A trophic transfer study: accumulation of multi-walled carbon nanotubes associated to green algae in water flea Daphnia magna

Toxicity and Biotransformation of Carbon-Based Nanomaterials in Marine Microalgae Heterosigma akashiwo

This work is related to the environmental toxicology risk assessment and evaluation of the possible transformation of carbon-based nanomaterials (CNMs) after contact with marine microalgae. The materials used in the study represent common and widely applied multi-walled carbon nanotubes (CNTs), fullerene (C₆₀), graphene (Gr), and graphene oxide (GrO). The toxicity was evaluated as growth rate inhibition, esterase activity, membrane potential, and reactive oxygen species generation changes. The measurement was performed with flow cytometry after 3, 24, 96 h, and 7 days. The biotransformation of nanomaterials was evaluated after 7 days of microalgae cultivation with CNMs by FTIR and Raman spectroscopy. The calculated toxic level (EC₅₀ in mg/L, 96 h) of used CNMs reduced in the following order: CNTs (18.98) > GrO (76.77) > Gr (159.40) > C₆₀ (414.0). Oxidative stress and membrane depolarization were the main toxic action of CNTs and GrO. At the same time, Gr and C₆₀ decreased the toxic action with time and had no negative impact on microalgae after 7 days of exposure even at the concentration of 125 mg/L. Moreover, C₆₀ and Gr after 7 days of contact with microalgae cells obtained structural deformations.

Evaluation of bioaccumulation of nanoplastics, carbon nanotubes, fullerenes, and graphene family materials

Bioaccumulation is a key factor in understanding the potential ecotoxicity of substances. While there are well-developed models and methods to evaluate bioaccumulation of dissolved organic and inorganic substances, it is substantially more challenging to assess bioaccumulation of particulate contaminants such as engineered carbon nanomaterials (CNMs; carbon nanotubes (CNTs), graphene family nanomaterials (GFNs), and fullerenes) and nanoplastics. In this study, the methods used to evaluate bioaccumulation of different CNMs and nanoplastics are critically reviewed. In plant studies, uptake of CNMs and nanoplastics into the roots and stems was observed. For multicellular organisms other than plants, absorbance across epithelial surfaces was typically limited. Biomagnification was not observed for CNTs and GFNs but were observed for nanoplastics in some studies. However, the reported absorption in many nanoplastic studies may be a consequence of an experimental artifact, namely release of the fluorescent probe from the plastic particles and subsequent uptake. We identify that additional work is needed to develop analytical methods to provide robust, orthogonal methods that can measure unlabeled (e.g., without isotopic or fluorescent labels) CNMs and nanoplastics.

Impact of weathered multi-walled carbon nanotubes on the epithelial cells of the intestinal tract in the freshwater grazers Lymnaea stagnalis and Rhithrogena semicolorata

Freshwater grazers are suitable organisms to investigate the fate of environmental pollutants, such as weathered multi-walled carbon nanotubes (wMWCNTs). One key process is the uptake of ingested materials into digestive or absorptive cells. To address this, we investigated the localization of wMWCNTs in the intestinal tracts of the mud snail Lymnaea stagnalis (L. stagnalis) and the mayfly Rhithrogena semicolorata (R. semicolorata). In L. stagnalis, bundles of wMWCNTs could be detected in the midgut lumen, whereas only single wMWCNTs could be detected in the lumina of the digestive gland. Intracellular uptake of wMWCNTs was detected by transmission electron microscopy (TEM) but was restricted to the cells of the digestive gland. In larvae of R. semicolorata, irritations of the microvilli and damages in the apical parts of the epithelial gut cells were detected after feeding with 1 to 10 mg/L wMWCNTs. In both models, we detected fibrillar structures in close association with the epithelial cells that formed peritrophic membranes (PMs). The PM may cause a reduced transmission of wMWCNT bundles into the epithelium by forming a filter barrier and potentially protecting the cells from the wMWCNTs. As a result, the uptake of wMWCNTs into cells is rare in mud snails and may not occur at all in mayfly larvae. In addition, we monitor physiological markers such as levels of glycogen or triglycerides and the RNA/DNA ratio. This ratio was significantly affected in L. stagnalis after 24 days with 10 mg/L wMWCNTs, but not in R. semicolorata after 28 days and 10 mg/L wMWCNTs. However, significant effects on the energy status of R. semicolorata were analysed after 28 days of exposure to 1 mg/L wMWCNTs. Furthermore, we observed a significant reduction of phagosomes per enterocyte cell in mayfly larvae at a concentration of 10 mg/L wMWCNTs (p < 0.01).

