Diversity and Distribution of Methane Functional Microorganisms in Sedimentary Columns of Hongfeng Reservoir in Different Seasons

Freshwater ecosystem is a significant natural source of CH₄ emission in the atmosphere. To fully understand the dynamics of methane emissions in reservoirs, it is essential to grasp the temporal and vertical distribution patterns, as well as the factors that influence the methanogenic bacterial communities within the sediments. This study investigates the methane dynamics, carbon isotope fractionation (δ¹³CH₄), and abundance of functional microorganisms along the geochemical gradient in the in situ sedimentary column of Hongfeng Reservoir (China). Notably, the methane concentration in sediment in summer ranged in 15.39-127.22 µmol/L, which is twice as high as wintertime concentrations in the surface layer near the sediment-water interface (0-10 cm depth). Illumina sequencing of the sediments identified 11 genera affiliated with methanogenic archaea, with dominant genus Methanosaeta reaching a relative abundance of 34.95% in summer. The total carbon (TOC) content in sedimentary columns in different seasons is positively correlated with Methanosarcina (P < 0.05). In addition, seasonal discrepancies are observed in the sediment profiles for total nitrogen (TN), sulfate (SO₄^2-), and ferrous iron (Fe²⁺) concentrations. The concentration of total nitrogen (TN) is higher in summer than in winter. In summer, sulfate accumulates in the middle layer of the sedimentary column, while in winter, the maximum concentration of sulfate in the surface layer reaches 0.65 mmol/L. These geochemical gradients drive the biological transformation of nitrogen, sulfur, and iron, may also be linked to the consumption of methane. Thus, it is established that the temporal and spatial dynamics of methanogenic communities in sediments significantly influence the fluctuations in methane release fluxes within reservoirs, highlighting the necessity to account for seasonal biological variations when assessing greenhouse gas emissions from reservoirs.