accumulation of well-dispersed HemNPs increased linearly with exposure time but decreased after reaching a maximum. The increase in HemNPs accumulation followed the order embryo > larva > adult. Compared with the waterborne route, the dietary accumulation of HemNPs in larval and adult zebrafish fluctuated, reaching a maximum after each food refreshment and then decreasing until the next food addition. Similar to waterborne exposure, adult fish accumulated less dietary HemNPs than did larvae. Nevertheless, dietary HemNPs mostly accumulated in the intestinal tract, with smaller amounts in the truncus, head, and gills, as compared with their waterborne counterparts. Moreover, in the gonad no dietary HemNPs were detected whereas accumulation via waterborne HemNPs was significant. Despite the low assimilation efficiency of dietary HemNPs, biodynamic modeling showed that the diet was the main source of particle accumulation in zebrafish. Thus, both the life stage and the exposure route should be considered in evaluations of the environmental risks of NPs.

Physiological endpoints in daphnid acute toxicity tests

Daphnids are freshwater crustaceans used in toxicity tests. Although lethality and immobilisation are the most commonly used endpoints in those tests, more sensitive parameters are required for determination of sublethal acute effects of toxicants. The use of various physiological endpoints in daphnids is considered as a low-cost and simple alternative that meets the 3R's rule (Replacement, Reduction, Refinement) criteria. However, currently there is no review-based evaluation of their applicability in toxicity testing. This paper presents the results on the most commonly determined physiological parameters of Daphnia in ecotoxicological studies and human drug testing, such as feeding activity, thoracic limb movement, heart rate, cardiac area, respiratory activity, compound eye, mandible movements and post-abdominal claw contractions. Furthermore, their applicability as promising endpoints in the assessment of water quality or drug testing is discussed.