After the industrial revolution, increasing anthropogenic CO2 emission causes a number of changes in seawater. These changes are known as ocean acidification and affect the seaweeds in various ways. Therefore, this study is aimed to determine the ecological succession of Ulva flexuosa Wulfen 1803 in future predicted CO2-induced low pH conditions alone and in combination with naturally relevant ultraviolet radiation (UVR). For this purpose, acidification experiments with and without UVR were conducted on U. flexuosa from the Mediterranean coast, and important physiological features of algae was investigated. In this study, the Fv/Fm ratios of U. flexuosa ranged from 0.718 +/- 0.01 to 0.754 +/- 0.009. While rETRmax values of samples exposed to elevated-CO2 were measured between 112.13 - 151.93 mu mol e(-)m(-2)s(-1), it was determined between 111.7 - 158.4 mu mol e(-)m(-2)s(-1) in samples exposed to ambient sea water. According to our results, increased CO2 concentration in seawater did not improve the photosynthetic efficiency of U. flexuosa. However, when the specimens were exposed to elevated-CO2, nitrate reductase activity of U. flexuosa was declined drastically. According to the results, it is suggested that the elevated CO2 may regulate the nitrogen preference of U. flexuosa. Besides, the data also show that U. flexuosa was not sensitive to UVR.