Methane emissions offset atmospheric carbon dioxide uptake in coastal macroalgae, mixed vegetation and sediment ecosystems.

Coastal ecosystems can efficiently remove carbon dioxide (CO) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH) emissions from these ecosystems may counterbalance atmospheric CO uptake. Still, knowledge of mechanisms sustaining such CH emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH in the range of 0.1 - 2.9 mg CH m d to the atmosphere, revealing in situ CH emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO-equivalent CH emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO release of unvegetated sediments by 57%. Accounting for CH alongside CO sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.