Photodegradation is an important transformation pathway for macrolide antibiotics (MCLs) in aquatic environments, but the ecotoxicity of MCLs after phototransformation has not been reported in detail. This study investigated the effects of roxithromycin (ROX) before and after phototransformation on the growth and physio-biochemical characteristics of Chlorella pyrenoidosa, and its toxicity were explored using transcriptomics analysis. The results showed that 2 mg/L ROX before phototransformation (T0 group) inhibited algae growth with inhibition rates of 53.06%, 54.17%, 47.26%, 31.27%, and 28.38% at 3, 7, 10, 14, and 21 d, respectively, and chlorophyll synthesis was also inhibited. The upregulation of antioxidative enzyme activity levels and the malondialdehyde content indicated that ROX caused oxidative damage to C. pyrenoidosa during 21 d of exposure. After phototransformation for 48 h (T48 group), ROX exhibited no significant impact on the growth and physio-biochemical characteristics of the microalgae. Compared with the control group (without ROX and its phototransformation products), 2010 and 2988 differentially expressed genes were identified in the T0 and T48 treatment groups, respectively. ROX significantly downregulated genes related to porphyrin and chlorophyll metabolism, which resulted in the inhibition of chlorophyll synthesis and algae growth. ROX also significantly downregulated genes of DNA replication, suggesting the increased DNA proliferation risks in algae. After phototransformation, ROX upregulated most of the genes associated with the porphyrin and chlorophyll metabolism pathway, which may be the reason that the chlorophyll content in T48 treatment group showed no significant difference from the control group. Almost all light-harvesting chlorophyll a/b (LHCa/b) gene family members were upregulated in both T0 and T48 treatment groups, which may compensate part of the stress of ROX and its phototransformation products.

光降解是大环内酯类抗生素（MCL）在水生环境中的重要转化途径，但光转化后MCL的生态毒性尚未详细报道。本研究探讨了光转化前后罗红霉素（ROX）对蛋白核小球藻生长和生理生化特性的影响，并通过转录组学分析探讨了其毒性。结果表明，光转化前（T0组）2 mg/L ROX对藻类生长有抑制作用，3、7、10、14、21 d抑制率分别为53.06%、54.17%、47.26%、31.27%、28.38%。分别，叶绿素合成也受到抑制。抗氧化酶活性水平和丙二醛含量的上调表明，ROX 在暴露 21 d 期间对蛋白核糖藻造成了氧化损伤。光转化48小时后（T48组），ROX对微藻的生长和生理生化特性没有表现出明显的影响。与对照组（不含ROX及其光转化产物）相比，T0和T48处理组分别鉴定出2010个和2988个差异表达基因。 ROX显着下调与卟啉和叶绿素代谢相关的基因，从而抑制叶绿素合成和藻类生长。 ROX 还显着下调 DNA 复制基因，表明藻类中 DNA 增殖风险增加。光转化后，ROX上调了大部分与卟啉和叶绿素代谢途径相关的基因，这可能是T48处理组叶绿素含量与对照组无显着差异的原因。 T0和T48处理组中几乎所有捕光叶绿素a/b (LHCa/b)基因家族成员均上调，这可能补偿ROX及其光转化产物的部分应激。