Multiple Pathways Drive CH 4 and N 2 O Emissions in Coastal Macroalgal Ecosystems

Coastal macroalgal ecosystems, while recognized for their capacity to sequester CO 2, remain poorly understood as potential sources of potent greenhouse gases methane (CH 4 ) and nitrous oxide (N 2 O). Here, we show that the surface waters dominated by Ulva prolifera during green tide in the Yellow Sea exhibited 346% and 147% higher CH 4 and N 2 O 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 CH 4 release: during the growth stage (DO > 5.0 mg/L), CH 4 release was driven primarily by algal photosynthesis and reactive oxygen species; whereas during decay (DO > 4.0 mg/L), CH 4 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 CH 4 production. For N 2 O, 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 CH 4 and N 2 O emissions in coastal macroalgal ecosystems, highlighting previously unrecognized non-CO 2 greenhouse gas fluxes that should be considered alongside carbon sequestration in evaluating the climate impacts of macroalgae.