Microbial Communities in Carbonate-Containing Bottom Sediments of the Bolshaya Zelenovskaya Oil-Methane Seep (Central Baikal)

A layer (260–270 cm) of soft carbonate nodules containing numerous valves of ancient diatoms was discovered in bottom sediments of the Bolshaya Zelenovskaya oil-methane seep (Central Baikal). Various methods confirmed the presence of a siderite phase in the carbonate; their enrichment in the heavy isotope δ13C (+14‰ VPDB) and the light isotope δ18O (−8.43‰ VPDB) also has been detected. Carbonate precipitation could have been facilitated by the high organic matter content of ancient diatoms, dissolved inorganic carbon, or CO2, supplied by the flow of mineralized fluids and formed during methanogenesis. High-throughput sequencing of the variable region of the 16S rRNA gene revealed the dominance of archaea of the family Methanoregulaceae (11–35%) and of the class Bathyarchaeia (16–54%) with a significant contribution of representatives of Nitrososphaeria (18%) and Woesearchaeales (10%) in the surface sample, as well as of Hadarchaeota (12%) in the deep carbonate-containing sediment. Bacterial dominants were represented by the phyla Chloroflexota (14–30%) and Acidobacteriota (5.8–11%), as well as representatives of Pseudomonadota (32%) in the surface sample and of Artibacterota (28.5%) in the carbonate-containing sediment. Based on the isotopic composition of δ18O and δ13C, chemical composition of pore waters, and microbial diversity, the manuscript discusses the possible scenario of carbonate precipitation process.