Influence of natural freshwater bacteria on the cytotoxic effects of azithromycin in Chlorella vulgaris

Abstract The intensified use of pharmaceuticals during the COVID-19 pandemic has increased their environmental release in aquatic environments. Among these emerging contaminants, antibiotics are of particular concern owing to their widespread occurrence and potential ecological impacts. Given the ecotoxicological relevance of the antibiotic azithromycin (AZT) and the ecological importance of microalgae-bacteria interactions in aquatic systems, this study compares the cytotoxicity of AZT on the freshwater microalga Chlorella vulgaris in axenic cultures and in consortium with bacteria isolated from a freshwater reservoir. The results indicate that AZT induces severe damage to C. vulgaris cells, primarily compromising cellular energy metabolism by targeting chloroplasts and mitochondria. The presence of aquatic bacteria in consortium cultures amplifies AZT toxicity, leading to overproduction of intracellular reactive oxygen species (ROS). Furthermore, co-culturing with bacteria causes severe cytoplasmic membrane damage in C. vulgaris cells exposed to AZT, namely, membrane depolarization and loss of cell viability, toxicity not observed under axenic conditions. While AZT activates an apoptosis-like cell process in axenic cultures, co-culturing with bacteria shifts the mode of cell death towards a necrotic-like process. Overall, these results emphasize the environmental relevance of using microalga-bacteria consortia for ecotoxicological studies, providing a more realistic framework for assessing the cytotoxicity of emerging contaminants.