Effects of aqueous stable fullerene nanocrystals (nC60) on the food conversion from Daphnia magna to Danio rerio in a simplified freshwater food chain

Daphnia as a model organism to probe biological responses to nanomaterials-from individual to population effects via adverse outcome pathways

The importance of the cladoceran Daphnia as a model organism for ecotoxicity testing has been well-established since the 1980s. Daphnia have been increasingly used in standardised testing of chemicals as they are well characterised and show sensitivity to pollutants, making them an essential indicator species for environmental stress. The mapping of the genomes of D. pulex in 2012 and D. magna in 2017 further consolidated their utility for ecotoxicity testing, including demonstrating the responsiveness of the Daphnia genome to environmental stressors. The short lifecycle and parthenogenetic reproduction make Daphnia useful for assessment of developmental toxicity and adaption to stress. The emergence of nanomaterials (NMs) and their safety assessment has introduced some challenges to the use of standard toxicity tests which were developed for soluble chemicals. NMs have enormous reactive surface areas resulting in dynamic interactions with dissolved organic carbon, proteins and other biomolecules in their surroundings leading to a myriad of physical, chemical, biological, and macromolecular transformations of the NMs and thus changes in their bioavailability to, and impacts on, daphnids. However, NM safety assessments are also driving innovations in our approaches to toxicity testing, for both chemicals and other emerging contaminants such as microplastics (MPs). These advances include establishing more realistic environmental exposures via medium composition tuning including pre-conditioning by the organisms to provide relevant biomolecules as background, development of microfluidics approaches to mimic environmental flow conditions typical in streams, utilisation of field daphnids cultured in the lab to assess adaption and impacts of pre-exposure to pollution gradients, and of course development of mechanistic insights to connect the first encounter with NMs or MPs to an adverse outcome, via the key events in an adverse outcome pathway. Insights into these developments are presented below to inspire further advances and utilisation of these important organisms as part of an overall environmental risk assessment of NMs and MPs impacts, including in mixture exposure scenarios.

Alleviative Effects of C60 on the Trophic Transfer of Cadmium along the Food Chain in Aquatic Environment

C₆₀ could enhance the accumulation of pollutants in organisms, but their effects on higher trophic levels remain unknown. In the present study, the transfer of C₆₀ from Daphnia magna to zebrafish (Danio rerio) and its effects on Cd transfer were investigated. The results showed that C₆₀ could be transferred from D. magna to zebrafish through dietary exposure and accumulate mainly in the intestines, but biomagnification was not observed. The presence of C₆₀ promoted accumulation of Cd in D. magna. However, it decreased Cd burden in the higher trophic level (zebrafish), displaying an alleviative effect on the trophic transfer of Cd along the food chain. To explore the underlying mechanisms, the release of Cd from D. magna in digestive fluids and changes in zebrafish digestive physiology were further investigated. The results showed that C₆₀ did not inhibit Cd release from D. magna, but stimulated the digestive tracts of zebrafish to excrete Cd earlier and in a greater amount, which consequently lowered assimilation efficiency of Cd in zebrafish. Overall, the present study showed the trophic transfer of C₆₀ in the aquatic food chain and revealed the effects of C₆₀ on trophic transfer of Cd along the food chain in 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.

Mechanisms underlying the acute toxicity of fullerene to Daphnia magna: Energy acquisition restriction and oxidative stress

The toxicity of fullerene (C₆₀) to Daphnia magna has been a subject with increasing concerns. Nevertheless, the underlying mechanisms are still poorly understood. In the present study, we evaluated various aspects of the toxicological impacts of C₆₀ on daphnia. After a 72-h exposure, the 50% effective concentration of C₆₀ was 14.9 mg/L for immobilization, and 16.3 mg/L for mortality. Daphnia exhibited a quick uptake of C₆₀ with a body burden value of 413 μg/g in wet weight in the 1 mg/L C₆₀ treatment group. Transmission electron microscopy observations revealed that C₆₀ had mainly accumulated in the guts of organisms. The feeding rate, gut ultra-structural alterations, and digestive enzyme activities of daphnia in response to C₆₀ treatment were evaluated. The results revealed a significant reduction in the digestion and filtration rates, as well as gut impairment and inhibition of digestive enzymes (cellulose, amylase, trypsin, and β-galactosidase) activity of C₆₀ exposed daphnia. In addition, the changes in superoxide dismutase (SOD) and malondialdehyde (MDA) levels in daphnia under C₆₀ exposures were also discovered. These results, for the first time, provide systematic evidence that C₆₀ caused a restriction in energy acquisition and increased oxidative damage in daphnia, which might be related to the bioaccumulation of C₆₀ and finally led to the immobility and mortality.

Biological effect of aqueous C60 aggregates on Scenedesmus obliquus revealed by transcriptomics and non-targeted metabolomics

This work evaluated biological effect of nC60 on Scenedesmus obliquus. The cells were exposed to various concentrations of nC60 for 7days. Low-dose of nC60 was found to have a minor growth inhibitory effect. The transcriptomics and metabolomics were integrated to examine intricate molecular and cellular effects of nC60 on Scenedesmus obliquus. We found that Scenedesmus obliquus cells exposed to nC60 had several significant alterations in cellular transcription and biochemical processes. During the 7-day exposure to nC60, 2234 and 2,448 unigenes were differentially expressed by 0.1mg/L and 1mg/L nC60-treated groups compared with the control, including 2085 or 2247 up-regulated genes and 149 or 201 down-regulated genes, respectively. We successfully identified 22 metabolites, including 6 significantly changed metabolites, such as sucrose, d-glucose, and malic acid. The citrate cycle (TCA cycle) (ko00020) was the main target of both differentially expressed genes and metabolic change. However, accumulation of sucrose (end-product) could have induced feedback inhibition of photosynthesis in Scenedesmus obliquus, explaining the slight growth inhibition observed. The results provided a mechanistic understanding of the growth inhibition of nC60 toxicity. These genes and metabolites are useful biomarkers for future studies and offer new insights into the early detectable changes in Scenedesmus obliquus with nC60 exposure.

Fullerene inhibits benzo(a)pyrene Efflux from Cyprinus carpio hepatocytes by affecting cell membrane fluidity and P-glycoprotein expression

P-Glycoprotein (P-gp) can protect cells by pumping out toxic compounds, and has been found widely expressed in fish tissues. Here, we illustrate the P-gp efflux ability for benzo(a)pyrene (BaP) in the hepatocytes of common carp (Cyprinus carpio) after exposing to fullerene aqueous suspension (nC60). The results revealed that nC60 increased the membrane fluidity by decreasing the ratio of saturated to unsaturated fatty acids, and increased the cholesterol contents. These findings, combined with 10-38% and 70-75% down-regulation of P-gp mRNA and protein respectively, suggested that nC60 caused inhibition on P-gp efflux transport system. Therefore, we further investigated the cellular efflux ability for BaP. Results showed unequivocally that nC60 is a potent P-gp inhibitor. The retaining BaP amounts after efflux were elevated by 1.7-2.8 fold during the 10 day exposure. Meanwhile, 5mg/L humic acid (one of the important fractions of natural organic matter, which is ubiquitous in aquatic environment) alleviated the nC60 damage to hepatocytes in terms of oxidative damage, cholesterol increment, and P-gp content reduction; and finally attenuated the suppressed P-gp efflux ability. Collectively, this study provides the first evidence of nC60 toxicity to P-gp functionality in fish and illustrates the possible mechanism of the suppressed P-gp efflux ability for BaP.