Antibiotics and antibiotic resistance genes increase cyanobacterial blooms by altering microbial nitrogen...

The effects of antibiotics and antibiotic resistance genes (ARGs) on endogenous nitrogen (N) biogeochemistry, particularly in relation to cyanobacterial bloom dynamics, remain poorly understood. Therefore, laboratory microcosms simulating the full life cycle of cyanobacteria were established to investigate the effects of clindamycin (CLIN), tetracycline (TC), sulfamethoxazole (SMX), and their mixture (MIX) on N cycling across the sediment-water interface. Antibiotics accelerated ammonia oxidation and thus nitrification in overlying water during cyanobacterial bloom and decline. ARGs may influence N-cycling functional microbiota and the associated N transformation processes indirectly through mechanisms such as MGEs-mediated horizontal gene transfer or modulation of microbial community structure. Antibiotics increased the ammonia supply in overlying water, thereby supporting cyanobacterial blooms, by increasing N-fixing genes and cyanobacteria and associated N fixation. In sediments, N-cycling genes and ARGs were core regulators of NO₂^--N and NH₃-N, suppressing organic N mineralization while promoting denitrification. At low ambient concentrations, CLIN and MIX stimulated cyanobacterial growth and bloom formation by increasing photosynthesis, strengthening N fixation capacity, and alleviating pressure from competing bacteria. By contrast, high concentrations caused toxic, inhibitory effects. Tetracycline increased cyanobacterial growth by reducing nutrient competition and increasing NH₃-N uptake at all concentrations. Sulfamethoxazole at 1000 ng/L sustained a longer duration of cyanobacterial bloom, possibly because it alleviates bacterial competition via folate synthesis inhibition. Therefore, by mediating bacterial communities and functional genes, antibiotics and their ARGs regulate endogenous N cycling and consequently influence cyanobacterial blooms. These findings provide novel insights into the mechanisms behind cyanobacterial bloom outbreaks in eutrophic lakes and thus have important implications for managing endogenous pollution.