Methane emission fluxes and associated microbial community characteristics in a temperate seagrass meadow

Seagrass meadows are acknowledged as blue carbon ecosystems, yet they are also ideal habitats for methane (CH₄) release, offsetting their ability to mitigate climate change. The global CH₄ fluxes in seagrass meadows remain highly uncertain due to regional and species biases, and the microbial mechanisms driving methane release are poorly understood. Here, we investigated CH₄ air-sea fluxes, sediment CH₄ emission potential and microbes involved in CH₄ release using geochemical techniques combined with qPCR and Illumina sequencing in a temperate Zostera japonica and Zostera marina mixed meadow. The CH₄ air-sea fluxes fluctuated from -0.42 to 11.42 μmol·m^-2·d^-1, showing a strong seasonal variation. CH₄ emission potential was significantly higher in seagrass vegetated sediments (10.34 ± 2.72 nmol·g^-1·d^-1) than in the adjacent bare sediments (1.55 ± 1.15 nmol·g^-1·d^-1), primarily attributed to variations in sediment organic matter content. Diverse methanogens occurred in the seagrass meadow, with Methanolobus dominating in seagrass sediments, while Methanococcoides, Methanosarcina, and Methanoculleus being prevalent in bare sediments. Meanwhile, a variety of methylotrophic groups were detected, including aerobic Gammaproteobacteria, anaerobic Desulfobacterota and Methylomirabilota, as well as archaea Candidatus Methanoperedens. The co-occurrence of these functional groups implied the presence of complex CH₄ production and oxidation pathways, which regulated the CH₄ budget in the seagrass ecosystems. Taken together, our findings enhance the comprehension of the methane emission process and driving mechanism in seagrass ecosystems.