Floating iron biofilms as hidden barriers to methane emissions in wetlands

Wetland methane emissions are primarily mitigated by microbial oxidation at the sediment-water interface (SWI). Yet, this paradigm fails to explain persistent methane oversaturation in surface waters and lower-than-expected emissions. Here, we demonstrate an overlooked methane barrier formed at water–air interfaces (WAI) by self-organized iron biofilms. Organic matter inputs destabilize the SWI, enabling ferrous iron to diffuse upward and become oxidized, resulting in the formation of iron biofilms ranging in thicknesses from nanometers to centimeters at the WAI. This iron film barrier reduces methane emissions by 2.4- to 6.9-fold through physical entrapment and microbial oxidation (e.g., pmoA gene abundance: 2.1×10⁸ copies m⁻²), involving communities dominated by aerobic and facultative heterotrophs, as well as nitrate- and sulfur-respiring taxa. The dual methane-barrier paradigm explains emission paradoxes in straw-amended environments. This mechanism, absent from the Intergovernmental Panel on Climate Change models, could revise global methane budget estimates and guide nature-based solutions to mitigate wetland methane emissions.