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To investigate the contribution of macroalgae to biogeochemical nutrients and carbon cycles, we measured the uptake rates of nutrients and CO 2 by Undaria pinnatifida using an incubation method in an acrylic chamber. From January to March 2010, U. pinnatifida was sampled at Ilkwang, a well-known area of macroalgae culture in Korea. The initial and final concentrations of nutrients, dissolved oxygen, total alkalinity, and pH of the chamber water were measured, and production/uptake rates were calculated using concentration changes, chamber volume, and incubation time. The production rate of dissolved oxygen by U. pinnatifida (n = 32) was about 5.4 ± 4.0 µmol g fw -1 h -1 . The uptake rate of total dissolved inorganic carbon (TDIC), calculated by total alkalinity and pH, was 7.9 ± 6.5 μmol g fw -1 h -1 . Nutrients uptake averaged 141.7 ± 119.2 nmol N g fw -1 h -1 and 15.0 ± 9.1 nmol P g fw -1 h -1 . A positive linear correlation (r 2 = 9.6) existed between the production rate of dissolved oxygen and the uptake rate of total dissolved inorganic carbon, suggesting that these two factors serve as good indicators of U. pinnatifida photosynthesis. The relationships between fresh weight and uptake rates of nutrients and CO 2 suggested that younger specimens (<~50 g fresh weight) are much more efficient at nutrients and CO 2 uptake than are specimens >50 g. The amount of carbon uptake by the total biomass of U. pinnatifida in Korea during the year of 2008 was about 0.001-0.002% of global ocean carbon uptake. Thus, more research should be focused on macroalgae-based biogeochemical cycles to evaluate the roles and contributions of macroalgae to the global carbon cycle.

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