Tetracycline and ciprofloxacin reduce nitrification and denitrification activity and alter microbial community composition and activity in microalgal-bacterial aerobic granular sludge.

Microalgal-bacterial aerobic granular sludge (MB-AGS) systems offer promising potential for wastewater treatment under chemical stress. However, their performance in the presence of antibiotics remains poorly understood. This study evaluated the response of MB-AGS to 1000 µg/L of tetracycline and ciprofloxacin in two separate bioreactors operated under alternating dark (60 min) and light (170 min) cycles at 20 °C. Chemical oxygen demand (COD) removal remained stable at 90 ± 4% (tetracycline) and 91 ± 6% (ciprofloxacin) over 80 days, suggesting that COD conversion was not impacted by antibiotic exposure. However, phosphate removal declined from ∼63% (antibiotic-free bioreactors) to 45 ± 6% (under tetracycline exposure) and 38 ± 8% (under ciprofloxacin exposure) after addition of antibiotics. Ciprofloxacin inhibited nitrification (declined to ∼50% NH4+ removal), associated with reduced abundance of Nitrosomonas, while tetracycline impacted denitrification, evidenced by a lower Thauera abundance. Despite these impacts, the system removed 88.3 ± 5.6% of tetracycline and 69.5 ± 12.4% of ciprofloxacin, primarily through biosorption (for both antibiotics were more than 80%). Extracellular polymeric substances content increased by ∼19% under antibiotics exposure. Metagenomic analysis indicated changes in microbial community composition and function, while the overall antibiotic resistance gene profile remained relatively stable despite dynamic changes in individual resistance genes under antibiotic exposure. These findings demonstrate the strong potential of MB-AGS systems for effective organic carbon removal, while also highlighting opportunities to further enhance nutrient removal and mitigate antibiotic resistance genes under antibiotic stress.