Ecotoxicity effects of roxithromycin before and after phototransformation on the physiological, biochemical characteristics and gene transcription in algae Chlorella pyrenoidosa

We report the effects of roxithromycin (ROX) before and after phototransformation on the growth and physio-biochemical characteristics of Chlorella pyrenoidosa, and the toxicity mechanisms were explored based on transcriptomic analysis. The results showed that ROX (2 mg/L, 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. ROX after phototransformation for 48 h (T48 group) exhibited no significant impact on the growth and physio-biochemical characteristics of the microalgae. Compared with the CK group (without ROX and its phototransformation products), 2010 and 2988 differentially expressed genes were identified in the T0 and T48 treatment groups, respectively. ROX downregulated the genes related to porphyrin, chlorophyll metabolism and DNA replication, and exposure to ROX inhibited the chlorophyll synthesis and algae growth and increased the DNA proliferation risks in algae. ROX after phototransformation upregulated most genes related to porphyrin and chlorophyll metabolism pathway, which may be the reason that the chlorophyll content in T48 treatment group showed no significant difference from the CK group. Almost all light-harvesting chlorophyll a/b (LHCa/b) gene family members were upregulated in both T0 and T48 treatment groups, suggesting that cell resistance against the stress of ROX and its phototransformation products can be enhanced in microalgae.