Concentration-dependent effects of azithromycin on Microcystis aeruginosa: Insights into growth, oxidative stress,...

The widespread use of azithromycin has led to its residues in surface waters, but its effects on microalgae across a wide concentration range remain unclear, posing potential threats to aquatic ecosystems. In this study, acute toxicity tests were conducted to investigate the biological effects of different concentrations of azithromycin on Microcystis aeruginosa, including algal growth, photosynthesis, intracellular polysaccharide synthesis, oxidative stress, and gene expression. The results showed that the biological effects of azithromycin on Microcystis aeruginosa were concentration-dependent. Environmentally relevant low concentrations (50, 500 ng/L) did not significantly inhibit growth, but stimulated intracellular polysaccharide synthesis and induced fluctuations in superoxide dismutase activity. Under low concentrations, the expression of photosynthesis genes (psaB, rbcL), DNA damage repair genes (recA, grpE), and the microcystin transporter gene (mcyH) was upregulated. In contrast, environmentally relevant high concentrations (5, 50, 500 μg/L) significantly inhibited growth, with a maximum inhibition of 77.2% at 50 μg/L after 96 h. Photosynthesis and intra- and extracellular polysaccharide synthesis were severely suppressed. Superoxide dismutase activity and malondialdehyde content increased significantly, and both intra- and extracellular concentrations of Microcystin-LR and Microcystin-RR were markedly elevated. Meanwhile, the expression of psaB, rbcL, recA, grpE, and mcyH was downregulated. These findings deepen the comprehensive understanding of azithromycin's effects on Microcystis aeruginosa across a broad concentration range and help elucidate toxicological mechanisms of antibiotics in microalgae.