Response of the photosynthetic physiology of Ulva lactuca to Cu toxicity under ocean acidification

Ocean acidification can significantly affect the physiological performance of macroalgae. While copper (Cu) is an essential element for macroalgae and has been extensively studied, the interactive effects of ocean acidification and Cu on these organisms remain less understood. In this study, we measured the photosynthetic characteristics of Ulva lactuca exposed to varying Cu concentrations at two CO₂ levels (415 ppmv, low concentration; 1000 ppmv, high concentration). The results indicated that during chronic toxicity testing, the growth of juvenile U. lactuca significantly increased at Cu concentrations of 0.001 μM, 0.01 μM, and 0.1 μM regardless of low CO₂ concentrations or high CO₂ concentrations condition. In acute toxicity tests, elevated Cu concentrations negatively impacted the growth rate, yield, and photosynthetic rate of U. lactuca under low CO₂ concentrations. Conversely, high CO₂ concentrations enhanced the photosynthetic capacity of U. lactuca with increased Cu concentrations, while the growth rate significantly decreased at Cu concentration of 1.5 μM. Additionally, the activities of peroxidase (POD) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) increased, with an enhancement of malondialdehyde (MDA) content at 1.5 μM Cu under high CO₂ conditions. However, the structure of the chloroplast thylakoid was disrupted by elevated Cu concentrations. These findings suggest that low Cu concentrations promote the growth of U. lactuca, whereas high Cu concentrations inhibit algal growth, and ocean acidification may exacerbate the adverse effects of Cu on U. lactuca in acute toxicity tests.