Abstract: In order to understand the impact of macroscopic algae aquaculture on the content of seawater dissolved inorganic carbon (DIC), and clarify the change process of DIC content in different conditions (light, dark, alternation of light and dark), Sargassum thunbergii and Hizikia fusiforme collected from Wenzhou Dongtou offshore were cultivated in laboratory, three treatments (light, dark, alternation of light and dark) were set in the experiment, DIC contents were measured and DIC change rates were analyzed. The results showed that: DIC consumption rate of Sargassum thunbergii was higher than that of Hizikia fusiforme under light condition. DIC release rate of Sargassum thunbergii was higher than that of Hizikia fusiforme under dark condition. DIC consumption rate of both Sargassum thunbergii and Hizikia fusiforme increased at first and then reduced under the light and dark alternation condition, and DIC consumption rate of Sargassum thunbergii was higher than that of Hizikia fusiforme. With the carrying out of the light and dark alternated process, photosynthetic consumed DIC and dark respiration released DIC converted continuously. Overall, the aquaculture of macroscopic algae decreased the DIC content of seawater and promoted the atmospheric CO₂ transfer to seawater. With the harvest of macroscopic algae, a “removable carbon sink” was formed. The development of macroscopic algae aquaculture had important carbon sink ecological significance.