[Effects of ocean acidification on growth, phosphate and nitrate uptake of macroalgae]

The ocean acidification caused by elevated CO2 concentration can affect the growth, physiology and ecology, and nutrient uptake of macroalgae. In this paper, the growth and nutrient (PO4(3-) and NO3(-)) uptake of three species of macroalgae [Ulva pertusa, Scytosiphon lomentaria and Corallina pilulifera (calcified algae)] at three pHs (8.2, 7.9 and 7.6) were investigated under conditions of monoculture and mixed culture. Under the condition of monoculture, the percents of increased wet weights of U. pertusa, S. lomentaria and C. pilulifera on day 10 were the highest at the pHs of 7.9, 7.6 and 8.2, respectively, when compared with those on day 0, and the relative growth rates of C. pilulifera at pH 7.6 were significantly lower than those at pH 8.2. The data of mixed culture experiments suggested that lower pH was beneficial for the growth of S. lomentaria, while higher pH was beneficial for the growth of C. pilulifera. In both monoculture and mixed culture, the PO4(3-) and NO3(-) concentrations at the three pHs decreased with the time. The PO4(3-) concentrations decreased sharply by 71.9% - 99.0% from day 0 to day 2, and then decreased smoothly. Under the conditions of monoculture, the PO4(3-) uptake rates of U. pertusa, S. lomentaria and C. pilulifera were the highest at pHs of 8.2, 8.2 and 7.6, respectively. The NO3(-) uptake rates of U. pertusa and C. pilulifera in monoculture were the highest at pHs of 8.2 and 7.6, respectively. Under the conditions of mixed culture, the PO4(3-) uptake rates of U. pertusa + S. lomentaria, U. pertusa + C. pilulifera, S. lomentaria + C. pilulifera were the highest at pHs of 7.6, 8.2 and 8.2, respectively. The NO3(-) uptake rates of U. pertusa + C. pilulifera at pH 7.6 were the highest among the three pHs. The lower growth and higher nutrient (PO4(3-) and NO3(-)) uptake rates of monoculture C. pilulifera at pH 7.6 showed that the uptake and assimilation of PO4(3-) and NO3(-) were not coupled. The species composition of algae was changed due to ocean acidification although the dominant species was not change. Therefore, the different responses of growth and nutrient uptake of macroalgae to long-term ocean acidification in natural environment might lead to the change in macroalgal community.