Differentially Charged Nanoplastics Induce Distinct Effects on the Growth and Gut of Benthic Insects (Chironomus kiinensis) via Charge-Specific Accumulation and Perturbation of the Gut Microbiota.

Nanoplastics (NPs), as an emerging contaminant, have usually been found charged in the environment, posing threats to aquatic animals. However, the underlying mechanisms governing the gut toxicity of differentially charged NPs to benthic insects are not well understood. In this study, the gut toxicity in larvae of Chironomus kiinensis exposed to negatively charged NPs (PS-COOH, 50 nm) and positively charged NPs (PS-NH2, 50 nm) at 0.1 and 1 g/kg was investigated through fluorescence imaging, histopathology, biochemical approaches, and 16S rRNA sequencing. The results showed that PS-NH2 caused more adverse effect on the larval growth performance and induced more severe oxidative stress, epithelial damage, and inflammatory responses in the gut than PS-COOH. The stronger impact caused by PS-NH2 was because the gut accumulated PS-NH2 more readily than PS-COOH for its negatively charged cell membrane. In addition, PS-NH2 were less agglomerated compared with PS-COOH, leading to an increased interaction with gut cell membranes and microbiota. Furthermore, alpha diversity and relative abundance of the keystone microbiota related to gut barrier and nutrient absorption were markedly lower exposed to PS-NH2 than PS-COOH, indirectly exacerbating stronger gut and growth damage. This study provides novel insights into the effect mechanisms underlying differentially charged NPs on benthic insects.

Insights into adsorption behavior and mechanism of Cu(II) onto biodegradable and conventional microplastics: Effect of aging process and environmental factors.

The widespread promotion attempt of biodegradable plastics is considered as an effective solution to address conventional plastic pollution. However, the interaction of microplastics (MPs) easily broken down from biodegradable plastics with the coexisting pollutants in aquatic environments has gained less attention. Herein, we investigated the effects of the aging process and environmental factors on copper (Cu(II)) adsorption behavior by biodegradable polylactic acid and conventional polystyrene MPs. Results demonstrated that the aging process significantly altered physicochemical properties of both types of MPs, and PLA showed less resistance to aging. The aged polylactic acid MPs (aged-PLA) exhibited the far highest Cu(II) maximum adsorption capacity (7.13 mg/g) mainly due to its abundant oxygen-containing functional groups (OCFGs), followed by pristine polylactic acid (PLA, 6.08 mg/g), aged polystyrene (aged-PS, 0.489 mg/g) and pristine polystyrene (PS, 0.365 mg/g). The adsorption kinetics of Cu(II) on PLA MPs were controlled by film and intraparticle diffusion, while film diffusion governed the Cu(II) adsorption onto PS MPs. In addition to roles of rougher surface structure, greater surface area and pore filling, the complexation of OCFGs and electrostatic interaction were critical to the adsorption mechanism of aged-PLA and aged-PS, and cation-π interaction was associated with adsorption of aged-PS. Moreover, the adsorption capacity of Cu(II) on aged MPs gradually grew with the increasing pH from 4 to 7. Besides, humic acid significantly promoted the adsorption of Cu(II) at a low concentration (0-20 mg/L) due to the formation of binary mixtures of MPs-HA but inhibited the adsorption at a high concentration (50 mg/L) because of its competitive effect, suggesting the dual roles of humic acid in the adsorption process. Overall, our findings provide a better understanding of the adsorption behavior of metals on biodegradable MPs and emphasize their non-negligible risk as carriers of contaminant.

Effects of microplastics on sedimentary greenhouse gas emissions and underlying microbiome-mediated mechanisms: A comparison of sediments from distinct altitudes.

Microplastics (MPs) are emerging contaminants in aquatic ecosystems that can profoundly affect carbon and nitrogen cycling. However, the impact mechanisms of MPs on sedimentary greenhouse gas (GHG) emissions at distinct altitudes remain poorly elucidated. Here, we investigated the effects of polyvinyl chloride (PVC) and polylactic acid (PLA) on sedimentary CO2, CH4, and N2O emissions at distinct altitudes of the Yellow River. PVC increased the relative abundance of denitrifiers (e.g., Xanthobacteriaceae, Rhodocyclaceae) to promote N2O emissions, whereas PLA reduced the abundance of AOA gene and denitrifiers (e.g., Pseudomonadaceae, Sphingomonadaceae), impeding N2O emissions. Both PVC and PLA stimulated the growth of microbes (Saprospiraceae, Aquabacterium, and Desulfuromonadia) associated with complex organics degradation, leading to increased CO2 emissions. Notably, the concurrent inhibition of PLA on mcrA and pmoA genes led to its minimal impact on CH4 emissions. High-altitude MQ sediments, characterized by abundant substrate and a higher abundance of functional genes (AOA, AOB, nirK, mcrA), demonstrated higher GHG emissions. Conversely, lower microbial diversity rendered the low-altitude LJ microbial community more susceptible to PVC, leading to a more significant promotion on GHG emissions. This study unequivocally confirms that MPs exacerbate GHG emissions via microbiome-mediated mechanisms, providing a robust theoretical foundation for microplastic control to mitigate global warming.

