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.

Molecular assembly of extracellular polymeric substances regulating aggregation of differently charged nanoplastics and subsequent interactions with bacterial membrane.

Extracellular polymeric substances (EPS) represent an interface between microbial cells and aquatic environment, where nanoplastics acquire coatings to alter their fate and toxicity. However, little is known about molecular interactions governing modification of nanoplastics at biological interfaces. Molecular dynamics simulations combining experiments were conducted to investigate assembly of EPS and its regulatory roles in the aggregation of differently charged nanoplastics and interactions with bacterial membrane. Driven by hydrophobic and electrostatic interactions, EPS formed micelle-like supramolecular structures with hydrophobic core and amphiphilic exterior. Different components, depending on their hydrophobicity and charge, were found to promote or suppress EPS assembly. Neutral and hydrophobic nanoplastics showed unbiased adsorption of EPS species, while cationic and anionic nanoplastics were distinct and attracted specific molecules of opposite charges. Compared with isolated EPS, assembled EPS concealed hydrophobic groups to be less adsorbed by nanoplastics. Aggregation of nanoplastics was alleviated by EPS due to electrostatic repulsion plus steric hindrance. ESP suppressed binding of cationic nanoplastics to the bacterial membrane through reducing the surface charge. Neutral and anionic nanoplastics showed weak membrane association, but their binding interactions were promoted by EPS. The structural details revealed here provided molecular level insights into modifications of nanoplastics at the eco-environment interface.

Benzo[a]pyrene and heavy metal ion adsorption on nanoplastics regulated by humic acid: Cooperation/competition mechanisms revealed by molecular dynamics simulations.

Nanoplastics adsorb pollutants and organic matter to aggravate or alleviate impact to the eco-environment and human health. However, the interaction mechanisms remain unclear and difficult to study using current experimental techniques. By means of molecular dynamics simulation, here we investigate adsorption of benzo[a]pyrene (BaP) and heavy metal ions (Cu2+) on nanoplastics of different materials and surface charges regulated by humic acid (HA). Among considered materials, polystyrene shows the highest capacity of adsorbing BaPs via forming sandwiched π-stacking structures with benzene rings. Driven by hydrophobic, electrostatic and hydrogen bonding interactions, HAs spontaneously aggregate into micelle-like structures with hydrophobic core and charged exterior accessible to BaPs and Cu2+, respectively. Cationic and neutral nanoplastics adsorb more HAs to form eco-coronas, which modulate BaP and Cu2+ adsorption via following cooperation/competition mechanisms. On one hand, the direct binding of BaPs to nanoplastics is hindered by HAs through BaP encapsulation plus competitive adsorption. On the other hand, adsorbed HAs expose carboxyl groups to offer rich binding sites to promote Cu2+ adsorption on neutral and cationic nanoplastics, while unbound HAs compete with anionic nanoplastics to inhibit Cu2+ adsorption. These results provide molecular level insights into transport, transformation and accessibility of nanoplastics with coexisting contaminants in the aqueous environment.

Molecular modeling of nanoplastic transformations in alveolar fluid and impacts on the lung surfactant film.

Airborne nanoplastics can be inhaled to threaten human health, but research on the inhaled nanoplastic toxicity is in its infancy, and interaction mechanisms are largely unknown. By means of molecular dynamics simulation, we employed spherical nanoplastics of different materials and aging properties to predict and elucidate nanoplastic transformations in alveolar fluid and impacts on the lung surfactant (LS) film at the alveolar air-water interface. Results showed spontaneous adsorption of LS molecules on nanoplastics of 10 nm in diameter, and the adsorption layer can be defined as coronas, which increased the particle size, reduced and equalized the surface hydrophobicity, and endowed nanoplastics with negative surface charges. Nanoplastics of polypropylene and polyvinylchloride materials were dissolved by LS, which could increase bioavailability of polymers and toxic additives. Aging properties represented by the nanoplastic size, polymer's molecular weight and surface chemistry altered nanoplastic transformations through modulating competition between polymer-LS and polymer-polymer interactions. Upon transferred to the alveolar air-water interface through vesicle fusion, nanoplastics could interfere with the normal biophysical function of LS through disrupting the LS ultrastructure and fluidity, and prompting collapse of the LS film. These results provide new molecular level insights into fate and toxicity of airborne nanoplastics in human respiratory system.