Uranium increases toxin biosynthesis gene expression and cell toxicity in the cyanobacteria Microcystis aeruginosa

The present of uranium (U) in aquatic environments may be toxic to microorganisms. However, the effect and toxicity mechanism of U to the cyanobacterium Microcystis aeruginosa is poorly understood. In this study, a M. aeruginosa strain was exposed to different U doses, and then its physiological and transcriptomic responses to U were investigated. Results showed that the growth, chlorophyll a contents and photosynthesis efficiencies were inhibited by relative higher U doses (4.0 and 8.0 mg/L). U exposure also triggered the generation of reactive oxygen species (ROS), resulting in elevated superoxide dismutase (SOD) activity at relative lower doses and short-time exposures, and subsequently reduced at high doses and long-time exposures. The levels of microcystins (MCs) increased with the increase of U concentrations and times, but decreased at 4 day treatment. Transcriptomic analysis of M. aeruginosa revealed that the many photosynthesis-related genes were differentially expressed, while the antioxidant enzyme genes upregulated to resist the oxidative stress caused by U exposure. Most of genes involved in N metabolism were upregulated to improve the N absorption and utilization. MC biosynthesis-related genes have similarity decreasing trends with MC level changes at 4 day exposure. These findings underscore the physiological influence of U on M. aeruginosa and highlight its role in stimulating the MC generation, thereby increasing ecological risks in aquatic systems.