Colloidal Behavior and Biodegradation of Engineered Carbon-Based Nanomaterials in Aquatic Environment

Carbon-based nanomaterials (CNMs) have attracted a growing interest over the last decades. They have become a material commonly used in industry, consumer products, water purification, and medicine. Despite this, the safety and toxic properties of different types of CNMs are still debatable. Multiple studies in recent years highlight the toxicity of CNMs in relation to aquatic organisms, including bacteria, microalgae, bivalves, sea urchins, and other species. However, the aspects that have significant influence on the toxic properties of CNMs in the aquatic environment are often not considered in research works and require further study. In this work, we summarized the current knowledge of colloidal behavior, transformation, and biodegradation of different types of CNMs, including graphene and graphene-related materials, carbon nanotubes, fullerenes, and carbon quantum dots. The other part of this work represents an overview of the known mechanisms of CNMs' biodegradation and discusses current research works relating to the biodegradation of CNMs in aquatic species. The knowledge about the biodegradation of nanomaterials will facilitate the development of the principals of "biodegradable-by-design" nanoparticles which have promising application in medicine as nano-carriers and represent lower toxicity and risks for living species and the environment.

Trophic transfer and toxicity of silver nanoparticles along a phytoplankton-zooplankton-fish food chain

This study evaluated the bioconcentration metrics, organ-specific distribution, and trophic consequences of silver nanoparticles along a Dunaliella salina-Artemia salina-Poecilia reticulata food chain. To this end, accumulation, tissue-specific distribution, bioconcentration and biomagnification factors, and trophic toxicity of AgNPs were quantitatively investigated along di- and tri-trophic food chains. Overall, silver accumulation increased markedly in intestine and liver tissues, carcass, and embryos of guppy fish with rising exposure concentrations and reducing trophic levels. Following trophic and waterborne exposure, AgNPs illustrated a regular tendency in following order: intestine > liver > embryos > carcass. BCF displayed values of 826, 131, and ≈ 1000 for microalgae, brine shrimp, and guppy fish, respectively. Moreover, BMF showed values <1.00 for 48-h post-hatched nauplii and guppy fish received AgNPs-exposed phytoplankton, yet >1.00 for the liver and whole body of guppy fish treated with AgNPs-exposed nauplii through algae and water, indicating that AgNPs could be biomagnified from the second to third trophic level, but not from the first to second or third levels. Furthermore, the waterborne and trophic exposure of AgNPs considerably induced oxidative stress and reproductive toxicity. Together, this study demonstrated that AgNPs could be biomagnified across trophic chain and consequently cause trophic toxicity.

Toxicity assessment of TiO2-conjugated Carbon-based nanohybrid material on a freshwater bioindicator cladoceran, Daphnia magna

The application of nanocomposite materials fabricated from titanium dioxide nanoparticles (TiO₂ NPs) and different carbon (C) allotropes have gained popularity in water treatment applications due to their synergistic properties. Studies to date have focused on simple forms of nanomaterials (NMs), however, with the technology development, there is a dramatic increase in production and application of these complex NMs which could result in toxicological impacts on organisms when released into aquatic environments. This raises serious concerns about their safety and the need to ascertain their potential adverse effects on aquatic organisms. While conjugated TiO₂ NPs/carbon-based nanohybrids (TiO₂/C-NHs) may exhibit enhanced photocatalytic activity, there is no research in the scientific community regarding their toxicological effects on D. magna, which are indicators of freshwater pollution. In this study, two under-represented TiO₂/C-NHs (i.e., TiO₂- conjugated carbon nanofiber (CNF), and TiO₂-conjugated multi-walled carbon nanotube (CNT)) were investigated for their toxic effects on D. magna, through a series of acute toxicity tests with a set of sublethal biochemical biomarkers of oxidative stress. The lethal toxicity and oxidative stress formation of TiO₂/C-NHs over 48 h revealed a concentration-dependant increase in D. magna mortality. The primary mechanism identified was the generation of ROS, which was in line with toxicity results. Light microscopy and CytoViva® images visualized D. magna interaction with the NPs, which accumulated and appeared as dark materials in the lines of the gut tract. The collective results indicate that TiO₂/C-NHs have the potential to cause an effect on freshwater organisms when released into the environment. However, the relevance of TiO₂/C-NHs effects needs further chronic toxicity studies since they show promise to be used in nano-bioremediation materials to treat wastewaters.

