Abstract: This study adopted two species of marine macroalgae, Gracilaria lemaneiformis and Ulva conglobata as experimental materials, which were cultured under outdoor conditions. The treatments were designated as ambient (390 μL·L^-1) and elevated (800 μL·L^-1) CO₂ concentrations, and low (10 μmol·L^-1) and high N (150 μmol·L^-1) availability, in order to examine the effects of elevated CO₂ on growth and photosynthesis in marine macroalgae grown at different N supplies. During the cultivation, the biomass of the algae was measured. After culture for 18 days, the photosynthesis was examined with seawater in equilibrium with ambient (390 μL·L^-1) air and CO₂enriched (800 μL·L^-1) air, respectively. The results showed that elevated CO₂  significantly enhanced the relative growth rate (RGR) in both the macroalgal species regardless of the N levels in culture. However, the enhancement of growth was more pronounced in G. lemaneiformis, but less pronounced in U. conglobata, under low than high N conditions. Shortterm exposure to CO₂ enrichment stimulated photosynthesis of both the macroalgal species. Such stimulation of photosynthesis in G. lemaneiformis could be fully remained with prolonged growth with enrichedCO₂. However, long-term growth condition of CO₂ enrichment had an inhibitory effect on photosynthesis in U. conglobata. Additionally, the enriched N supply significantly increased photosynthesis and respiration in both the macroalgal species, regardless of the CO₂ concentration in culture.

Key words: non-structural carbohydrate, graft, nitrogen deficiency, photosynthesis