Effects of Ocean Acidification and Temperature Coupling on Photosynthetic Activity and Physiological Properties of Ulva fasciata and Sargassum horneri

To investigate the ecological impacts of macroalgae in the framework of shifting global CO₂ concentrations, we conducted a study utilizing <i>Ulva fasciata</i> and <i>Sargassum horneri</i> specimens sourced from the Ma'an Archipelago in Zhejiang Province on how ocean acidification (OA) and temperature changes interact to affect the photosynthetic physiological responses of macroalgae. The results of the study showed that OA reduced the tolerance of <i>U. fasciata</i> to bright light at 20 °C, resulting in more pronounced photoinhibition, while 15 °C caused significant inhibition of <i>U. fasciata</i>, reducing its growth and photosynthetic activity, but OA alleviated the inhibition and promoted the growth of the alga to a certain extent. The tolerance of <i>S. horneri</i> to bright light was also reduced at 20 °C; the inhibition was relieved at 15 °C, and the OA further improved the algal growth. The Relative Growth Rate (<i>RGR</i>), photosynthetic pigment content, and the release of the dissolved organic carbon (DOC) of <i>U. fasciata</i> were mainly affected by the change in temperature; the growth of the alga and the synthesis of metabolites were more favored by 20 °C. A similar temperature dependence was observed for <i>S. horneri</i>, with faster growth and high metabolism at 15 °C. Our results suggest that OA reduces the tolerance of macroalgae to high light at suitable growth temperatures; however, at unsuitable growth temperatures, OA effectively mitigates this inhibitory effect and promotes algal growth.