Recent studies show that triclosan (TCS) exposure causes reduction in pigments,suppres-sion of photosynthesis,and induction of oxidative stress at the physiological level,resulting in morphological alteration and growth inhibition in algae including Raphidocelis subcapitata(R.subcapitata,a freshwater model green alga).However,the underlying molecular mecha-nisms remain to be elucidated,especially at environmentally relevant concentrations.The present study uncovered the transcriptional profiles and molecular mechanisms of TCS tox-icity in R.subcapitata using next-generation sequencing.The algal growth was drastically inhibited following a 7-day exposure at both 75 and 100 μg/L TCS,but not at 5 μg/L (environ-mentally realistic level).The transcriptomic analysis shows that molecular signaling path-ways including porphyrin and chlorophyll metabolism,photosynthesis-antenna proteins,and photosynthesis were suppressed in all three TCS treatments,and the perturbations of these signaling pathways were exacerbated with increased TCS exposure concentrations.Additionally,signaling of replication-coupled DNA repair was only activated in 100 μg/L TCS treatment.These results indicate that photosynthesis systems were sensitive targets of TCS toxicity in R.subcapitata,which is distinct from the inhibition of lipid synthesis by TCS in bacteria.This study provides novel knowledge on molecular mechanisms of TCS toxicity in R.subcapitata.