Nanoplastics amplify ARG colonization and Alter microbial communities in the phycosphere of Chlorella pyrenoidosa under antibiotic stress

The widespread use of antibiotics has led to the accumulation of antibiotic resistance genes (ARGs) in aquatic environments, raising significant concerns for both ecosystem stability and human health. The phycosphere is an important site for the enrichment of bacteria and ARGs. Simultaneously, increasing plastic pollution has introduced nanoplastics (NPs) into water systems globally. Limited research exists on whether NPs might influence antibiotic effects on phycosphere bacteria and ARG dynamics. This study addresses this gap by employing the green algae Chlorella pyrenoidosa as a model to examine the effects of combined NP and antibiotic exposure-specifically sulfamethoxazole (SMX) and ciprofloxacin (CIP)-on algal growth, phycosphere and free-living bacterial communities, and ARG abundance. The results demonstrate that NPs alone have minimal effects on C. pyrenoidosa growth and physiology. However, combined NP and antibiotic exposure significantly enhances antibiotic toxicity to the algae. Additionally, NP exposure notably increases p_Cyanobacteria abundance in the phycosphere while negatively affecting p_Proteobacteria , which are vital for water purification. While NPs do not directly promote ARG proliferation, they significantly amplify the antibiotic-induced abundance of ARGs under combined exposure in both phycosphere and free-living environment. Furthermore, the phycosphere harbors a higher total abundance of bacterial communities and ARGs than the free-living environment. It has a concentrated pattern of fewer bacterial species but multiple key genes. In contrast, the free-living environment features dispersed and diverse communities with fewer key genes. This suggests the phycosphere's critical role in ARG dissemination. These findings highlight NPs as amplifiers of ecological risk in antibiotic-contaminated environments and provide insights into ARG dissemination mechanisms in aquatic ecosystems. • NPs amplify SMX and CIP toxicity to Chlorella pyrenoidosa in aquatic environments. • NPs boost Cyanobacteria and reduce Proteobacteria abundance within the phycosphere. • NPs enhance ARG level in phycosphere and free-living environments under antibiotic stress. • Phycosphere has higher ARG and bacterial abundance, crucial for ARG spread. • NPs act as risk amplifiers in antibiotic-contaminated aquatic ecosystems.