Humic acids alleviate and reverse sulfonamide antibiotic toxicity to green algae: A mechanism study from inhibition to stimulation.

The widespread occurrence of sulfonamide antibiotics (SAs) in aquatic environments poses a global ecological threat. Conventional international aquatic environment antibiotic risk assessment typically focuses on the toxicity of single antibiotics, ignoring the influence of inherent environmental substances. However, in natural aquatic environments, antibiotics frequently coexist with natural dissolved organic matter (such as humic acid, HA). This study aimed to elucidate the mechanisms through which HA influences the toxicity of SAs at the molecular, cellular, physiological, and biochemical levels. Through integrated toxicity testing, spectroscopic analysis (FTIR, EEM), ultrastructural observation, and physiological assessment, this study suggests that HA can mitigate and even reverse the toxic effects of SMZ under specific mixture conditions, transforming > 70% growth inhibition into up to 47% stimulation. The findings indicated that the pEC₅₀ of 20 SAs-HA mixture groups against Raphidocelis subcapitata (green algae) ranged from 2.82 to 3.69 at 96 h exposure and from 3.01 to 3.85 after 144 h exposure. 65% of mixtures demonstrated antagonistic effects. Therefore, incorporating dissolved organic matter interactions into antibiotic global ecological risk assessment could more accurately reflect actual toxicological effects under compound pollution conditions and improve the environmental relevance and reliability.