Pollution characteristics, risk assessment, and environmental associations of microplastics and antibiotic resistance genes in tropical marine aquaculture: A comparative study across different farming systems in Hainan.

Microplastics (MPs) and antibiotic resistance genes (ARGs) were compared across four marine aquaculture systems in Hainan (fish, shrimp, fish-shrimp mixed, and snail farms). MPs averaged 4.90 ± 5.50 items·L-1, with polyethylene, rayon, polypropylene-polyethylene copolymer, and polystyrene as dominant polymers. Fibers (58.90%) and 100-500 μm particles prevailed. Snail farms showed the highest MP concentration (6.62 ± 1.68 items·L-1), while others ranged from 2.02 to 2.90 items·L-1. The Pollution Load Index indicated low contamination overall, but snail farms posed the highest potential ecological risk. A total of 130 ARGs belonging to 11 classes and 13 MGEs belonging to 6 classes were detected, with fish farms showing higher ARG abundance and fish-shrimp mixed systems showing higher MGE abundance. MGEs showed strong correlations with ARGs, indicating potential associations with ARG mobility rather than direct evidence of horizontal gene transfer. Salinity and specific conductance showed potential associations with ARG distribution, but these relationships should be interpreted as exploratory rather than statistically confirmed drivers. Risk Quotient (RQ) analysis revealed the highest combined ARG risks in shrimp and mixed farms, followed by fish, then snail farms. Tetracycline resistance genes dominated risk, reaching an RQ of 1772.14 in shrimp farms. The study provides system-specific insights into MP and ARG pollution and risks, supporting targeted monitoring and management in tropical marine aquaculture.