Effects of triclosan on energy metabolism and fatty acid composition of the protist Euglena gracilis

Triclosan (TCS) is a widely utilized and effective antibacterial agent that can be found in various aquatic environments. However, the toxicity and underlying molecular pathways of TCS on protist are not well known. In this study, the acute (96 h) and subacute toxicity (8 days) of TCS to the protist Euglena gracilis were determined, and photosynthetic, transcriptional and lipidomic alterations were investigated to reveal the molecular mechanisms of TCS toxicity. E. gracilis exhibited lower sensitivity to TCS compared to microalgae, with a half maximal inhibitory concentration (IC₅₀) value of 1.20 mg/L. After 4 days of exposure, the photosynthetic efficiency was reduced in a dose-dependent manner. After 0.94 mg/L TCS treatment for an 8-day period, TCS targeted many differentially expressed genes that encode proteins involved in energy metabolism, lipid metabolism, cellular responses to antioxidases and reactive oxygen species (ROS). TCS significantly impacted metabolic activities as well as the biosynthesis of fatty acids. After TCS treatment, the medium-and long-chain fatty acid amount considerably declined, as shown by lipidomic analysis. Cellular response and regulation of ROS, especially superoxide dismutase and peroxidase activities were upregulated, suggesting oxidative stress in protist cells. This study demonstrates that low-dose TCS may inhibit the growth and photosynthesis of E. gracilis and exert oxidative stress at the earlier stage of TCS exposure but stimulate genes involved in energy and lipid metabolism, leading to the resilience and recovery of E. gracilis to TCS.