4.5 Relation of C-N-S function genes
Through the above analysis, C-N-S related functional genes are usually enriched in the stagnant zone, indicating that these functional genes were related to each other, as cluster analysis shown in heatmap Fig.11a.
The increased relative abundance of N cycling genes could increase nutrient availability in stagnant coal reservoirs which could in turn affect C metabolism. The C metabolism were also related with each other.
Breakdown of polysaccharides into simple sugars is the primary source of energy and carbon for the microbial community. Degradation pathways for the monosaccharides, glucose, galactose and xylose were also prevalent in the Picrust2 data, include the genes associated with mannose metabolism, carbohydrate hydrolases, lactose and galactose uptake and utilization, L-fructose utilization, xylose utilization (Huang et al., 2017).
In the anaerobic coal reservoir, especially in the stagnant area, inorganic terminal electron acceptors (nitrate, sulfate) are rare, fermentation and acetogenesis are essential pathways for the further degradation of monosaccharides, and supply the substrates for methanogenesis. Fermentation produces low-molecular-weight alcohols and organic acids such as ethanol, propionate, acetate and lactate, as well as hydrogen and carbon dioxide. These genes were particularly abundant in the stagnant area, suggesting increased metabolic substrate production. These substrates such as H2 and acetic acid could be used by the methanogens, as a result, methanogens were more enriched in stagnant area.
Analysis of these function genes and their abundances and expression enabled us to identify correlations between specific microbial populations and biogeochemistry, and revealed key populations that drive the mineralization of organic matter from polysaccharides through to simple sugars, and the greenhouse gases CO2 and CH4.
Fig.10 shows the genes’ relative abundance, including C decomposition genes, sulfur metabolism, nitrogen metabolism and methane metabolism. C decomposition genes and nitrogen metabolism genes were more abundant than sulfur metabolism and methane metabolism genes. This suggested fermentation was important in the study area, different microorganisms such as methanogen, nitrate reduction and sulfate reduction bacteria might compete for limited fermentation substrates. The relative low abundance of methanogens was an important reason that biogenic gas was not enriched in the study area. According to the above analysis, the microbial C-N-S cycle pattern in Shizhuangnan Block was built (Fig.13), indicating denitrification, methanogenesis and sulfate reduction, were increased in the stagnant area.
Although the significant increase in abundance of the genes involved in nutrient cycling processes observed in stagnant area may potentially enhance the rates of nutrient cycling, more in-depth studies are necessary to determine the rates and extent of stimulation of different nutrient-cycling processes in the future.