Macroalgae play a significant role in global carbon sequestration. Substantial macroalgal organic carbon inputs and subsequent degradation can cause deoxygenation; however, the impact of oxygen deficiency on carbon fate remains understudied, which is critical for assessing the climate mitigation role of macroalgae. Here, we investigated changes in the carbon pool and non-CO₂ greenhouse gases (N₂O and CH₄) to assess the influence of oxygen levels on the carbon sink capacity of macroalgae. The microbial remineralization rate of macroalgal organic matter was not consistently slower under anoxic conditions (AK) compared to oxic conditions (OK). Total organic carbon (TOC) concentrations in the water column were 530±94 (OK) and 282±38 (AK) µmol kg^-1. For dissolved inorganic carbon (DIC), concentrations on day 30 were 4585±197 (OK) and 5200±492 (AK) µmol kg^-1, while those for total alkalinity (TA) were 2684±18 (OK) and 4523±671 (AK) µmol kg^-1. Following a 30-day sealed incubation, the bags were opened to reach atmospheric equilibrium. Subsequently, DIC dropped to 1837±79 (OK) and 3744±354 (AK) µmol kg^-1, and TA fell to 2059±14 (OK) and 4431±657 (AK) µmol kg^-1. Ultimately, relative to the control group (seawater only, OS) under air-sea equilibrium, the ΔDIC values were -22±76 and 1885±351 µmol kg^-1 in the OK and AK treatments, respectively, while ΔTA values were -57±11 and 2315±655 µmol kg^-1. The emissions of N₂O and CH₄ did not substantially offset the climate effect of carbon sequestration. These results suggest that, beyond the traditional focus on organic carbon preservation, anaerobic respiration under anoxic conditions may also contribute to macroalgal carbon sequestration by generating alkalinity that enhances the retention and stabilization of DIC.

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