Ocean warming and coastal eutrophication could intensify nitrous oxide emissions by bloom-forming green macroalgae.

Nitrous oxide (N₂O) is a potent and ozone-depleting greenhouse gas, for which the contribution of marine macroalgae to coastal N₂O dynamics remains poorly understood. Here, we investigated N₂O emissions from incubations with the bloom-forming green macroalga Ulva lactuca under simulated warming and eutrophication scenarios. Specimens were exposed to three temperatures (24, 27, and 30 °C) and three nutrient conditions representing no nutrient enrichment, natural eutrophication, and anthropogenic eutrophication (0, 10, and 50% von Stosch's Enrichment Solution [VSES], respectively) in a fully crossed laboratory experiment. Gas samples were collected over 2-h static incubations and analyzed by gas chromatography to determine biomass-normalized N₂O fluxes. N₂O emissions were significantly affected by temperature, nutrient levels, and their interaction. The highest flux was recorded at 30 °C under 10% VSES, reaching 673.41 ± 46.07 ng N₂O-N g-1 h-1. N₂O concentrations (ppb) were positively and significantly correlated with macroalgal nitrate uptake rates. Although N₂O emissions increased under some enriched conditions, greater nutrient availability did not consistently result in higher emissions. These findings provide experimental evidence that U. lactuca may contribute to coastal N₂O dynamics under environmental conditions relevant to Ulva blooms. However, the magnitude of this contribution under natural bloom conditions remains to be quantified. The contribution of surface-associated microorganisms to the measured N₂O fluxes was not independently quantified. Given the increasing frequency of coastal macroalgal blooms worldwide, these findings highlight the potential relevance of Ulva-associated N₂O emissions to coastal nitrogen cycling and greenhouse gas dynamics.