Seagrass and Macroalgae Detritus Drive Distinct Methane Production Dynamics and Microbial Succession in Coastal Sediments

Seagrass- and macroalgae-vegetated coastal sediments are critical carbon sinks but also methane (CH4) sources. Vegetation shifts between seagrasses and macroalgae modulate sediment biogeochemistry, yet their impacts on CH4 production and microbial controls remain poorly resolved. We conducted 106 day incubations of coastal sediments amended with graded loadings of seagrass, macroalgae, and their mixtures. Integrated CH4 monitoring, Gompertz modeling, 16S rRNA gene amplicon sequencing, PCR amplification of mcrA genes and mcrA transcript demonstrated that macroalgae amendment resulted in more rapid dissolved organic carbon release, earlier CH4 accumulation, higher CH4 yields, and stronger methanogen enrichment. Conversely, seagrass decomposition proceeded more gradually and was associated with comparatively weaker methanogenic responses. Mixed organic matter inputs enhanced CH4 production relative to single-source amendments, although this positive priming effect weakened with increasing macroalgae proportions. SHAP analysis further suggested that organic carbon availability and nutrient stoichiometry jointly influenced microbial carbon utilization and priming responses. Microbial succession closely tracked changes in sediment geochemistry and CH4 accumulation. These findings suggest that shifts in coastal vegetation composition may substantially alter CH4 production potential by modifying organic matter decomposition dynamics and microbial succession. This study establishes a quantitative framework for predicting CH4 emissions under scenarios of expanding, mixed seagrass-macroalgae ecosystems.