Photoaging of biodegradable nanoplastics regulates their toxicity to aquatic insects (Chironomus kiinensis) by impairing gut and disrupting intestinal microbiota.

Biodegradable plastic, a widely used ecofriendly alternative to conventional plastic, easily form nanoplastics (NPs) upon environmental weathering. However, the effects and underlying mechanisms governing the toxicity of photoaged biodegradable NPs to aquatic insects are not understood. In this study, we investigated the photoaging of polylactic acid nanoplastics (PLA-NPs, a typical biodegradable plastic) that were placed under xenon arc lamp for 50 days and 100 days and compared the toxicity of virgin and photoaged PLA-NPs to Chironomus kiinensis (a dominant aquatic insect). The results showed that photoaged PLA-NPs significantly decreased the body weight, body length and emergence rate of C. kiinensis. Additionally, photoaged PLA-NPs induced more severe gut oxidative stress, histological damage, and inflammatory responses than virgin PLA-NPs. Furthermore, the alpha diversity of gut microbiota was lower in photoaged PLA-NPs group than virgin PLA-NPs. The relative abundance of key gut bacteria related to intestinal barrier defense, immunity, and nutrient absorption was reduced more significantly in photoaged PLA-NPs group than virgin PLA, indirectly leading to stronger gut damage and growth reduction. A stronger impact of photoaged PLA-NPs on the gut and its microbiota occurred because photoaging reduced the size of NPs from 255.5 nm (virgin PLA) to 217.1 nm (PLA-50) and 182.5 nm (PLA-100), induced surface oxidation and enhancement of oxidative potential, and improved the stability of NPs, thereby exacerbating toxicity on the gut and its microbiota. This study provides insights into the effects of biodegradable NPs on aquatic insects and highlights the importance of considering biodegradable nanoplastic aging in risk assessments.

Bioavailability of micro/nanoplastics and their associated polycyclic aromatic hydrocarbons to Daphnia Magna: Role of ingestion and egestion of plastics.

Aquatic ecosystems are ubiquitously polluted and deteriorated by micro/nanoplastics (MPs/NPs) and their associated contaminants. However, the bioavailability of MPs/NPs and their associated hydrophobic organic contaminants (HOCs) remains largely unknown. This study employs passive dosing systems to study the bioavailability of differently-sized MPs (3 and 20 µm)/NPs (80 nm) and their associated polycyclic aromatic hydrocarbons (PAHs) to Daphnia magna, a model species in aquatic ecosystem. At constant concentrations of freely dissolved PAHs, the presence of MPs/NPs raises the immobilization of D. magna to 71.1-80.0 %, far higher than their counterparts caused by PAHs (24.4 %) or MPs (20.0-24.4 %)/NPs (15.5 %). It demonstrates that the MPs/NPs-associated PAHs are bioavailable, acting as a key contributor (37.1-50.0 %) for the overall immobilization. Interestingly, although the immobilization of D. magna caused by MPs is higher than NPs, the bioavailability of MPs/NPs-associated PAHs declines with plastic size. Such a trend is due to the fact that MPs are actively ingested but hardly egested; while NPs are passively ingested and rapidly egested, leading to a continuous and higher accessibility of NPs-associated PAHs to D. magna. These findings clarify an integrated role of ingestion and egestion in controlling the bioavailability of MPs/NPs and their associated HOCs. Further, this study suggests that MPs/NPs-associated HOCs should be primarily concerned in chemical risk assessment in aquatic ecosystem. Accordingly, both ingestion and egestion of MPs/NPs by aquatic species should be addressed in future studies.

Bioavailability quantification and uptake mechanisms of pyrene associated with different-sized microplastics to Daphnia magna.

Microplastics (MPs) are the significant environmental factor for bioavailability of hydrophobic organic contaminants (HOCs) in aquatic environments. Nevertheless, the bioavailability of microplastic-associated HOCs remains unclear. In this research, the freely dissolved pyrene concentrations were kept stable with passive dosing devices, and the pyrene content in D. magna tissues as well as D. magna immobilization were analyzed to quantify bioavailability of pyrene (a representative HOC) associated with naturally-aged polystyrene (PS) MPs. Furthermore, the uptake mechanisms of pyrene associated with MPs of different sizes were explored by investigating the distribution of MPs in D. magna tissues with scanning electron microscopy. Especially, a new schematic model of bioavailability process was established. The results demonstrated that a part of pyrene associated with 0-1.5 µm MPs could directly cross cell membrane through endocytosis from intestine and exposure solutions to D. magna tissues except the 10-60 and 60-230 µm MPs. The bioavailability of microplastic-associated pyrene was ordered as 0-1.5 µm (20.0-21.6%) > 10-60 µm (10.7-13.8%) > 60-230 µm MPs (6.0-9.8%), which were essentially resulted from the difference in uptake mechanisms of pyrene associated with MPs of different sizes. This work suggests that the bioavailability of microplastic-associated HOCs should be considered when assessing water quality and environmental risk of HOCs in natural waters.

