Horizontal Gene Transfer of Antibiotic Resistance Genes in Algae-Bacteria Wastewater Biofilms: Mechanisms, Drivers, and Implications

The increasing prevalence of antibiotic resistance genes (ARGs) in wastewater environments has raised significant environmental and public health concerns. Algae-bacteria biofilms have emerged as promising systems for wastewater treatment due to their synergistic interactions and high pollutant removal efficiency. However, their complex structure and microbial composition may also create favorable conditions for the persistence and dissemination of ARGs. This review provides a focused overview of ARG occurrence, horizontal gene transfer (HGT) mechanisms, and key influencing factors in algae-bacteria biofilms. Current evidence indicates that ARGs are present in both intracellular and extracellular forms, with the extracellular polymeric substances (EPS) matrix playing a critical role in their retention and stability. Conjugation is likely the dominant HGT pathway, while transformation and transduction may also contribute under biofilm conditions. Factors such as antibiotic pressure, extracellular DNA availability, algal-driven redox gradients, nutrient conditions, and microbial community interactions collectively regulate ARG transfer. HGT can enhance biofilm resilience under stress but may also alter microbial community structure and affect treatment performance, particularly nitrogen removal. Overall, algae-bacteria biofilms represent both effective treatment systems and potential reservoirs for ARG dissemination. Further research is needed to clarify HGT pathways, identify ARG hosts, and better understand algal-bacterial interactions to support safer wastewater treatment applications.