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 CO ₂ 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 CO ₂ concentrations or high CO ₂ concentrations condition. In acute toxicity tests, elevated Cu concentrations negatively impacted the growth rate, yield, and photosynthetic rate of U. lactuca under low CO ₂ concentrations. Conversely, high CO ₂ 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 CO ₂ 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.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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