Inhibitory effects and oxidative target site of dibutyl phthalate on Karenia brevis

The inhibitory action and possible damage mechanism of dibutyl phthalate (DBP) on the red tide algae Karenia brevis were investigated. The results showed that the algae experienced oxidative stress after exposure to 5 mg L−1 DBP. Malondialdehyde (MDA) peaked after 72 h, with a value approximately 2.3 times higher than that observed for untreated cells. The superoxide dismutase (SOD) and catalase (CAT) activities significantly increased as an adaptive reaction after 48 h. DBP induced the overproduction of reactive oxygen species (ROS), the OH concentration showed a peak of 33 U mL−1 at 48 h, and the highest H2O2 content was approximately 250 nmol/107 cells at 72 h; these latter two values were 2.5 and 4.4 times higher than observed for the control, respectively. TEM images showed that a number of small vacuoles or apical tubers were commonly found around the cell membrane, and the membrane structure was ultimately disintegrated. Further experiments were carried out to locate the original ROS production sites following DBP exposure. The activity of CuZn-SOD (a mainly cytosolic isoform, with some also found in chloroplasts) under DBP exposure was approximately 2.5 times higher than the control, whereas the Mn-SOD (mitochondrial isoform) activity was significantly inhibited. No significant difference was observed in the activity of Fe-SOD (chloroplastic isoform). In addition, dicumarol (an inhibitor of the electron transport chain in the plasma membrane) stimulated DBP-induced ROS production, whereas rotenone (an inhibitor of the mitochondria electron transport chain complex I) decreased DBP-induced ROS production. These results suggested that mitochondria could be the main target sites for DBP attack.