Inhibition mechanisms of perchlorate on the photosynthesis of cyanobacterium Synechocystis sp. PCC6803: Insights from physiology and transcriptome analysis

Perchlorate (ClO4−) is a type of novel persistent inorganic pollutant that has gained increasing attention because of its high solubility, poor degradability, and widespread distribution. However, the impacts of perchlorate on aquatic autotrophs such cyanobacterium are still unclear. Herein, Synechocystis sp. PCC6803 (Synechocystis) was used to investigate the response mechanisms of perchlorate on cyanobacterium by integrating physiological and transcriptome analyses. Physiological results showed that perchlorate mainly damaged the photosystem of Synechocystis, and the inhibition degree of photosystem II (PSII) was severer than that of photosystem I (PSI). When the exposed cells were moved to a clean medium, the photosynthetic activities were slightly repaired but still lower than in the control group, indicating irreversible damage. Furthermore, perchlorate also destroyed the cellular ultrastructure and induced oxidative stress in Synechocystis. The antioxidant glutathione (GSH) content and the superoxide dismutase (SOD) enzyme activity were enhanced to scavenge harmful reactive oxygen (ROS) in Synechocystis. Transcriptome analysis revealed that the genes associated with "photosynthesis" and "electron transport" were significantly regulated. For instance, most genes related to PSI (e.g., psaf, psaJ) and the "electron transport chain" were upregulated, whereas most genes related to PSII (e.g., psbA3, psbD1, psbB, and psbC) were downregulated. Additionally, perchlorate also induced the expression of genes related to the antioxidant system (sod2, gpx, gst, katG, and gshB) to reduce oxidative damage. Overall, this study is the first to investigate the impacts and mechanisms of cyanobacterium under perchlorate stress, which is conducive to assessing the risk of perchlorate in aquatic environments.