<p>Green-tide forming Ulva meridionalis exhibits high growth rate and even doubles a day at an optimized culturing condition. U. meridionalis is expected to be a source for fixation of carbon dioxide due to high ability in photosynthesis and formation of cell wall polysaccharides. We used ¹³C-tracer to understand the mechanism how U. meridionalis rapidly capture carbon. U. meridionalis were cultivated for 12 h. under light or dark conditions using enriched artificial seawater medium (EASW) containing ¹³C-NaHCO₃ as a sole carbon source. Within 12 h of cultivation, U. meridionalis exhibited 1.8-fold increase in its dry weight and 41.2% incorporation of ¹³C. FT-ICR-MS analysis demonstrated that ¹³C-labelled nucleotide diphosphate (NDP) sugars started to appear within 4 hours of culturing under light with m/z ranging between m₀ (monoisotopic mass of NDP-hexose) and m₀₊₁₅. The semiquantitative ¹³C-NMR analysis revealed that ¹³C also rapidly transfers to matrix polysaccharide.</p>