Nematode Uptake Preference toward Different Nanoplastics through Avoidance Behavior Regulation.

Expounding bioaccumulation pathways of nanoplastics in organisms is a prerequisite for assessing their ecological risks in the context of global plastic pollution. Invertebrate uptake preference toward nanoplastics is a key initial step of nanoplastic food chain transport that controls their global biosafety, while the biological regulatory mechanism remains unclear. Here, we reveal a preferential uptake mechanism involving active avoidance of nanoplastics by Caenorhabditis elegans and demonstrate the relationship between the uptake preference and nanoplastic characteristics. Nanoplastics with 100 nm in size or positive surface charges induce stronger avoidance due to higher toxicity, causing lower accumulation in nematodes, compared to the 500 nm-sized or negatively charged nanoplastics, respectively. Further evidence showed that nematodes did not actively ingest any types of nanoplastics, while different nanoplastics induced defense responses in a toxicity-dependent manner and distinctly stimulated the avoidance behavior of nematodes (ranged from 15.8 to 68.7%). Transcriptomics and validations using mutants confirmed that the insulin/IGF signaling (IIS) pathway is essential for the selective avoidance of nanoplastics. Specifically, the activation of DAF-16 promoted the IIS pathway-mediated defense against nanoplastics and stimulated the avoidance behavior, increasing the survival chances of nematodes. Considering the genetical universality of this defense response among invertebrates, such an uptake preference toward certain nanoplastics could lead to cascaded risks in the ecosystem.

Epidermal microorganisms contributed to the toxic mechanism of nZVI and TCEP in earthworms by robbing metal elements and nutrients.

Disrupting effects of pollutants on symbiotic microbiota have been regarded as an important mechanism of host toxicity, with most current research focusing on the intestinal microbiota. In fact, the epidermal microbiota, which participates in the nutrient exchange between hosts and environments, could play a crucial role in host toxicity via community changes. To compare the contributions of intestinal and epidermal symbiotic microorganisms to host toxicity, this study designed single and combined scenarios of soil contamination [nano zero-valent iron (nZVI) and tris (2-chloroethyl) phosphate (TCEP)], and revealed the coupling mechanisms between intestinal/epidermal symbiotic bacterial communities and earthworm toxicological endpoints. Microbiome analysis showed that 15% of intestinal microbes were highly correlated with host endpoints, compared to 45% of epidermal microbes showing a similar correlation. Functional comparisons revealed that key species on the epidermis were mainly heterotrophic microbes with genetic abilities to utilize metal elements and carbohydrate nutrients. Further verifications demonstrated that when facing the co-contamination of nZVI and TCEP, certain symbiotic microorganisms became dominant and consumed zinc, copper, and manganese along with saccharides and amino acids, which may be responsible for the nutritional deficiencies in the host earthworms. The findings can enrich the understanding of the coupling relationship between symbiotic microorganisms and host toxicity, highlighting the importance of epidermal microorganisms in host resistance to environmental pollution.

Earthworms enhance the bioremediation of tris(2-butoxyethyl) phosphate-contaminated soil by releasing degrading microbes.

The escalating awareness of the environmental risks posed by organophosphorus flame retardants (OPFRs), e.g., tris(2-butoxyethyl) phosphate (TBOEP), necessitates the development of effective approaches to mitigate their adverse ecological effects. However, research on the remediation of OPFR-contaminated soil remains limited. In this study, a strategy is proposed to enhance the microbial remediation of TBOEP-contaminated soil through the introduction of exotic earthworms (Eisenia fetida). The presence of earthworms led to a substantial increase in the 28-d removal rates of TBOEP at concentrations of 0.05, 0.5, and 5 mg/kg, with improvements of 32.3 ± 2.0%, 33.2 ± 1.3%, and 33.0 ± 5.6% compared to rates in the absence of earthworms, respectively. The underlying mechanisms for this enhancement include the earthworm-mediated enrichment of TBOEP-degrading bacteria, particularly Rhodococcus, Flavobacterium, and Pseudomonas, and the transfer of Rhodococcus from the earthworm gut to the soil, resulting in an increased relative abundance within the soil. Concurrently, the earthworms stimulated soil peroxidase activity, facilitating the oxidative degradation of TBOEP. Furthermore, the rise in dissolved organic matter content following earthworm treatment fostered the growth of degrading bacteria in the soil. Rhodococcus emerged as a dominant contributor to soil TBOEP removal, consuming humic-like compounds in dissolved organic matter. This investigation underscores the significance of gut microbes and offers valuable insights for the application of earthworm-based remediation strategies in OPFR-contaminated soil.

