Effects of Cadmium and Nickel Mixtures on Multiple Endpoints of the Microalga Raphidocelis subcapitata

It is crucial to investigate the effects of mixtures of contaminants on aquatic organisms, because they reflect what occurs in the environment. Cadmium (Cd) and nickel (Ni) are metals that co-occur in aquatic ecosystems, and information is scarce on their joint toxicity to Chlorophyceae using multiple endpoints. We evaluated the effects of isolated and combined Cd and Ni metals on multiple endpoints of the chlorophycean Raphidocelis subcapitata. The results showed that Cd inhibited cell density, increased reactive oxygen species (ROS) production (up to 308% at 0.075 mg L^-1 of Cd), chlorophyll a (Chl a) fluorescence (0.050-0.100 mg L^-1 of Cd), cell size (0.025-0.100 mg L^-1 of Cd), and cell complexity in all concentrations evaluated. Nickel exposure decreased ROS production by up to 25% at 0.25 mg L^-1 of Ni and Chl a fluorescence in all concentrations assessed. Cell density and oxygen-evolving complex (initial fluorescence/variable fluorescence [F₀/F_v]) were only affected at 0.5 mg L^-1 of Ni. In terms of algal growth, mixture toxicity showed antagonism at low doses and synergism at high doses, with a dose level change greater than the median inhibitory concentration. The independent action model and dose-level-dependent deviation best fit our data. Cadmium and Ni mixtures resulted in a significant increase in cell size and cell complexity, as well as changes in ROS production and Chl a fluorescence, and they did not affect the photosynthetic parameters. Environ Toxicol Chem 2024;43:1855-1869. © 2024 SETAC.