Harmful effects of the semiconductor copper tungstate (CuWO₄) on multiple photosynthetic parameters of the green microalga Raphidocelis subcapitata

The semiconductor copper tungstate (CuWO₄) may end up in aquatic ecosystems since it has the potential for water decontamination. The toxic effects of CuWO₄ are totally unknown for eukaryotic organisms. In view of this, we aimed to evaluate the toxicity of CuWO₄ particles (size of 161.5 nm) on the cosmopolitan green microalga Raphidocelis subcapitata (a standardized test-organism for ecotoxicological assays), analyzing the growth and multiple photosynthetic parameters obtained by pulse amplitude modulated (PAM) fluorometry. At 1.2 mg l^-1, the growth was affected, and there was an increase in reactive oxygen species (ROS) after 72 h The effective efficiency (ɸ_M') of photosystem II (PSII) was affected at 13.1 mg l^-1, while the efficiency of the oxygen-evolving complex (OEC), responsible for the water-splitting process, decayed at 5.6 mg l^-1. According to quenching parameters, energy allocated to photosynthetic processes (qP) decreased, indicating a malfunctioning of the PSII. We also observed a 50 % increase in the non-regulated energy dissipation by heat and fluorescence (Y(NO)), and a 50 % decrease in the regulated energy dissipation (NPQ), suggesting difficulties for algae to cope with light. Rapid light curves (RLC) were the second most sensitive parameter, as we observed a decay of the relative maximum electron transport rate (rETRmax) at 2.8 mg l^-1. Therefore, since the microalga R. subcapitata was affected by concentrations up to 1.2 mg l^-1 (0.01 mg l^-1 dissolved Cu), it is important to evaluate carefully the use of CuWO₄ to decontaminate natural waters, considering the protection of the aquatic biota.