Regulatory Mechanisms of Plant Growth-Promoting Bacteria in Alleviating Microplastic and Heavy Metal Combined Pollution: Insights from Plant Growth and Metagenomic Analysis

The co-occurrence of microplastics and heavy metals in soil can lead to synergistic interactions that may exert more pronounced toxic effects on plant growth. Previous studies have demonstrated the promising potential of plant growth-promoting bacteria (PGPB) in mitigating the combined toxicity of microplastics and heavy metals. However, the rhizosphere microbial mechanisms underlying this alleviation remain unclear. Metagenomic sequencing offers significant advantages for microbial functional analysis, yet it has been underutilized in studies involving combined microplastic and heavy metal contamination. In this study, a pot experiment was conducted to evaluate the effects of inoculating sorghum with two plant growth-promoting bacterial (PGPB) strains, Bacillus sp. SL-413 and Enterobacter sp. VY-1, on plant tolerance to co-contamination with 13 μm polyethylene (PE) microplastics (0.5%, w/w) and cadmium (Cd, 10 mg kg−1). The impact on rhizosphere microbial community structure and function was assessed using metagenomic analysis. The results showed that PE-Cd co-contamination, compared to Cd alone, caused varying degrees of reduction in sorghum height and biomass, indicating an enhanced toxic effect due to the combined pollutants. Inoculation with PGPB effectively alleviated the PE-Cd combined toxicity, resulting in increases in sorghum height by 4.81–12.50%, biomass by 0.43–38.40%, and Cd accumulation by 6.20–38.07%. Both Cd and PE-Cd treatments, as well as PGPB inoculation, significantly altered the composition of rhizosphere soil bacterial communities, particularly affecting the relative abundances of Ramlibacter, Solirubrobacter, and Streptomyces. Metagenomic analysis further revealed that PE-Cd co-contamination suppressed microbial functional potential in the rhizosphere. However, inoculation with Bacillus sp. SL-413 and Enterobacter sp. VY-1 alleviated the functional stress induced by PE-Cd co-contamination and significantly enhanced microbial gene functions in the soil. Specifically, genes involved in nitrogen and phosphorus cycling increased by 3.35–5.32% and 2.26–7.38%, respectively, compared to the PE-Cd treatment without inoculation. This study provides fundamental data and scientific evidence for understanding the ecotoxicological effects of microplastic and heavy metal co-contamination, as well as the potential for microbial remediation using PGPB.