Alkalinity (Bicarbonate) Pumping by Coastal Macroalgal Forests

Abstract Macroalgal ecosystems remain underrepresented in blue carbon frameworks, largely due to uncertainties surrounding the fate of macroalgal carbon during decomposition. Laboratory experiments on Sargassum horneri , Codium fragile , and Ulva australis revealed concurrent increases in total alkalinity and dissolved inorganic carbon, indicating substantial bicarbonate (HCO 3 − ) production via previously unrecognized mechanisms in oxygenated environments. This pattern was corroborated by field data from Korean coastal macroalgal habitats and Yellow Sea macroalgal blooms. Contrary to the conventional view that macroalgal decomposition primarily releases CO 2 , over half of the inorganic carbon is released as HCO 3 − in oxygenated waters. This bicarbonate surge—potentially driven by sulfate reduction in anoxic macroalgal aggregates or intracellular HCO 3 − release—not only raises seawater alkalinity but also enhances carbon sequestration potential and mitigates ocean acidification. These findings highlight the critical role of macroalgal habitats in the global carbon cycle and call for their inclusion in blue carbon strategies.