The Effects of Inorganic Carbon and Irradiance on the Photosynthetic Performance and Growth of the Macroalga Sargassum horneri

Sargassum horneri is a highly productive macroalgal species capable of assimilating dissolved inorganic carbon (DIC) and converting CO2 into carbohydrates, making it a promising solution for carbon capture and biomass enhancement. Owing to its wide distribution and natural abundance, the utilization of S. horneri may help mitigate rising oceanic CO2 concentrations. This study evaluated the combined effects of inorganic carbon availability (2000, 4000, and 8000 μM NaHCO3) and irradiance (100, 150, and 200 µmol photons m−2 s−1) on photosynthetic performance and growth across short-term (24 h) and long-term (4-week) experimental trials. Carbon enrichment and light intensity interacted to significantly influence growth and carbon assimilation. The highest growth rate (35.83 ± 3.95%) was observed under 8000 μM DIC (0.75 g L−1) at 200 µmol photons m−2 s−1, corresponding to an optimal mean growth condition of 19 ± 0.04% (p < 0.05). These findings demonstrate that elevated inorganic carbon enhances photosynthetic efficiency by supplying sufficient substrate for carbon fixation, thereby supporting the feasibility of Sargassum horneri as a viable species for CO2 absorption and carbon capture applications.