nZVI-induced iron poisoning aggravated the toxicity of TCEP to earthworm in soil.

Tris (2-chloroethyl) phosphate (TCEP) is a newly developed organophosphorus flame retardant that has been increasingly detected in soil as a contaminant. Nanoremediation is a potential solution for the control of TCEP, while the effectiveness and ecological risks are poorly understood. Here, we investigated the physicochemical interactions and joint toxicity of nano zero-valent iron (nZVI) (50-5000 mg/kg) and TCEP (50-5000 µg/kg) at environmental relevant concentrations to earthworms (Eisenia fetida) in soil. During a 28-d exposure, TCEP in soil was neither self-degraded nor removed by nZVI, and the individual toxicity of TCEP on the physiology of earthworms was significantly higher than that of nZVI. Notably, nZVI was found to synergize the toxicity of TCEP to earthworms without showing the classical "Trojan horse effect". Mechanically, TCEP mainly induced a typical neurotoxicity, and indirectly inhibited the food ingestion and growth performance of earthworms; nZVI induced iron poisoning aggravated the intestinal damage and directly inhibited the energy metabolism, therefore exacerbated the TCEP-induced malnutrition. Our findings provide new insights into the toxic mechanisms of nZVI-TCEP co-exposure to soil organisms, and emphasize the necessity of risk assessment and cautious usage of nanoremediation in newly emerged contaminations.

Biological responses of Eisenia fetida towards the exposure and metabolism of tris (2-butoxyethyl) phosphate.

The toxicity of various organophosphorus flame retardants (OPFRs) is of increasing concern. However, there is still a lack of research on the toxicity of OPFRs to terrestrial invertebrates and its metabolism in vivo. Herein, earthworms (Eisenia fetida) were exposed to soil spiked with 0, 0.05, 0.5, and 5 mg/kg tris(2-butoxyethyl) phosphate (TBOEP, a typical alkyl OPFRs) for 28 d to study the biological responses to the exposure and metabolism of TBOEP. TBOEP exposure inhibited the activity of acetyl-cholinesterase (64.4-68.6% of that in the control group), increased the energy consumption level, and affected calcium-dependent pathways of E. fetida, which caused a 3.6-12.4% reduction in the weight gain rate (developmental toxicity), a 10.6-69.4% reduction in the number of juveniles (reproduction toxicity), and neurotoxicity to E. fetida. The 5 mg/kg TBOEP exposure caused a significant accumulation of malondialdehyde (1.68 times higher than that in the control group) in E. fetida, which indicated that the balance of oxidation and anti-oxidation of E. fetida was broken. Meanwhile, E. fetida maintained the absorption and metabolic abilities to TBOEP under the environmental condition. The removal rate of soil TBOEP was increased by 25.1-35.5% by the presence of E. fetida. Importantly, TBOEP could accumulate in E. fetida (0.09-76.0 µg/kg) and the activation of cytochrome P450 and glutathione detoxification pathway promoted the metabolism of TBOEP in E. fetida. These findings link the biological responses and metabolic behavior of earthworms under pollution stress and provide fundamental data for the environmental risk assessment and pollution removal of OPFRs in soil.