• Antibiotics were detected in over 90% of samples from a tropical river. • Presence of genes tet W, sul II, and erm B could indicate persistent human pollution. • Antibiotic residues could shift microbial composition and ecosystem functions. • Integrated data show links between land use, ARGs, and pollution. • Improved monitoring and mitigation actions are needed to reduce ARG impacts. Antibiotic contamination and the spread of antimicrobial resistance genes in tropical freshwater ecosystems have become an emerging environmental concern, yet their ecological implications remain poorly understood. However, little data is available in the tropics compared with template regions. This study presents a chemical and microbiological characterization of the Peñas Blancas River, a tropical watershed in Costa Rica subject to varying degrees of anthropogenic influence. A total of 38 surface water samples from eight sampling points, representing a gradient of anthropogenic influence, were analyzed for the presence of 27 antibiotics and 11 antimicrobial resistance genes (ARGs). According to these results, the microbial community composition was characterized, and an ecotoxicological and resistance risk assessment was calculated. Fluoroquinolones, such as ciprofloxacin and norfloxacin, were detected in over 90% of samples, with concentrations up to 29 ng/L, posing a potential ecotoxicological threat. Concurrently, ARGs such as tet W (63.27%), tet Q and sul II (each 61.22%), qnr S (44.90%), tetA and erm B (each 42.86%)were frequently detected, suggesting ongoing selective pressure despite the absence of corresponding antibiotic residues. The microbiome analysis revealed a highly diverse bacterial community, comprising over 1,400 identified genera. Significant correlations were observed between antibiotic concentrations and shifts in microbial composition, including an increased abundance of genera such as Aurantimicrobium and Sphingomonas in contaminated sites. No negative correlations were found between antibiotics and alpha diversity. Norfloxacin showed significant positive correlations with Shannon, Simpson, and Faith’s PD indices, while ciprofloxacin and moxifloxacin correlated positively with Shannon and Simpson, suggesting possible microbial modulation without diversity loss. These findings highlight the complex relationships between chemical pollutants and microbial ecology, underscoring the need for comprehensive monitoring strategies in tropical freshwater ecosystems. Our data support the need for mitigation actions such as improving wastewater treatment, reducing antibiotic misuse, and controlling agricultural discharges to protect tropical freshwater ecosystems.
Vargas-Villalobos et al. (Sun,) studied this question.