Cu is considered to be toxic to macroalgae at higher levels. Ocean acidification can also alter the physiological performances of macroalgae. However, little is known regarding the interactive effects of Cu and ocean acidification on macroalgae. In this study, a green tide macroalga, Ulva prolifera, was cultured at the conditions of three levels of Cu (control, 0.5 µM, and 2 µM) and pCO2 (ambient, 1000 µatm, and 1400 µatm) to investigate the responses of U. prolifera to interaction of Cu exposure and ocean acidification. The relative growth rate of thalli decreased with the rise of Cu for all pCO2 conditions except the 1000 ?atm pCO2. Compared with the control, 2 µM Cu reduced the net photosynthetic rate for all pCO2 conditions while 0.5 µM Cu only reduced it at 1400 µatm pCO2. The inhibition rate of Cu on the relative growth rate and net photosynthetic rate was reduced at 1000 µatm pCO2 but was magnified at 1400 ?atm pCO2. Contrary to growth, the dark respiration rate was enhanced by 0.5 µM Cu at ambient pCO2 and by 2 µM Cu at ambient and 1000 µatm pCO2, although it was reduced by 2 µM Cu at 1400 µatm pCO2 compared to the control. The 0.5 µM Cu did not affect the relative electron transport rate (rETR) for any pCO2 condition but 2 µM Cu decreased it for all pCO2 conditions except 1000 µatm pCO2. The mute effect of 0.5 µM Cu on the net photosynthetic rate and rETR at ambient pCO2 may be due to more Chl a and Chl b being synthesized. In addition, 2 µM Cu and 1400 µatm pCO2 led to branched thalli, which may be a defense mechanism against the stress of high Cu and pCO2. Our data, for the first time, demonstrate that a modest increase of pCO2 can alleviate the toxicity of Cu to U. prolifera whilst a further increase exacerbates it. U. prolifera can respond to the stress of Cu pollution and ocean acidification via physiological and morphological alterations.
