Azithromycin at Environmentally Relevant Concentrations Increases the Growth and Microcystin Content in the Cyanobacterium Microcystis aeruginosa
摘要
Azithromycin is a widely used macrolide antibiotic reported globally in aquatic environments, where its concentrations increased during the COVID-19 pandemic. Although usually present in subinhibitory concentrations in surface water (≤ 3 µg L⁻1), it can impact non-target organisms, such as algae and cyanobacteria, the primary producers of aquatic food webs. This study evaluated the effects of environmentally relevant concentrations of azithromycin on the bloom-forming cyanobacterium Microcystis aeruginosa. The cyanobacterial physiological response was evaluated by estimating growth (biovolume), cellular chlorophyll-α quota, photosynthetic activity, and microcystin content (extra and intracellular concentrations and cellular quota). Exposure to azithromycin (0.3 and 0.6 µg L⁻1) resulted in a significant increase in biovolume (60–94%; p < 0.05), accompanied by a reduction in the cellular chlorophyll-α quota (13–48%; p < 0.01), without photosynthetic impairment. Additionally, a peak (fourfold; p < 0.01) in cellular microcystin quota was observed after 2d exposure to the higher azithromycin concentration, followed by a gradual decrease (20–42%; p < 0.0001). Despite this, total (both extracellular and intracellular) microcystin concentrations were higher in the treated cultures than in the controls, due to increased cell densities. The drop in cellular toxin and chlorophyll quotas in the antibiotic treatments may be attributed to the higher cell division rate, leading to a dilution of their cellular content. These findings suggest that antibiotics in the aquatic environment may contribute to the increase in harmful cyanobacterial blooms, posing significant risks to both public and environmental health.