Increasing atmospheric CO₂ is causing global public concern and seabed sequestration is one possible method of carbon reduction. However, studies on the potential risk of CO₂ leakage and its possible effects on the marine environment are still very limited. To investigate such possible effects on sensitive marine organisms, coralline algae, Corallina pilulifera, were cultured under controlled conditions: 20℃, 100μmol photons m^-2 s^-1 and a light period of 12h. Three treatments were set at acidities of pH 8.1, 6.8 and 5.5, by aerating natural seawater with pure gaseous CO₂. After 24 hours, photosynthesis and calcification rates of C. pilulifera cultured at different pH levels were determined. The rate of photosynthetic carbon fixation was enhanced at the pH of 6.8 and was inhibited at the pH of 5.5, compared with the algae grown in the seawater control (pH 8.1). The rate of calcified carbon fixation was depressed with decreasing pH, and even exhibited a negative value at pH 5.5. Additionally, with the decrease in pH, the ratio of particulate inorganic carbon (PIC) to particulate organic carbon (POC) content in the algae, measured with a vario TOC cube, decreased remarkably, which reflected the comprehensive effects of CO₂-induced seawater acidification on photosynthesis and calcification. Rapid light curves of algae cultured at different pH levels, which indicated the responses of electron transport rates (ETR) in photosystem II (PS II) to irradiance, were determined by pulse amplitude modulated chlorophyll fluorescence (PAM). The results showed that the photoinhibition term (a) increased with the decrease in pH, indicating that algae grown at lower pH levels experience greater photoinhibition. The light saturation point (I_k) decreased significantly under the CO₂-induced acidification conditions, though a significant difference was not found between pH of 6.8 and 5.5. The initial slope of the rapid light curve (α), reflecting the efficiency of the electron transport rate at low irradiance, was lower at pH 5.5 than at the other two levels, while there was no significant difference between pH 8.1 and 6.8 levels. The maximum relative electron transport rate (rETR_max) exhibited the highest value in algae cultured at pH 6.8 and the lowest at pH 5.5. According to these results, we concluded that CO₂-induced seawater acidification noticeably affected the photosynthesis and calcification of C. pilulifera, and different degrees of acidification caused different responses of photosynthesis and calcification. At the lowest pH level (pH 5.5), both the photosynthesis and calcification of C. pilulifera were significantly inhibited. These results provide a reference for studies on the risk of CO₂ leakage from seabed sequestration methods on the physiology and ecology of marine coralline algae.