Abstract: To investigate the biotransformation characteristics of polyethylene terephthalate (PET) by marine-derived microorganisms and the effect of PET crystallinity on degradation efficiency, strain SCSIO 43976 was isolated from seagrass bed sediments in Sanya Bay, Hainan, and identified using 16S rRNA gene sequence analysis. Waste PET bottle flakes were pretreated by melt-quenching to reduce crystallinity. Two culture systems were established with original PET and low-crystallinity PET, respectively. After 60 days of incubation, changes in PET materials were analyzed by weight loss measurement, scanning electron microscopy (SEM), gel permeation chromatography (GPC), and Fourier-transform infrared spectroscopy (FTIR). Transcriptomics and untargeted metabolomics were employed to elucidate the bacterial response mechanisms. Results showed that strain SCSIO 43976 shared 97.09% 16S rRNA gene sequence similarity with Mesorhizobium qingshengii CCBAU 33460T and was identified as Mesorhizobium sp. SCSIO 43976. Melt-quenching treatment reduced PET crystallinity from 31.75% to 7.05%. After 60 days of cultivation, the weight loss rate of the low-crystallinity PET inoculated group (T1) reached 1.33% ± 0.30%, significantly higher than that of the original PET inoculated group (T2) at 0.112% ± 0.174%. Meanwhile, the T1 group exhibited surface cracks, pits, and lamellar exfoliation on PET films, accompanied by changes in molecular weight and characteristic chemical structures. Transcriptomic analysis revealed 950 differentially expressed genes (DEGs) between the T1 group and the control group, primarily enriched in aromatic compound degradation pathways. Functional genes including pcaG, pcaH, pcaC (involved in aromatic compound transformation) and glcD, glcE, glcF, glcB (involved in ethylene glycol metabolism) were detected. Metabolomic analysis identified 129 differential metabolites between T1 and T2 groups, mainly associated with polycyclic aromatic hydrocarbon degradation pathways. Metabolites related to terephthalic acid transformation (C06318) and ethylene glycol metabolic product (1,4-dioxane-2,5-diol) were also detected. These findings demonstrate that Mesorhizobium sp. SCSIO 43976 possesses biotransformation potential for waste PET bottle flakes, and reducing PET crystallinity can enhance strain-mediated material transformation and related product metabolism, providing evidence for enhancing microbial PET conversion through low-crystallinity pretreatment strategies.

Key words: polyethylene terephthalate, PET biodegradation, Mesorhizobium sp., melt-quenching, transcriptomics, untargeted metabolomics