Impacts of the hydrochemical characteristics of coal seam water on the structures and metabolic functions of microbial communities

Background The core of coalbed methane (CBM) bioengineering is to inject bio-fracturing fluids rich in efficient methanogenic bacteria into coal reservoirs to promote the conversion of coals to methane. However, the relationships of the hydrochemical characteristics of coal seam water with the structures and metabolic functions of microbial communities remain unclear.

Methods This study investigated coal seam water from 12 typical mining areas in Ningxia. Based on microbial classification and sequencing, as well as statistical analysis, this study explored the relationships of pH, anions, cations, and chemical oxygen demand (COD) of coal seam water with the structures and metabolic functions of microbial communities.

Results and Conclusions The results indicate that the bacterial communities consist primarily of hydrolytic bacteria and bacteria enabling acidogenic fermentation, with dominant genera including Proteiniborus, Clostridium_sensu_stricto_1, and Thauera. The metabolism of methanogenic archaea occurs primarily through mixotrophy, with Methanosarcina identified as the dominant genus of these archaea. The coal seam water exhibits pH values ranging from 7.4 to 8.5. A decrease in pH value corresponds to increased diversity of the bacterial communities. However, the pH value exerts a small impact on the archaeal communities and an insignificant impact on the microbial metabolic functions. The diversity and abundance of the bacterial communities are positively correlated with the mass concentrations of anions Cl⁻ and \mathrmSO_4^2- when their mass concentrations are less than 905 mg/L and 1 974 mg/L, respectively. In contrast, the mass concentrations of cations Ca²⁺ and Mg²⁺ are significantly negatively correlated with microbial cell motility, intracellular transport, secretion, and vesicular transport, as well as the metabolism of inorganic ions, when their mass concentrations range from 5.6 mg/L to 411.0 mg/L and from 30.3 mg/L to 697.0 mg/L, respectively. Additionally, the COD in coal seam water exhibits significant positive correlations with the energy generation and transformation, along with the carbon cycle, involving microbial communities. Higher COD is associated with richer organic matter in water, thereby enhancing the involvement of microbial communities in the carbon cycle. The results of this study reveal the mechanisms behind the impacts of the hydrochemical characteristics of coal seam water on microbial communities while also laying a scientific basis for optimizing the formulation of bio-fracturing fluids in CBM bioengineering. Moreover, these results help understand the potential impacts of environmental changes on underground microbial communities during CBM production.