Complex co-contaminant responses of Chlorella sp. and its phycosphere microbiota under co-exposure to PET microfibers and oxytetracycline

Microplastics (MPs) and antibiotics co-occur in aquatic environments, yet their joint ecological effects remain poorly understood. We examined polyethylene terephthalate microplastic fibers (PET-MFs) and oxytetracycline (OTC, 2 mg/L), alone and in combination, on Chlorella sp. and its phycosphere microbiome over 28 days. Microscopy revealed extensive algal adhesion to fibers and aggregated clusters. PET-MFs dose-dependently inhibited algal growth by lowering photosynthetic efficiency, disrupting pigment synthesis, and inducing oxidative stress, and shifted the phycosphere microbiome by reducing Rhodobacter and Brevundimonas and enriching Paucibacter . Transcriptomics showed a strong dose response: LMF (5000 particles/L MFs) induced 216 DEGs, versus 2920 DEGs in HMF (50,000 particles/L MFs); OTC alone caused 2443 DEGs and suppressed glycolysis/pyruvate metabolism and photosynthesis related gene expression despite limited biomass effects. Co-exposure amplified disturbance to 7126 DEGs in LMO (LMF and OTC) and 12,880 DEGs in HMO (HMF and OTC), exceeding either single treatment. Although total bacterial abundance changed little, ARGs/MGEs increased, with intI2 elevated in all MF-containing groups and tet genes promoted by OTC. Correlation analyses support an algae–microbiome–resistome linkage: algal traits (OD680, Chl-a, Fv/fm) tracked shifts in dominant genera, which co-varied with int and tet modules, while soluble protein/EPS aligned with higher int / tet signals. Together, these results indicate complex, non-linear PET-MF–OTC interactions, with algal physiology likely mediating microbiome structure and ARG dynamics. This study advances mechanistic understanding of pollutant–microalgae–microbiome crosstalk and highlights ecological risks from co-occurring MPs and antibiotics. • Microplastic fibers dose dependently suppress algal growth and photosynthesis. • Microplastic fibers restructure the phycosphere and increase antibiotic resistance genes. • Oxytetracycline alters metabolism and antioxidant defenses with minor biomass impact. • >Co-exposure antagonizes algal growth yet amplifies transcriptomic stress. • Algal physiology likely shapes microbiome structure and antibiotic resistance genes.