Responses of Raphidocelis subcapitata exposed to Cd and Pb: Mechanisms of toxicity assessed by multiple endpoints.

Microalgae have been widely used in ecotoxicological studies in order to evaluate the impacts of heavy metals in aquatic ecosystems. However, there are few studies that analyze the effects of metals in an integrative way on photosynthetic apparatus of freshwater microalgae in the generation of reactive oxygen species (ROS) and biochemical composition. Therefore, this study aimed to assess cadmium (Cd) and lead (Pb) toxicity using synchronously physiological and biochemical endpoints, specially detecting lipidic classes for the very first time during Cd and Pb-exposure to Raphidocelis subcapitata. Here we show that analyzing the algae growth, the IC50-72 h for Cd was 0.04 µM and for Pb was 0.78 µM. In general, the Cd affected the biochemical parameters more, leading to an increase in total lipid content (7.2-fold), total carbohydrates (3.5-fold) and ROS production (3.7-fold). The higher production of lipids and carbohydrates during Cd-exposure probably acted as a defense mechanism, helping to reduce the extent of damage caused by the metal in the photosynthetic apparatus. For Pb, the physiological parameters were more sensitive, which resulted in changes of chlorophyll a synthesis and a reduction of both efficiency of oxygen-evolving complex and quantum yields. Besides that, we observed changes in the lipid class composition during Cd and Pb-exposure, suggesting these analyses as great biomarkers to assess metal toxicity mechanisms in ecological risk assessments. Thereby, here we demonstrate the importance of using multiple endpoints in ecotoxicological studies in order to obtain a better understanding of the mechanisms of metal toxicity to R. subcapitata.