Interactions between nano/micro plastics and suspended sediment in water: Implications on aggregation and settling.

Interactions between nano/microplastics and suspended sediment (SS) in natural waters are important for the environmental fate of plastic particles. This study investigated the effect of heteroaggregation between nano/microplastics and SS on the settling of aggregates. In NaCl solutions (0.05-0.5 M), large SS (100-500 µm in diameter) significantly increased the settling ratio of polystyrene nanoplastics (PSNPs) with an average diameter of 100 nm due to the formation of PSNPs-SS aggregates. The settling ratio of the heteroaggregates increased significantly when the NaCl concentration increased from 50 to 200 mM. This was primarily because higher ionic strength reduced the electrostatic repulsion between large SS and PSNPs, and subsequently increased the heteroaggregation rate. No obvious differences in settling ratios were observed in 200 or 500 mM NaCl solutions because the heteroaggregation entered the diffusion-controlled regime. However, in HA solutions (10-50 mg L-1), the surface adsorption of HA on PSNPs and large SS reduced the heteroaggregation of PSNPs-SS and thus led to the low co-settling ratio due to the steric hindrance according to the DLVO theory. In contrast, polyethylene microplastics (PEMPs) with diameters of 1.0-1.2 mm were found to always float on water surface (up to 8 months), even after addition of 500 mg L-1 small SS (<10 µm in diameter). Clearly, the heteroaggregation of PEMPs and small SS had minor effect on the settling of PEMPs due to the overwhelming boyanccy. These results provided new insight into the fate and distribution of nano/microplastics in aquatic environment, which affect the bioavailability of plastic particles in natural waters.

Bioavailability of adsorbed phenanthrene by black carbon and multi-walled carbon nanotubes to Agrobacterium.

Carbonaceous sorbents including black carbon (BC) and carbon nanotubes have attracted research attention around the world because of their effects on bioavailability of hydrophobic organic compounds (HOCs) in sediments and soils. In this research, (14)C-labeled and unlabeled phenanthrene were spiked into three artificial sediments: (i) a sediment sample without amorphous organic carbon (OC) and with BC collected from the Yangtze River (BC-YR), (ii) a sediment without OC and with multi-walled carbon nanotubes (MWCNTs), and (iii) a sediment without OC and with fresh wood char. Biodegradation and mineralization of adsorbed phenanthrene by Agrobacterium and XAD-2 assisted abiotic desorption of adsorbed phenanthrene were studied. The results showed that microbes could utilize a fraction of adsorbed phenanthrene by BC and MWCNTs after aging for 21-40d. With aging for 28d, the biodegradation efficiencies of phenanthrene after incubation for 21d were 83.8%, 73.5% and 54.2% for BC-YR, char and MWCNTs, respectively; with aging for 40d, the mineralization rates of (14)C-labeled phenanthrene after incubation for 25d were 38.3%, 25.1% and 24.6%, respectively. The desorption and biodegradation processes showed similar residual concentration of phenanthrene. However, the biodegradation rates were higher than the desorption rates during the fast biodegradation stage, suggesting that bacteria could promote desorption or access and utilize the sorbed phenanthrene. The biodegradation and mineralization efficiencies of phenanthrene associated with MWCNTs were significantly lower than with BC (p<0.01), implying adsorption by MWCNTs may lead to a greater decrease of HOCs bioavailability in the environment.

Effect of suspended-sediment concentration on nitrification in river water: importance of suspended sediment-water interface.

High suspended sediment (SPS) concentrations exist in many Asian river systems. In this research, the effects of SPS concentration on nitrification in river water systems were studied. With orwithout introducing ammonium-oxidizing bacteria isolated from the water and sediment samples of the Yellow River, the microbially mediated nitrification rate increased with SPS concentration as described by the power function y = a x x(b), where y is the nitrification rate, x is the SPS concentration, and a and b are constants. With an indigenous ammonium-oxidizing bacteria, nitrification rate constants, i.e., K4 (micromax/Ks) values obtained from the Monod model, were 0.0016, 0.0036, 0.0040, 0.0063, 0.0066, 0.0071, and 0.0077 day(-1) microM(-1) for the systems with SPS concentrations of 0, 0.2 1.0, 5.0, 10, 20, and 40 g/L, respectively. The sorption percentage of NH4+-N increased with SPS concentration as a power function. Bacteria tend to attach onto SPS, and the maximum specific growth rate at the SPS-water interface was about twice that in the water phase. An increase of bacterial population and nitrification rate constant with SPS as a power function resulted in an increase of nitrification rate with SPS as a power function. Therefore, the high SPS concentration caused by erosion and bottom sediment resuspension and other factors will accelerate ammonium oxidation in many turbid river systems. This has useful implications for nitrogen removal from river systems.