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.