Microplastic fragmentation by rotifers in aquatic ecosystems contributes to global nanoplastic pollution.

The role of aquatic organisms in the biological fragmentation of microplastics and their contribution to global nanoplastic pollution are poorly understood. Here we present a biological fragmentation pathway that generates nanoplastics during the ingestion of microplastics by rotifers, a commonly found and globally distributed surface water zooplankton relevant for nutrient recycling. Both marine and freshwater rotifers could rapidly grind polystyrene, polyethylene and photo-aged microplastics, thus releasing smaller particulates during ingestion. Nanoindentation studies of the trophi of the rotifer chitinous mastax revealed a Young's modulus of 1.46 GPa, which was higher than the 0.79 GPa for polystyrene microparticles, suggesting a fragmentation mechanism through grinding the edges of microplastics. Marine and freshwater rotifers generated over 3.48 × 105 and 3.66 × 105 submicrometre particles per rotifer in a day, respectively, from photo-aged microplastics. Our data suggest the ubiquitous occurrence of microplastic fragmentation by different rotifer species in natural aquatic environments of both primary and secondary microplastics of various polymer compositions and provide previously unidentified insights into the fate of microplastics and the source of nanoplastics in global surface waters.

Mechanistic Understanding toward the Maternal Transfer of Nanoplastics in Daphnia magna.

Nanoplastics induce transgenerational toxicity to aquatic organisms, but the specific pathways for the maternal transfer of nanoplastics remain unclear. Herein, laser scanning confocal microscopy (LSCM) observations identified the specific pathways on the maternal transfer of polystyrene (PS) nanoplastics (25 nm) in Daphnia magna. In vivo and in vitro experiments showed that PS nanoplastics could enter the brood chamber through its opening and then be internalized to eggs and embryos using LSCM imaging (pathway I). In addition, PS nanoplastics were observed in the oocytes of the ovary, demonstrating gut-ovary-oocyte transfer (pathway II). Furthermore, label-free hyperspectral imaging was used to detect the distribution of nanoplastics in the embryos and ovary of Daphnia, again confirming the maternal transfer of nanoplastics through the two pathways mentioned above. The contribution from pathway I (88%) was much higher than pathway II (12%) based on nanoflow cytometry quantification. In addition, maternal transfer in Daphnia depended on the particle size of PS nanoplastics, as demonstrated by using LSCM and hyperspectral imaging. Unlike 25 nm nanoplastics, 50 nm PS nanoplastics could enter the brood chamber and the eggs/embryos (pathway I), but were not detected in the ovary (pathway II); 100 nm PS nanoplastics were difficult to be internalized by eggs/embryos and could not enter the ovary either. These findings provide insight into the maternal transfer mechanisms of nanoplastics in Daphnia, and are critical for better understanding the transgenerational toxicity of aquatic organisms.

Transfer of CeO2 nanoparticles between freshwater omnivorous organisms: Effect of feces and necrophagy.

Transfer of CeO2 engineered nanoparticles (NPs) through feces was investigated between two omnivorous organisms, red crucian carp (Carassius auratus red var.) and crayfish (Procambarus clarkii). Upon water exposure (5 mg/L, 7 days), the highest bioaccumulation was observed in carp gills (5.95 µg Ce/g D.W.) and crayfish hepatopancreas (648 µg Ce/g D.W.), with the bioconcentration factors (BCFs) at 0.45 and 3.61, respectively. In addition, 97.4% and 73.0% of ingested Ce were excreted by carp and crayfish, respectively. The feces of carp and crayfish were collected and fed to crayfish and carp, respectively. After feces exposure, bioconcentration was observed in both carp (BCF, 3.00) and crayfish (BCF, 4.56). After feeding crayfish with carp bodies (1.85 µg Ce/g D.W.), CeO2 NPs were not biomagnified (biomagnification factor, 0.28). Upon water exposure, CeO2 NPs were transformed into Ce(III) in the feces of both carp (24.6%) and crayfish (13.6%), and the transformation was stronger after subsequent feces exposure (100% and 73.7%, respectively). Feces exposure lowered histopathological damage, oxidative stress, and nutritional quality (e.g., crude proteins, microelements, amino acids) to carp and crayfish in comparison with water exposure. This research highlights the importance of feces exposure on the transfer and fate of NPs in aquatic ecosystems.

Heteroaggregation between graphene oxide and titanium dioxide particles of different shapes in aqueous phase.

Graphene oxide (GO) inevitably interacts with engineered and/or natural nanoparticles (NPs) after releasing into aquatic environments. It is observed that GO could form heteroaggregates with differently shaped TiO2 NPS at pHs 4.0 and 7.0 rather than pH 10.0. Optimal pHs for the maximal heteroaggregation were around 6.63 and 4.92 for TiO2 spheres and fibers, respectively, which was dominated by electrostatic attraction. The optimal concentration ratio for TiO2 spheres was 10 times higher than TiO2 fibers, due to cross-linking configuration and stronger van der Waals force of TiO2 fibers with GO. Pre-homoaggregation of TiO2 NPs could promote subsequent heteroaggregation while the promotion was negligible at high ionic strength (e.g., > critical coagulant concentrations), and a longer pre-homoaggregation time showed a stronger promotion (750 s > 150 s > 0 s). Natural organic matter (NOM) obviously inhibited GO-TiO2 heteroaggregation at pH 4.0 compared with pH 7.0 due to stronger steric hindrance. Moreover, strong disturbance (e.g., sonication) could break down the heteroaggregates, and the disaggregation and re-aggregation processes were shape-independent. This work provides new insights into the role of NPs shapes and homoaggregation on the interaction of GO with both engineered and natural particles/minerals, and the fate of GO in aquatic environments.