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Combined microplastic and cadmium pollution reshapes plastisphere–rhizosphere bacterial microbiomes and regulates the growth of sorghum plants for bioenergy
Summary
Microplastics and toxic heavy metals like cadmium often pollute farmland together, and this study found that this combo stunts the growth of sorghum (a crop used for biofuel) more than either pollutant alone, especially when the plastic type is PVC. The silver lining: certain soil bacteria adapted to this pollution by evolving ways to break down plastic and resist heavy metals, and these bacteria actually helped the plants grow better despite the contamination, hinting at a possible future strategy for cleaning up polluted soil using plants and microbes together.
The dual contamination of soil–bioenergy plant systems by heavy metals (HMs) and microplastics (MPs) is becoming increasingly severe. However, the microbial selection patterns under this dual pressure remain unclear. In this study, the effects of combined contamination by cadmium (Cd) and microplastics (polyethylene terephthalate (PET) and polyvinyl chloride (PVC)) at environmentally relevant concentrations (1% MPs and 10 mg·kg −1 Cd) were investigated. The effects on the growth of the bioenergy plant sorghum and on the bacterial communities in both the soil and the plastisphere were examined. The results revealed that combined microplastic-heavy metal (MP-HM) contamination had a more significant inhibitory effect on bioenergy plant growth, with PVC-Cd resulting in stronger inhibition than PET-Cd. Metagenomic sequencing and network analysis revealed that the combined MPs-HM contamination significantly reshaped the bacterial microbiome community structure, with distinct differences between plastisphere microorganisms and rhizosphere soil microorganisms. Compared with the PVC plastisphere, the PET plastisphere exhibited greater microbial diversity and a more complex interaction network, highlighting the differences in ecological niche selection pressure between plastic types. Various metal resistance genes (MRGs) and microplastic degradation genes (MDGs) were detected in both the rhizosphere soil and the plastisphere, with higher abundances in the plastisphere than in the rhizosphere soil. Under dual stress from microplastics and heavy metals, a series of bacterial genera, including Bradyrhizobium , Rhizobium , Sphingomicrobium , Ramlibacter , and Hydrogenophaga , enriched in both the rhizosphere soil and plastisphere, carried abundant MRGs (e.g., tupC and fbpC ) and MDGs (e.g., ACADM and paaG ), which were significantly positively correlated with the same mobile genetic elements (MGEs), such as transposase, integrase, and recombinase. Structural equation modeling (SEM) indicated that the synergy between core microorganisms, MDGs, and MRGs significantly promoted sorghum biomass. This study provides correlational evidence for the potential coselection of MRGs and MDGs within a specific system of energy sorghum–farmland soil–two types of microplastics (PET and PVC) under combined Cd‑MP stress. We acknowledge that this inference is based on correlation analyses and has not been experimentally verified. These results not only deepen our understanding of the ecological effects of MP-HM combined pollution but also provide an important theoretical basis for bioenergy plant remediation technologies targeting MP-HM composite contamination.