Effects of Roxithromycin Exposure on the Nitrogen Metabolism and Environmental Bacterial Recruitment of Chlorella pyrenoidosa

The ecotoxicity induced by macrolides has attracted widespread attention, but their impacts on the nitrogen metabolism and symbiotic environmental bacteria of microalgae remain unclear. This study examined the effects of roxithromycin (ROX) on the growth, chlorophyll levels, and nitrogen metabolism of <i>Chlorella pyrenoidosa</i>; investigated the changes in the composition and functions of environmental bacterial communities; and finally, analyzed the relationship between microalgae and environmental bacteria. The results indicated that all concentrations of ROX (0.1, 0.25, and 1 mg/L) inhibited microalgae growth, but the inhibition rates gradually decreased after a certain exposure period. For instance, the inhibition rate in the 1 mg/L treatment group reached the highest value of 43.43% at 7 d, which then decreased to 18.93% at 21 d. Although the total chlorophyll content was slightly inhibited by 1 mg/L ROX, the Chl-a/Chl-b value increased between 3 and 21 d. The nitrate reductase activities in the three treatments were inhibited at 3 d, but gradually returned to normal levels and even exceeded that of the control group at 21 d. Under ROX treatment, the consumption of NO₃^- by microalgae corresponded to the nitrate reductase activity, with slower consumption in the early stage and no obvious difference from the control group in the later stage. Overall, the diversity of environmental bacteria did not undergo significant changes, but the abundance of some specific bacteria increased, such as nitrogen-fixing bacteria (<i>unclassified-f-Rhizobiaceae</i> and <i>Mesorhizobium</i>) and organic contaminant-degrading bacteria (<i>Limnobacter</i>, <i>Sphingopyxis</i>, and <i>Aquimonas</i>). The 0.25 and 1 mg/L ROX treatments significantly enhanced the carbohydrate metabolism, cofactor and vitamin metabolism, amino acid metabolism, and energy metabolism of the environmental bacteria, but significantly downregulated nitrogen denitrification. This study provides new insights into the environmental bacteria-driven recovery mechanism of microalgae under antibiotic stress.