Norfloxacin is one of the widely used antibiotics, often detected in aquatic ecosystems, and difficultly degraded in the environment. However, how norfloxacin affects the photosynthetic process of freshwater phytoplankton is still largely unknown, especially under varied light conditions. In this study, we investigated photosynthetic mechanisms of Microcystis aeruginosa in responses to antibiotic norfloxacin (0-50 μg/L) for 72 h under low (LL; 50 μmol photons m⁻² s⁻¹) and high (HL; 250 μmol photons m⁻² s⁻¹) growth light regimes. We found that environmentally related concentrations of norfloxacin inhibited the growth rate and operational quantum yield of photosynthesis system II (PSII) of M. aeruginosa more under HL than under LL, suggesting HL increased the toxicity of norfloxacin to M. aeruginosa. Further analyses showed that norfloxacin deactivated PSII reaction centers under both growth light regimes with increased minimal fluorescence yields only under HL, suggesting that norfloxacin not only damaged reaction centers of PSII, but also inhibited energy transfer among phycobilisomes in M. aeruginosa under HL. However, non-photosynthetic quenching decreased in the studied species by norfloxacin exposure under both growth light regimes, suggesting that excess energy might not be efficiently dissipated as heat. Also, we found that reactive oxygen species (ROS) content increased under norfloxacin treatments with a higher ROS content under HL compared to LL. In addition, HL increased the absorption of norfloxacin by M. aeruginosa, which could partly explain the high sensitivity to norfloxacin of M. aeruginosa under HL. This study firstly reports that light can strongly affect the toxicity of norfloxacin to M. aeruginosa, and has vitally important implications for assessing the toxicity of norfloxacin to aquatic microorganisms.

诺氟沙星是广泛使用的抗生素之一，通常在水生生态系统中被发现，并且在环境中难以降解。但是，诺氟沙星如何影响淡水浮游植物的光合作用仍然是未知的，特别是在变化的光照条件下。在这项研究中，我们研究了铜绿微囊藻在低（LL； 50μmol光子m ^-2 s ^-1）和高（HL； 250μmol光子）对抗生素诺氟沙星（0-50μg/ L）反应72 h后的光合作用机制。m ^-2 s ^-1）生长光态。我们发现与环境有关的诺氟沙星浓度抑制了光合作用系统II（PSII）的生长速率和可操作的量子产率。HL下的铜绿假单胞菌比LL下的铜绿假单胞菌更多，这表明HL增加了诺氟沙星对铜绿假单胞菌的毒性。进一步的分析表明，在两种生长光模式下，诺氟沙星都使PSII反应中心失活，仅在HL下增加了最小荧光产量，这表明诺氟沙星不仅破坏了PSII的反应中心，而且抑制了铜绿假单胞菌中藻胆体之间的能量转移。根据HL。但是，在两种生长光条件下，诺氟沙星暴露都会使非光合猝灭作用减弱，这表明多余的能量可能无法有效地作为热量消散。同样，我们发现在诺氟沙星处理下活性氧（ROS）含量增加，而在HL下与LL相比，HL下的ROS含量更高。另外，HL增加了铜绿假单胞菌对诺氟沙星的吸收，这部分可以解释HL对铜绿假单胞菌对诺氟沙星的高敏感性。这项研究首先报道光可以强烈影响诺氟沙星对铜绿假单胞菌的毒性，并且对于评估诺氟沙星对水生微生物的毒性具有至关重要的意义。