Interactive effects of warming and copper toxicity on a tropical freshwater green microalga Chloromonas augustae (Chlorophyceae).

Microalgae, the primary producers in aquatic ecosystems, are highly susceptible to heavy metal contamination. In this study, the interactive effects of warming and copper (Cu) toxicity on the physiology (cell density, photosynthetic efficiency, reactive oxygen species (ROS) production, and metal uptake in the biomass) and biochemistry (metabolite) of a freshwater green microalga, Chloromonas augustae (UMACC246), were elucidated. The microalgae were exposed to culture media supplemented with copper (II) sulfate pentahydrate (CuSO4·5H2O) at different concentrations (50, 150, 250 μM) at two temperatures, 25 °C (control) and 30 °C (sub-optimal), for 24 h. The results indicated that C. augustae exhibited a concentration- and temperature-dependent decrease in the cell density. Copper greatly affected the photosynthetic efficiency of C. augustae by reducing the maximum rate of relative electron transport (rETRm), light harvesting efficiency (α), and saturation irradiance (Ek). Warming increased ROS production remarkably in the microalga. Untargeted metabolomics indicated that temperature contributed to the most significant variations in the cultures (p < 0.05) in comparison with Cu toxicity or both factors combined. Compounds such as amino acids and amines were significantly dysregulated in response to warming and Cu toxicity. Pathway analyses showed that the glutathione metabolism, sulfur metabolism, and mechanisms in the amino acid metabolism were regulated, suggesting that the microalga underwent primary metabolism restructuring for survival and adaptation. Overall, the data showed that warming enhanced Cu toxicity in the cultures. This implied that increasing water temperature and metal toxicity due to global warming and anthropogenic activities will probably exacerbate existing threats to the primary producers. [ABSTRACT FROM AUTHOR]