Sedimentary microorganisms play crucial roles in maintaining the functional stability of aquatic ecosystems. However, their taxonomic composition and assembly processes are not well known in estuarine–coastal margins because of their complex environment. We investigated microbial communities, co-occurrence relationships, and underlying mechanisms in 33 surface sediment samples collected in the Jiulong River Estuary and the Taiwan Strait to reveal their composition dynamics. The abundance, diversity, and composition of microorganisms demonstrated obvious spatial variables. Methanobacterium and Methanosarcina, as well as Candidatus_Nitrosopumilus and Nitrososphaeraceae were the main methanogenic and ammonia-oxidizing archaea, with an average abundance of more than 5.91% and 4.27%, respectively. Along with a salinity gradient increase, the relative abundance of methanogenic archaea (from 42.9% to 16.6%) contrasted with the trend of ammonia-oxidizing archaea (from 6.04% to 18.7%). The number of methanogenic archaea gradually decreased with increasing geographic distance (p < 0.05), whereas ammonia-oxidizing archaea showed no significant change (p > 0.05). In co-occurrence patterns, closer inter-taxa connections were observed among archaea–archaea and bacteria–bacteria than in archaea–bacteria, which indicated that coexistence within the same kingdom was greater than interaction between different kingdoms in shaping the community structure along the salinity gradient. Furthermore, null model analyses of the microbial community showed that undominated was the most prominent process, explaining over 44.9% of community variation, followed by heterogeneous selection and dispersal limitation, which contributed to 27.7% and 16.3%, respectively. We demonstrated that stochasticity, rather than determinism, regulates community assembly. These results further highlight that intra-kingdom co-occurrence and stochastic processes shape the structure and assembly of microbial communities in estuarine–coastal margins.
Archaea participate in biogeochemical cycles in aquatic ecosystems, and deciphering their community dynamics and assembly mechanisms is key to understanding their ecological functions. Here, sediments from 12 selected reservoirs from the Wujiang and Pearl River basins in southwest China were investigated using 16S rRNA Illumina sequencing and quantitative PCR for archaeal abundance and richness in all seasons. Generally, archaeal abundance and α-diversity were significantly correlated with temperature; however, β-diversity analysis showed that community structures varied greatly among locations rather than seasons, indicating a distance-decay pattern with geographical variation. The null model revealed the major contribution of stochasticity to archaeal community assembly, which was further confirmed by the neutral community model that could explain 71.7% and 90.2% of the variance in archaeal assembly in the Wujiang and Pearl River basins, respectively. Moreover, sediment total nitrogen and organic carbon levels were significantly correlated with archaeal abundance and α-diversity. Interestingly, these nutrient levels were positively and negatively correlated, respectively, with the abundance of methanogenic and ammonia-oxidized archaea: the dominant sedimentary archaea in these reservoirs. Taken together, this work systematically characterized archaeal community profiles in reservoir sediments and demonstrated the combined action of stochastic processes and nutrient load in shaping archaeal communities in reservoir ecosystems.
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.
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