Depth-stratified microbial communities drive methane cycling and mercury methylation in East Dongting Lake sediments

16S rRNA amplicon sequencing and redundancy analysis to reveal the depth-stratified patterns of microbial communities driving methane cycling and mercury methylation in sediments of East Dongting Lake across a water depth gradient (deep: 25–35 m, middle: 15–25 m, shallow: 5–15 m). Methanogens,including Methanosaeta , were most abundant in the organic-rich middle layer (TOC: 11.56 ± 2.25 mg g⁻¹). Methylmercury concentrations were significantly higher in the middle (0.1966 ± 0.0868 μg kg⁻¹) and deep layers (0.1944 ± 0.0676 μg kg⁻¹) than in the shallow layer, even though the deep layer contained the lowest total mercury (THg: 47.90 ± 4.07 μg kg⁻¹). Mercury-methylating Bacillus was most abundant in the deep layer. Redundancy analysis revealed THg and sulfide were strongly associated with microbial community stratification across sediment depths. These results demonstrate that sediment depth couples methanogenesis and mercury methylation via redox and sulfur gradients, offering new insights into the co-regulation of greenhouse gas emissions and metal toxicity in freshwater ecosystems. • Methanogens, such as Methanosaeta , were most abundant in the organic-rich middle sediment layer. • Mercury-methylating Bacillus dominated in the deep sediment layer. • Total mercury and sulfide strongly correlated with microbial community stratification. • Methylmercury levels were high in deep sediments despite low total mercury. • Sediment depth links methanogenesis and mercury methylation via redox and sulfur gradients.