Copper (Cu) is vital for macroalgae's functions, but high concentrations can be toxic. Rising CO 2 levels affect algal growth and Cu bioavailability. In this study, the results reveal that at 5 °C, low Cu increased Ulva linza growth, while high Cu and elevated CO 2 decreased growth. At 10 °C, low Cu and elevated CO 2 enhanced growth, but high Cu did not have a significant impact. At 15 °C, high Cu reduced growth, but elevated CO 2 offset this effect. Furthermore, under elevated CO 2 conditions, the chloroplast structure of the algae appeared to be denser, accompanied by a large amount of starch granules, compared to low CO 2 conditions. These results emphasize that lower temperatures, in conjunction with elevated CO 2 concentration, could intensify the toxic effects of high Cu concentrations on thalli. However, at higher temperatures, elevated CO 2 concentration appeared to be capable of mitigating the detrimental effects of heavy metals on algae.