Polystyrene and Azithromycin Co-Contamination: Comparative Analysis of Microbial Shifts in the Rhizosphere vs. Bulk Soil under Wheat Growth

Abstract The high rate of entry of microplastics (MPs) and pharmaceuticals into the agroecosystem leads to progressive accumulation within the soil year after year, raising concerns about both soil ecological health and crop productivity. The combined contamination of macrolide antibiotics and nanoplastics in the rhizosphere remains poorly understood. In this work, microbial shifts in the rhizosphere and bulk soil under wheat growth as affected by the polystyrene (PS) and azithromycin antibiotics (AZM) co-contamination were investigated. In this study, the toxicity effects of different sizes and concentrations of PS (i.e., 100 nm; 300 nm; 50 and 500 mg/kg, respectively) and AZM (i.e., 5 and 20 mg/kg) were combined under wheat growth in the greenhouse for 110 days. A total of 15 treatments with three replicates each were prepared and arranged in a completely random design. Findings revealed that combined contamination (i.e., PS+AZM) negatively affected microbial richness and diversity compared to individual contamination, and it was more prominent in the rhizosphere than in bulk soil. This, in turn, affected the composition of the bacterial community, abundances, functions, and interactions. It also influenced functions involved in biogeochemical cycles within the system. The co-occurrence network showed that PS+AZM inhibited wheat growth in general and reduced bacterial interactions (nodes: 770, edges: 6184, positive interactions: 4656, negative interactions: 1528) compared to individual PS (nodes: 1301, edges: 18,190, positive interactions: 11,597, negative interactions: 6593) and AZM application (nodes: 866, edges: 7464, positive interactions: 5192, negative interactions: 2272). The same trend occurred in the rhizosphere vs bulk soil. PS+AZM drove distinct microbial community shifts, where the rhizosphere was enriched with mostly bacteria involved in nitrogen biogeochemical cycling (Microvirga; Pseudorhodoplanes; Polaromonas; Hydrogenophaga; and Bauldia) and bulk soil was enriched with bacteria adapted to low nutrients and stress resistance (i.e., Opitutus; Adhaeribacter; Blastococcus; Ilumatobacter; IS-44; Bryobacter; Subgroup_10; Gemmatimonas). These results shed light on the co-occurrence of MPs and antibiotics in agroecosystems. They may help in developing mitigation measures against the effects of MPs and antibiotics in agroecosystems.