Microplastic biofilm may shape microbial community enriched with antibiotic resistance genes to enhance nitrogen transformation under antibiotic stress

The response of nitrogen transformation to microplastic biofilm under antibiotics (ATs) stress as well as the interrelationships between functional genes and microorganisms in surface water are not very well understood and need further investigation. This study investigated the response of nitrogen transformation by analyzing changes in various nitrogen forms and explored the interaction between nitrogen transformation functions and antibiotic resistance genes (ARGs) under exposure to ATs (ciprofloxacin (CIP) and tetracycline (TC)) and PVC biofilm. Compared to the control, exposure to mature polyvinyl chloride (PVC) biofilm increased nitrate nitrogen (NO₃^--N) and ammonia nitrogen (NH₄⁺-N) removal by 12.48% and 8.79%, with the NO₃⁻-N removal rate constant reaching 0.17. However, co-exposure to CIP significantly inhibited nitrogen transformation, reducing the NO₃⁻-N removal rate constant to 0.08. In PVC biofilm, more active nitrogen transformation and enhanced horizontal transfer of ARGs led to a stronger positive correlation between nitrogen transformation genes (NTGs) and ARGs. Microorganisms carrying NTGs largely overlapped with ARGs host species, including Hydrogenophaga, Rhodococcus, and Ignavibacterium, which exhibited high abundance of both gene types. This indicated that PVC biofilm facilitated nitrogen transformation under ATs stress by enriching nitrogen transformation microorganisms carrying high ARGs abundance. These results extended knowledge of effects of ATs and microplastics (MPs) on nitrogen transformation in surface water and provided theoretical support for unique ecological effects of microplastic biofilm.