Temporal and spatial distribution of functional nitrogen genes associated with thermal stratification in the subtropical reservoir

Abstract Seasonal thermal stratification is an important hydrological feature of subtropical reservoirs and affects the biogeochemical cycle of nitrogen, which is highly dependent on microbial processes. However, the influence of seasonal thermal stratification on the temporal and spatial distribution of nitrification and denitrification microorganisms is still not clear. This study conducted a detailed survey of a large subtropical reservoir (Xin'anjiang reservoir, XAJR) in eastern China across time (April, July, October 2021, and January 2022) and space (four sampling points with different water depths, covering the epilimnion, thermocline, and isothermal). The research showed that XAJR was a typical subtropical monomer model. Thermal stratification had spatiotemporal heterogeneity, and the DO and pH in water also showed a similar stratification phenomenon. We investigated the temporal and spatial distribution of gene abundance of bacteria and archaea (16S rRNA), ammonia-oxidizing archaea (amoA-AOA), ammonia-oxidizing bacteria (amoA-AOB), and denitrifying bacteria (nirS) by quantitative PCR. We found that the abundance of nitrogen functional genes also showed seasonal stratification in the vertical profile, and the vertical changes of the relative abundance of amoA-AOA / archaea, amoA-AOB / bacteria, and nirS / bacteria genes were highly similar to the corresponding changes of single functional genes. The quantitative distribution of amoA and nirS genes had seasonal differences. The copies number of amoA gene in the stratified stage were greater than that in the mixed stage, while the copies number of the nirS gene in the early and mixed stages of thermal stratification were greater than that in the stratified mature stage. RDA and correlation analysis showed that a variety of environmental factors affected the seasonal distribution of amoA and nirS gene abundance, which may be due to environmental changes caused by thermal stratification. Our results emphasized the importance of seasonal thermal stratification in the reservoir and expanded the understanding of the temporal and spatial distribution patterns of nitrifying and denitrifying microorganisms in the ecosystem of subtropical reservoirs.