Dual environmental stressors: How pH modulates antibiotic toxicity in Danio rerio?

Aquatic ecosystems are increasingly subjected to environmental stressors, including pH fluctuations, and antibiotic contamination, which can disrupt essential biological functions such as metabolism, respiration, and reproduction. The interaction between these stressors presents significant ecological risks, as pH affects the toxicity, pharmacodynamics/kinetics of antibiotics by altering their ionization state and membrane permeability. This study assessed the toxicity of environmentally relevant concentrations of sulfamethoxazole (150 µg SMX/L), trimethoprim (30 µg TRIM/L), and their mixture (MIX: 150 µg SMX/L + 30 µg TRIM/L) under different pH conditions (6.5, 7.5, and 9.0) on Danio rerio juveniles. A multi-biomarker approach was used to assess D. rerio biological health status, including oxidative stress responses, lipid peroxidation, cholinergic neurotransmission, energetic metabolism, and DNA damage. Results revealed that SMX was marginally toxic across all pH scenarios, but caused more severe effects such as oxidative stress, lipid peroxidation, and DNA damage, under acidic pH. In contrast, TRIM toxicity increased at neutral and alkaline pH, causing severe alterations in antioxidant defenses and cellular integrity. The MIX treatment exhibited marginal toxicity at acidic and alkaline pH but was moderately toxic at neutral pH, leading to oxidative stress, lipid peroxidation, and DNA damage. These physiological and metabolic disruptions highlight how antibiotic mixtures, under varying pH conditions, can impair critical biological functions in aquatic organisms. These findings emphasize the urgent need for integrated research addressing multiple environmental stressors, particularly chemical contamination and climate change-driven abiotic factors. Ignoring these threats could lead to irreversible damage to aquatic ecosystems and biodiversity.