Physiological and biochemical responses of the marine dinoflagellate Prorocentrum minimum exposed to the oxidizing biocide chlorine

• Significant growth and pigment reduction of Prorocentrum minimum exposed above 0.5 mg L−1 Cl2.
• Cl2 induced marked photosystem inhibition as judged by chlorophyll autofluorescence.
• Significant increases of antioxidant enzymes SOD & CAT activities with increasing Cl2
• Cl2 concentrations discharged in marine environments cause damage to the microalgae.

Toxic effects of the commonly used biocide chlorine (Cl2) on the marine dinoflagellate Prorocentrum minimum were assessed using growth-, pigment- and enzyme activity-based endpoints. Cell count, chlorophyll a levels, carotenoids, and chlorophyll autofluorescence were monitored up to 72 h after exposure to Cl2, and these parameters showed a dose- and time-dependent decrease. The 72-h median effective concentration (EC50) based on growth rate was 1.177 mg L−1. Cl2 dose above 0.5 mg L−1 were toxic to P. minimum after 6-h exposure to Cl2; the effect increased with increase in exposure time as revealed by a significant reduction in growth rate and decreased chlorophyll fluorescence. Moreover, the activities of antioxidant enzymes, including superoxide dismutase and catalase, were altered proportionally with increasing Cl2 dose. The results of this study show that Cl2 concentrations as observed in power-plant discharges and in drinking-water systems cause physiological and biochemical damage to the microalgae.