Copper exposure induces the release of methyl iodide from coastal wetlands and seawater.

Methyl iodide (CH3I) is the main organic carrier for iodine transport from both marine and terrestrial ecosystems into the atmosphere, playing an important role in the global iodine cycle and ozone depletion. The widespread input of heavy metal copper (Cu(II)) deposition in marine environments due to anthropogenic activities may affect the production of CH3I, yet relevant studies are still lacking. Here, we explored the effects of Cu(II) on emission of CH3I in coastal wetlands and seawater. The results showed that Cu(II) addition significantly enhances CH3I emission from coastal wetland soils, with emissions under hydrogen peroxide (H2O2) co-addition reaching approximately five times those observed with Cu(II) alone. Notably, this promotional effect was more pronounced in coastal wetland soils with higher organic matter content. In seawater, addition of 400 nM Cu(II) further stimulated the abiotic processes, with emission of CH3I increased 136% (day 8) in filtered seawater (biologically reduced). In unfiltered seawater, despite 400 nM Cu(II) addition inhibition of biological activity evidenced by an 81.7% decrease in chlorophyll-a and a 49.2% reduction in bacterial abundance, the overall CH3I emission was further amplified, which we tentatively attributed to the release of organic matter from the decay of phytoplankton. These findings elucidate that elevated copper stress promote CH3I emission from both salt marsh soil and ocean into the atmosphere. This study provides an important scientific basis for assessing the impact of heavy metal pollution on CH3I emissions from coastal wetlands and seawater under climate change scenarios and anthropogenic perturbations.