Copper-nickel interaction effects on bioaccumulation and physiological responses of the acid-tolerant microalga Trebouxiophyceae sp. MA-1.

Copper (Cu) and nickel (Ni) coexist widely in industrial and municipal wastewater, especially acid mine drainage (AMD). However, research concerning the toxic effects and ecological risks of combined Cu/Ni stress on microalgae is scant, and a systematic comparison of microalgal responses between single and co-exposure conditions has yet to be systematically conducted. An indigenous acid-tolerant AMD microalga, Trebouxiophyceae sp. MA-1, was isolated and examined for its Cu and Ni tolerance and removal in this study. The algal growth was inhibited more severely by Cu than Ni. Co-exposure caused significant growth inhibition of the microalga. Intracellular uptake dominated Cu bioaccumulation in MA-1, yet 20 mg/L Ni drastically raised surface-adsorbed Cu fractions from 1.1 % to 35.5 % (0.8 mg/L Cu) and 13.1 % to 52.7 % (1.5 mg/L Cu). Functional groups associated with extracellular proteins and polysaccharides provided abundant binding sites, and protein-like substances in extracellular polymeric substances (EPS) were actively involved in Cu complexation. Ni promoted Cu bioaccumulation and aggravated oxidative stress; MDA levels reached 2.45- and 2.41-fold of the control under 1.5 mg/L Cu combined with 2 and 20 mg/L Ni, respectively. To mitigate this stress, MA-1 upregulated the activities of superoxide dismutase (SOD) and catalase (CAT) for reactive oxygen species (ROS) scavenging, and simultaneously enhanced the activities of enzymes related to the nitrogen metabolism pathway to assist in detoxification processes. Taken together, this study systematically elucidated the Cu/Ni bioaccumulation characteristics and detoxification responses of the acid-tolerant microalga MA-1, and highlighted the importance of accounting for interactions of multiple metals when assessing the ecological risks of heavy metal combined pollution.