关键词: 微塑料, 聚丙烯, 聚氯乙烯, 筛选, 降解菌

Abstract:

As a new type of pollutant, microplastics are ubiquitous in the environment. Biodegradation has received widespread attention for its high efficiency in removing microplastics. At present, available strains capable of degrading microplastics are comparatively rare, and their efficiency in degradation is insufficient. As such, the identification of microbial strains with high efficiency in microplastic degradation is urgently needed. Additionally, two or more types of microplastics are found to coexist in actual environmental settings. However, most strains reported in previous studies can only degrade a single type of microplastic. Research on the screening of bacterial strains capable of degrading multiple microplastics and their degradation effects remains limited. This study was conducted to screen and identify degrading bacteria from soil contaminated with polypropylene (PP) and polyvinyl chloride (PVC) microplastics. Strains capable of degrading either PP or PVC, as well as both PP and PVC, were identified using 16S rRNA gene sequencing. The degradation ability of bacteria on PP and PVC was determined by changes in weight, surface micromorphology, and functional groups, along with an analysis of the growth status and enzyme activity of the degrading bacteria. Results showed that strains PP1P and PP1Y could grow on PP as the sole carbon source and were identified as Roseomonas mucosa and Paenibacillus sp., respectively. PP1P exhibited rapid growth within the initial 10 days of cultivation, as demonstrated by the increase of 0.094 in OD₆₀₀ and 4.90×10⁸ in viable cell count. A moderated growth trend was observed thereafter, resulting in ΔOD₆₀₀ of 0.164 and the increment of viable cell counts of 7.80×10⁸ by day 50. PP1Y displayed slow growth during the first 10 days; it then proceeded to the logarithmic growth phase. By day 20, the ΔOD₆₀₀ reached 0.108 and the increase in viable cell count reached 4.27×10⁸, followed by a relatively slow increase in subsequent periods. Strain PVC1Y, identified as Priestia megaterium, could grow with PVC as the sole carbon source. It showed slow growth for the first 30 days, with increases in OD₆₀₀ and viable cells being 0.034 and 1.67×10⁸, respectively. Afterwards, PVC1Y entered a rapid growth phase, where the ΔOD₆₀₀ reached 0.089 and the increment of viable cells reached 4.20×10⁸ by day 50. Strain PPVC1W can grow using either PP or PVC as the carbon source and was identified as Bacillus sp. PPVC1W was able to enter a rapid growth phase following the start of cultivation, with PP as the carbon source. After 20 days, the ΔOD₆₀₀ and the increase of viable cell count reached 0.085 and 3.97×10⁸, respectively, and then the PPVC1W grew into the stationary phase. When PVC served as the carbon source, PPVC1W exhibited a consistent upward growth trend. At 50 days, the ΔOD₆₀₀ of PPVC1W was 0.038, and the increase of viable cell count reached 2.40×10⁸. After 50 days of incubation, the weight losses of PP caused by PP1P, PP1Y, and PPVC1W were 8.12%±2.52%, 8.06%±1.94%, and 6.03%±0.58%, respectively, while the weight losses of PVC caused by PVC1Y and PPVC1W were 5.15%±0.52% and 4.79%±1.43%, respectively. Compared with PP1Y and PPVC1W, PP1P achieved a higher PP weight loss and superior stability over a similar period, suggesting it may play a dominant role in the remediation of PP microplastic-contaminated environments. Based on the existing research finding, it is speculated that strains PP1Y and PPVC1W exhibit good compatibility in the remediation system of combined microplastic pollution, while showing great potential in terms of synergistic effects for improving the total degradation rate of mixed microplastics. PVC1Y and PPVC1W showed higher efficiency in the short-term degradation of PVC microplastics, making them suitable for application in PVC-contaminated environments with shorter remediation cycles and lower initial concentrations. In addition to effectively degrading PP, strain PPVC1W also showed considerable degradation ability to PVC. This indicated that strain PPVC1W had a broad substrate spectrum, and thus can serve as a superior microbial resource for the bioremediation of environments contaminated with multiple types of microplastics. As observed by scanning electron microscopy, the surface of PP microplastic in the control group was relatively smooth. After incubated with the degrading strains PP1P, PP1Y, and PPVC1W, the PP samples exhibited signs of bioerosion, including cracks, grooves, and rough surfaces. Pronounced surface alterations (e.g., numerous grooves, irregular depressions, and cracks) were observed on PVC after incubation with the degrading strains PVC1Y and PPVC1W, in contrast to the flat surface of the control group. The changes of functional groups of PP/PVC microplastics with the addition of PP1P, PP1Y, PPVC1W, and PVC1Y were determined by FTIR spectra. New absorption peaks (1639 cm⁻¹) appeared on the PP surface after 50 days of incubation with strain PP1P, attributed to the formation of −C=C. After treatment with strains PP1P, PPVC1W, and PP1Y, the intensities of the peaks at 3431, 2957, 2920, 2839, 1716, 1458, 1376, 1167, 972, and 841 cm⁻¹ became larger compared to those of the control group of PP. These peaks mainly correspond to functional groups such as −OH, −C−H, −C=O, and −C−C, indicating that the strains PP1P, PPVC1W, and PP1Y may promote the hydrolysis and cleavage of PP molecular chains during degradation. After the inoculation of PVC1Y and PPVC1W, the spectra of the PVC surface showed an increase in intensity at 3 424 cm⁻¹, which is attributed to the −OH. While a decrease in intensity was observed for the −C−Cl peak at 617 cm⁻¹. These indicated that both oxidation and dechlorination probably occurred during the degradation of PVC. After 50 days of cultivation with strains PP1P, PP1Y, and PPVC1W, significant increases in enzyme activities were observed in the culture systems with PP as the sole carbon source. The peroxidase activity increased by 8.00, 4.25, and 3.25 folds, and the catalase activity increased by 10.21, 7.55, and 5.19 folds, respectively. In the culture medium with PVC as the sole carbon source, the addition of strains PVC1Y and PPVC1W increased peroxidase activity by 7.5 and 3.25 folds, and enhanced catalase activity by 10.56 and 7.25 folds, respectively. All were significantly different from those before degradation. The order of changes in microplastic surface morphology, characteristic peaks, the increase in enzyme activity, growth-related ΔOD₆₀₀ and increase of viable cell count were consistent with the weight loss rate, ranked as PP1P-PP>PP1Y-PP>PPVC1W-PP, and PVC1Y-PVC>PPVC1W-PVC, indicating that the screened strains have good degradation ability for PP/PVC. This study provides bacterial sources and biotechnical support for the microbial remediation of microplastic pollution.

Key words: microplastics, polypropylene, polyvinyl chloride, screening, degrading bacteria