Methane dynamics controlled by subsurface metabolic architecture and tidal cycling in a mangrove wetland

Tidal wetlands play a crucial role in global methane cycling. The complex interplay among tidal changes, microbial communities, and methane emissions in the ecosystems remains poorly understood. We investigated the dynamics of methane emissions and oxidizing activities, and related geochemical and microbial characteristics in the Guandu wetland, a subtropical mangrove mudflat influenced by tidal cycles in northern Taiwan. Our results revealed that the Guandu wetland was readily transformed into anoxic conditions (at >3 mm depth) and acted as a net source of methane and CO₂, with emissions peaking at 5-9 h of exposure to the atmosphere. Elevated methane emissions were intimately linked to highly variable methane and sulfate concentrations in the subsurface during low tide. Contrastingly, the methane and sulfate gradients were greatly attenuated during high tide. Methane oxidation rates derived from incubations of surface sediments with methane were positively correlated with the abundances of pmoA genes in surface sediments. Surface communities with the shortest exposure were distinct from subsurface counterparts and surface communities with prolonged exposure or subject to high methane incubations. These lines of evidence suggest that vigorous methane and sulfate cycling and enhanced methane emissions jump start with the development of a strong redox gradient derived from subsurface metabolic architecture and are mediated by the complexly structured community compositions as the tide recedes. Multifaceted analyses at a high temporal resolution are required to better project greenhouse gas emissions from such dynamic environments in the context of climate change schemes.