Multiomics analysis reveals the molecular basis for increased body weight in silkworms (Bombyx mori) exposed to environmental concentrations of polystyrene micro- and nanoplastics

Abstract

Introduction: Micro- and nanoplastics (MNPs) are emerging environmental pollutants that have raised serious concerns about their potential impact on ecosystem and organism health. Despite increasing efforts to investigate the impacts of micro- and nanoplastics (MNPs) on biota little is known about their potential impacts on terrestrial organisms, especially insects, at environmental concentrations.

Objectives: To address this gap, we used an insect model, silkworm Bombyx mori to examine the potential long-term impacts of different sizes of polystyrene (PS) MNPs at environmentally realistic concentrations (0.25 to 1.0 μg/mL).

Methods: After exposure to PS-MNPs over most of the larval lifetime (from second to last instar), the endpoints were examined by an integrated physiological (growth and survival) and multiomics approach (metabolomics, 16S rRNA, and transcriptomics).

Results: Our results indicated that dietary exposures to PS-MNPs had no lethal effect on survivorship, but interestingly, increased host body weight. Multiomics analysis revealed that PS-MNPs exposure significantly altered multiple pathways, particularly lipid metabolism, leading to enriched energy reserves. Furthermore, the exposure changed the structure and composition of the gut microbiome and increased the abundance of gut bacteria Acinetobacter and Enterococcus. Notably, the predicted functional profiles and metabolite expressions were significantly correlated with bacterial abundance. Importantly, these observed effects were particle size-dependent and were ranked as PS-S (91.92 nm) > PS-M (5.69 µm) > PS-L (9.7 µm).

Conclusion: Overall, PS-MNPs at environmentally realistic concentrations exerted stimulatory effects on energy metabolism that subsequently enhanced body weight in silkworms, suggesting that chronic PS-MNPs exposure might trigger weight gain in animals and humans by influencing host energy and microbiota homeostasis.

Keywords: Gut microbiome; Metabolome; Model organism; Particle size-dependent effects; Silkworm; Transcriptome.

Graphical abstract

Fig. 1

Experiment design and characteristics of polystyrene micro- and nanoplastics (PS-MNPs). (A) Silkworm larvae from the second instar until the pre-pupal stage were exposed to three different sizes of PS-MNPs at environmental concentrations and the integrated biomarker responses were determined. CR, conventionally raised; ML, mulberry leaves; DPE, days post-exposure. (B) Physiochemical characteristics were determined by SEM and TEM, and (C) the chemical composition of the particles was reaffirmed by Py-GC/MS.

Fig. 2

Effects of different sizes of PS-MNPs at different concentrations on the general health of silkworms. (A) Larval body mass and (B) survivorship of the silkworms. Different letters indicate significant differences across the groups estimated by one-way ANOVA with Tukey’s post-hoc test. Survival curves were generated in GraphPad Prism (version 9.0) based on Kaplan-Meir (log-rank) analysis.

Fig. 3

PS-MNPs induced metabolomic alteration in the gut of silkworms. (A) PLS-DA scatter plot displays the patterns of metabolite changes between the CTR and exposure groups. (B) Metabolites clustering heatmap shows the significantly changed metabolites (SCMs) between the CTR and exposure groups. (C) The Venn diagram shows the distribution pattern of significantly changed metabolites (SCMs) across the different groups.

Fig. 4

PS-MNPs perturbed the metabolisms of (A) amino acids, (B) nucleosides, (C) carbohydrates, and (D) lipids, as well as affected (E) valeric acid (a short chain fatty acid) level in the gut contents of silkworms. The data are presented as mean values ± standard deviation and significant differences between groups were estimated using one-way ANOVA with Tukey’s post-hoc test, which is denoted by different letters.

Fig. 5

PS-MNPs changed the structure and composition of silkworm gut microbiota. (A) Dendrogram based on Bray-Curtis matrix showing clustering pattern of microbial communities under exposure to different particle sizes. (B) PCoA plot visualizes the gut bacterial community structure. (C) Bubble plots based on the phylum, family, and genus taxa show the differences between the CTR and exposure groups.

Fig. 6

Functional prediction and microbes-metabolites correlations. (A) The heatmap shows changes in the predicted functional potentials of the changed microbiome. (B) Significant correlations (r > 0.8) exist between bacteria (color teal) and metabolites (color lavender). Red lines indicate negative correlations, whereas positive correlations are denoted by blue lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

PS-MNPs-induced changes in the gene expression profiles of silkworms. (A) The heatmap depicts the relative expression level of differentially expressed genes (DEGs) between the CTR and exposure groups. (B) The upset plot displays the distribution pattern of DEGs across the exposure groups. The DEGs are involved in regulating the metabolism of (C) carbohydrates, (D) amino acids, (E) lipids, and (F) xenobiotics.

Fig. 8

Schematic of PS-MNPs-induced metabolic disorders in overall energy balance. Increased silkworm growth (weight gain) was attributed to enhanced energy reserves (ATPs) triggered by tricarboxylic acid (TCA) cycle that stimulated (A) amino acid metabolism, (B) lipid metabolism, (C) carbohydrates metabolism, and (D) nucleotide metabolism. (E) The PS-MNPs induced dysbiosis of gut microbiota, leading to significant differences in the predicted functional profiles.