Abstract <p>Precipitation of authigenic carbonates during diagenesis of bottom sediments is often a by-product of microbial activity, and the isotopic characteristics of such carbonate minerals are determined by the carbon isotope fractionation in enzymatic reactions. The composition of microbial communities of three mud volcanoes of Lake Baikal was assessed using the 16S rRNA gene metabarcoding. The chemical composition of pore waters of the bottom sediments of these mud volcanoes, containing siderites (FeCO₃) anomalously enriched in the isotope ¹³C (δ¹³C +7.24‰…+35.02‰ VPDB), for which formation mechanisms are currently unknown, was determined. The obtained data are consistent with the hypothesis that siderite precipitation may be associated with local zones of active biogenic degradation of organic matter; however, it is suggested that the function of microbial communities in this process is to reduce crystalline forms of Fe(III) oxyhydroxides to siderite or to replenish the Fe(II) pool, rather than that of dissolved inorganic carbon (DIC). Furthermore, the main factor determining the precipitation of authigenic siderites enriched in the ¹³C isotope may be the supply of DIC with a deep fluid flow or of gaseous CO₂, the isotopic characteristics of which will largely determine the δ¹³C values of carbonates in the sediments of the mud volcanoes of Lake Baikal.</p>