Two novel superoxide dismutase genes (CuZnSOD and MnSOD) in the toxic marine dinoflagellate Alexandrium pacificum and their differential responses to metal stressors

Superoxide dismutase (SOD) is an important antioxidant enzyme that is involved in the first line of defense against reactive oxygen species (ROS) within cells. Herein, we determined two novel CuZnSOD and MnSOD genes from the toxic marine dinoflagellate Alexandrium pacificum (designated as ApCuZnSOD and ApMnSOD) and characterized their structural features and phylogenetic affiliations. In addition, we examined the relative gene expression and ROS levels following exposure to heavy metals. ApCuZnSOD encoded 358 amino acids (aa) with two CuZnSOD-conserved domains. ApMnSOD encoded 203 aa that contained a mitochondrial-targeting signal and a MnSOD signature motif but missed an N-terminal domain. Phylogenetic trees showed that ApCuZnSOD clustered with other dinoflagellates, whereas ApMnSOD formed a clade with green algae and plants. Based on the 72-h median effective concentration (EC50), A. pacificum showed toxic responses in the order of Cu, Ni, Cr, Zn, Cd, and Pb. SOD expression levels dramatically increased after 6 h of Pb (≥6.5 times) and 48 h of Cu treatment (≥3.9 times). These results are consistent with the significant increase in ROS production in the A. pacificum exposed to Pb and Cu. These suggest that the two ApSODs are involved in the antioxidant defense system but respond differentially to individual metals.