Multiple Pathways Drive CH₄ and N₂O Emissions in Coastal Macroalgal Ecosystems

Coastal macroalgal ecosystems, while recognized for their capacity to sequester CO₂, remain poorly understood as potential sources of potent greenhouse gases methane (CH₄) and nitrous oxide (N₂O). Here, we show that the surface waters dominated by Ulva prolifera during green tide in the Yellow Sea exhibited 346% and 147% higher CH₄ and N₂O concentrations, respectively, compared to nonmacroalgal waters, despite dissolved oxygen (DO) levels exceeding 6.0 mg/L. Laboratory cultivation experiments revealed a two-stage dynamic of CH₄ release: during the growth stage (DO > 5.0 mg/L), CH₄ release was driven primarily by algal photosynthesis and reactive oxygen species; whereas during decay (DO > 4.0 mg/L), CH₄ release increased 2-3 fold, fueled by microbial degradation of algal-derived methanogenic precursors, including dimethylsulfoniopropionate and methylphosphonate. Methanogenic archaea were also detected in anoxic microniches within the macroalgal matrix, indicating additional CH₄ production. For N₂O, isotopic tracing experiment demonstrated that both ammonia oxidation and denitrification contribute to its production during macroalgal growth, while emissions declined to negligible levels during late decay due to nitrogen depletion. These findings reveal multiple aerobic pathways for CH₄ and N₂O emissions in coastal macroalgal ecosystems, highlighting previously unrecognized non-CO₂ greenhouse gas fluxes that should be considered alongside carbon sequestration in evaluating the climate impacts of macroalgae.