Multiwalled Carbon Nanotubes Promote Bacterial Conjugative Plasmid Transfer

Multiwalled carbon nanotubes (MWCNTs) regularly enter aquatic environments due to their ubiquity in consumer products and engineering applications. However, the effects of MWCNT pollution on the environmental microbiome are poorly understood. Here, we evaluated whether these carbon nanoparticles can elevate the spread of antimicrobial resistance by promoting bacterial plasmid transfer, which has previously been observed for copper nanomaterials with antimicrobial properties as well as for microplastics. Through a combination of experimental liquid mating assays between Pseudomonas putida donor and recipient strains with plasmid pKJK5::gfpmut3b and mathematical modeling, we here demonstrate that the presence of MWCNTs leads to increased plasmid transfer rates in a concentration-dependent manner. The percentage of transconjugants per recipient significantly increased from 0.21 ± 0.04% in absence to 0.41 ± 0.09% at 10 mg L-1 MWCNTs. Similar trends were observed when using an Escherichia coli donor hosting plasmid pB10. The identified mechanism underlying the observed dynamics was the agglomeration of MWCNTs. A significantly increased number of particles with >6 μm diameter was detected in the presence of MWCNTs, which can in turn provide novel surfaces for bacterial interactions between donor and recipient cells after colonization. Fluorescence microscopy confirmed that MWCNT agglomerates were indeed covered in biofilms that contained donor bacteria as well as elevated numbers of green fluorescent transconjugant cells containing the plasmid. Consequently, MWCNTs provide bacteria with novel surfaces for intense cell-to-cell interactions in biofilms and can promote bacterial plasmid transfer, hence potentially elevating the spread of antimicrobial resistance. IMPORTANCE In recent decades, the use of carbon nanoparticles, especially multiwalled carbon nanotubes (MWCNTs), in a variety of products and engineering applications has been growing exponentially. As a result, MWCNT pollution into environmental compartments has been increasing. We here demonstrate that the exposure to MWCNTs can affect bacterial plasmid transfer rates in aquatic environments, an important process connected to the spread of antimicrobial resistance genes in microbial communities. This is mechanistically explained by the ability of MWCNTs to form bigger agglomerates, hence providing novel surfaces for bacterial interactions. Consequently, increasing pollution with MWCNTs has the potential to elevate the ongoing spread of antimicrobial resistance, a major threat to human health in the 21st century.

A workflow to investigate the impacts of weathered multi-walled carbon nanotubes to the mud snail Lymnaea stagnalis

Although the development and application of nanomaterials is a growing industry, little data is available on the ecotoxicological effects on aquatic organisms. Therefore, we set up a workflow to address the potential uptake of weathered multi-walled carbon nanotubes (wMWCNTs) by a model organism, the pulmonary mud snail Lymnaea stagnalis (L. stagnalis), which plays an important role in the food web. It represents a suitable organism for this approach because as a grazer it potentially ingests large amounts of sedimented wMWCNTs. As food source for L. stagnalis, benthic biofilm was investigated by the use of a transmission electron microscope (TEM) and a scanning electron microscope (SEM) after exposure with wMWCNTs. In addition, isotopic labeling was applied with ¹⁴C-wMWCNTs (0.1 mg/L) to quantify fate, behavior, and enrichment of ¹⁴C-wMWCNTs in benthic biofilm and in L. stagnalis. Enrichment in benthic biofilm amounted to 529.0 µg wMWCNTs/g dry weight and in L. stagnalis to 79.6 µg wMWCNTs/g dry weight. A bioconcentration factor (BCF) for L. stagnalis was calculated (3500 L/kg). We demonstrate the accumulation of wMWCNTs (10 mg/L) in the digestive tract of L. stagnalis in an effect study. Moreover, the physiological markers glycogen and triglycerides as indicators for the physiological state, as well as the RNA/DNA ratio as growth indicator, were examined. No significant differences between exposed and control animals were analyzed for glycogen and triglycerides after 24 days of exposure, but a decreasing trend is recognizable for triglycerides. In contrast, the significant reduction in the RNA/DNA ratio of L. stagnalis indicated an inhibition of growth with a following recovery after depuration. The described workflow enables a comprehensive determination of the fate and the behavior of wMWCNTs specifically and in general all kinds of CNTs in the aquatic environment and therefore contributes to a holistic risk assessment of wMWCNTs.