Recovery of Alexandrium tamarense under chronic exposure of TiO2 nanoparticles and possible mechanisms

Harmful algal blooms (HAB), heavily influenced by human activities, pose serious hazard to aquatic ecology and human health. In this study, we monitored the physiological responses and paralytic shellfish poisoning toxins (PSTs) of the toxin-producing HAB species Alexandrium tamarense under titanium dioxide nanoparticles (nTiO2) exposure in the concentration range of 2-320 mg L-1 over a period of 13 days. The results showed the acute inhibition of nTiO2 on the algal growth, photosynthetic efficiency and esterase activity at all concentrations except 2 mg L-1. Nonetheless, they recovered after 13 days nTiO2 exposure from 20 to 80 mg L-1. The EC50 value increased from 85.1 mg L-1 in Day 4 to 140.9 mg L-1 in Day 13. The physiological recovery after prolonged exposure may result from the elimination of excess reactive oxygen species (ROS), a combined outcome of increased nTiO2 aggregation and algal antioxidant defense mechanisms. This observation is supported by the immediately increased antioxidant enzyme activities, including the superoxide dismutase (SOD) and catalase (CAT) activities upon nTiO2 exposure. Moreover, the production of PSTs in A. tamarense significantly increased by 1.41-1.76 folds after chronic nTiO2 exposure at all tested concentrations (p < 0.05), which might also be an adaptive response for the microalgae to overcome the stresses. In particular, the proportions of highly-toxic PSTs analogues GTX2/3, STX and dcSTX were significantly increased upon nTiO2 exposure (p < 0.05). Hence, the chronic nTiO2 exposure might aggravate the ecological impact of HABs. Furthermore investigations on different HAB species, especially those toxin-producing ones, and detail physiological responses are obviously needed.