Abstract: In this study, the green alga Ulva linza was selected to assess the effects of temperature on macroalgae under the ocean acidification condition. The juvenile U. linza was _cultured at two CO₂ concentrations \[normal, 390 μL·L^-1 (LC); elevated, 1000 μL·L^-1 (HC)\] and at 20 ℃, then the response of photosynthetic physiology of adult algae to temperature changes (5, 10, 15, 20, 25 and 30 ℃) was measured. Our results showed that high CO₂ significantly enhanced the relative growth rate of U. linza, with the enhancements of the nitrate reductase activity and respiration rate (R_d), although the net photosynthetic rates were not significantly changed. Moreover, the contents of Chl a and Chl b significantly decreased when the algae was cultured at the high CO₂ level. An instantaneous change in temperature resulted in different photosynthetic performance at the different CO₂ concentrations. When the temperature decreased, the maximum photosynthetic rate (P_max), respiration rate （R_d), light saturation point (I_k) and light compensation point (I_c) were reduced. However, photosynthetic efficiency (α) increased first and then decreased with the increase of temperature. High CO₂ concentration significantly enhanced α, R_d and I_c compared to the control treatment. P_max and I_k were enhanced with the temperature increase, while α, R_d and I_c showed an opposite trend. Moreover, α and R_d at high temperature were higher in HCgrown algae than in LCgrown algae, but I_c was lower in HCgrown algae. Compared to the LCgrown algae, the HCgrown algae had a better ability to adapt high temperature.

Key words: water source, precipitation variation, stable isotope, Nitraria sibirica, desert habitat