To investigate the effects of ocean acidification (OA) on macroalgae, we have performed in-situ experiments around CO₂ seep in Shimoda, Japan, during the period of June to July in 2019. A total 23 species of macroalgae were observed in control site (pH=8.27), including 2 species of green algae, 18 species of red algae and 3 species of brown algae, while in the near of CO₂ seep area (pH=7.9), only 10 species of macroalgae were observed, referring to 3 species of green algae, 4 species of red algae and 3 species of brown algae. With decrease of the density of macroalgae, our results implied that the increase of CO₂ (lower pH) significantly reduce the diversity and abundance of macroalgae. By using a GFS-3000 gas exchange system, the carbon assimilation rate of those tested macroalgae were identified to increase from control site to CO₂ seep area, while the K_0.5 (half saturation constant of CO₂) showed a significant decrease, suggesting that high CO₂ induced OA increased the carbon fixation of macroalgae but declined the activity of CO₂-concentrating mechanisms (CCMs). The further measurements of rapid light curves by a Dual-wavelength pulse-amplitude modulated Chlorophyll a fluorescence monitoring system showed that those macroalgae inhabited in control site exhibited a higher electron transport rate, a higher light utilization efficiency and a lower minimal saturating light intensity, compared with those macroalgae living in CO₂ seep area. Thus, we speculated that OA may modulate the effects of light fluctuations on macroalgae. To further verify this speculation, light-sensitivity of Gelidium elegans and Dictyopteris undulata, the dominated species around control site and CO₂ seep area, respectively, were analyzed. Results showed that OA significantly increased the photosystem II (PSII) damage rate of G. elegans, but decreased its repair rate during the high light treatment; D. undulata, inhabited in the CO₂ seep area (low pH), was not affected by high light. In conclusion, our present results demonstrate that effects of OA on macroalgae are species-specific.