UV-C Exposure Enhanced the Cd2+ Adsorption Capability of the Radiation-Resistant Strain Sphingomonas sp. M1-B02

Abstract

Microbial adsorption is a cost-effective and environmentally friendly remediation method for heavy metal pollution. The adsorption mechanism of cadmium (Cd) by bacteria inhabiting extreme environments is largely unexplored. This study describes the biosorption of Cd²⁺ by Sphingomonas sp. M1-B02, which was isolated from the moraine on the north slope of Mount Everest and has a good potential for biosorption. The difference in Cd²⁺ adsorption of the strain after UV irradiation stimulation indicated that the adsorption reached 68.90% in 24 h, but the adsorption after UV irradiation increased to 80.56%. The genome of strain M1-B02 contained antioxidant genes such as mutL, recA, recO, and heavy metal repair genes such as RS14805, apaG, chrA. Hydroxyl, nitro, and etceteras bonds on the bacterial surface were involved in Cd²⁺ adsorption through complexation reactions. The metabolites of the strains were significantly different after 24 h of Cd²⁺ stress, with pyocyanin, L-proline, hypoxanthine, etc., being downregulated and presumably involved in Cd²⁺ biosorption and upregulated after UV-C irradiation, which may explain the increase in Cd²⁺ adsorption capacity of the strain after UV-C irradiation, while the strain improved the metabolism of the antioxidant metabolite carnosine, indirectly increasing the adsorption capacity of the strains for Cd²⁺.

Keywords: Mount Everest; Sphingomonas; biosorption; heavy metal; metabolic analysis.

Figure 1

Screening and determination of the adsorption of Cd²⁺ by Sphingomonas sp. M1-B02 ((A), screening of optimal adsorption strain; (B), colonies of the Sphingomonas spp.; (C), physiological characteristics of Sphingomonas sp. M1-B02; (D), Optimal adsorption conditions for Sphingomonas sp. M1-B02).

Figure 2

Biosorption dynamic curves of Sphingomonas sp. M1-B02 ((A), Direct adsorption of Cd²⁺; (B), adsorption of Cd²⁺ after UV stress).

Figure 3

Electron microscopy comparison of strain M1-B02 before and after Cd²⁺ adsorption. (A1,B1,C1) Scanning electron microscopy (SEM) images of strain M1-B02 under different conditions. Red circles highlight regions of Cd²⁺. (A2,B2,C2) Energy-dispersive X-ray spectroscopy (EDS) analysis of corresponding samples showing the elemental composition. The table summarizes the mass percentage (Mass%) and atomic percentage (Atom%) of carbon (C), oxygen (O), and cadmium (Cd). The bottom images in (A2,B2,C2) map the spatial distribution of Cd²⁺ (green dots) on the bacterial surface.

Figure 4

The effect on the ultrastructure of Sphingomonas sp. M1-B02 under FTIR. (A1) corresponds to untreated Sphingomonas sp. M1-B02. (A2) represents the bacterial cells after Cd²⁺ adsorption. (A3) shows the cells exposed to UV treatment followed by Cd²⁺ adsorption.

Figure 5

Genome circos of Sphingomonas sp. M1-B02 with DNA repair and heavy metal repair genes.

Figure 6

Metabolomics analysis of Sphingomonas sp. M1-B02 ((A,B), PCA scoring charts of metabolites; (C), Venn diagram of differential metabolites; (D), KEGG compound classification chart; (E), KEGG pathway statistics chart).

Figure 7

Volcano plots illustrating the differential metabolites identified in pairwise comparisons of groups (B vs. A and C vs. B). (A) Volcano plot of metabolites in the comparison between group B and group A. (B) Volcano plot of metabolites in the comparison between group C and group B.

Figure 8

Box plot of distribution of significantly different metabolites in Group A, B, and C. The abundance of nine metabolites was compared across three groups (A, B, and C), with statistical significance indicated above each comparison. The boxplots represent the abundance values for (A) Pyocyanin, (B) Xanthine, (C) Hypoxanthine, (D) L-Proline, (E) Oxidized Glutathione, (F) Glycerol 2-phosphate, (G) 2-Oxoarginine, (H) Carnosine, and (I) N-Succinyl-L,L-2,6-diaminopimelate. Data points are visualized as boxplots, where the middle line represents the median, and the upper and lower bounds of the box correspond to the interquartile range (IQR). Groups are color-coded as A (grey), B (orange), and C (blue). *** represents p < 0.001.