Bacterial community composition
The obtained OTUs were assigned to 30 bacteria phyla, 54 classes, 150 orders, 267 families, 509 genera, and 783 species. The predominant phyla (relative abundance >1%) were Actinobacteria (41.8%), Firmicutes (29.3%), Proteobacteria (23.1%) and Bacteroidetes (2.7%) in the surface samples (Fig. 2A). The relative abundance of Firmicutes decreased from 67.9% in the 1S to 4.1% in the 8S. The relative abundance of Actinobacteria increased from 4.7% in the 1S to 77.7% in the 8S. The abundance of Cyanobacteria (2.0%) was the highest in the 2S. The abundance of Gemmatimonadete (2.2%) and Deinococcus-Thermus (0.90%) were the highest in the 1S. Calditrichaeota only existed in the 6S sample. Dependentiae, Latescibacteria, and Elusimicrobia only existed in the 1S, 2S and 3S samples. Synergistetes only existed in the 2S, 3S and 4S samples. Thermotogae only existed in the 1S, 2S, 3S and 4S samples. The above results suggest that the 1S, 2S, 3S, and 4S samples had higher bacterial community diversity than the 4S, 5S, 6S and 7S samples in the phylum level.
In the subsurface samples, the dominated bacterial phyla (relative abundance >1%) were Actinobacteria, Firmicutes, Proteobacteria, Bacteroidetes, Chloroflexi, Gemmatimonadete, Fusobacteria, and Acidobacteria (Fig. S1A). The relative abundance of Firmicutes decreased from 61.6% in the 1Su to 4.4% in the 8Su. The relative abundance of Actinobacteria increased from 5.7% in the 1Su to 75.4% in the 8Su. Dependentiae only existed in the 2Su samples. WPS-2 only existed in the 4Su samples. Entotheonellaeota only existed in the subsurface samples than surface samples.
At the genus level, the predominated genera in the samples were unclassifiedMicrococcaceae (36.6%), Bacillus (24.3%),Sphingomonas (4.1%), unclassified Burkholderiaceae(2.8%), Escherichia-Shigella (2.5%), Acinetobacter(2.5%), Microbacterium (1.8%),Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium (1.5%), andMethylobacterium (1.5%) (Fig. 2B). Besides, the relative abundance of Bacillus gradually decreased from north to south. The abundance of unclassified Micrococcaceae andSphingomonas gradually increased from north to south. The abundance of Gilliamella (14.9%) and Snodgrassella(10.7%) were significant higher in the 4S sample than those of other samples. The abundance ofAllorhizobium-Neorhizobium-Pararhizobium-Rhizobium (6.3%) was the highest in the 3S sample. Each sampling site has its unique genus. For example, Glycomyces , Constrictibacter ,Sulfurifustis, and Erysipelotrichaceae _UCG-003 only existed in the 1S sample. Candidatus_Saccharimonas andPromicromonospora only existed in the 2S sample.Megamonas , Acholeplasma and Perlucidibaca only existed in the 3S sample. Thermovirga ,Prochlorococcus _MIT9313, Succiniclasticum ,Shuttleworthia , Rosenbergiella , Aminobacterium, Barnesiella, and Rhizobacter only existed in the 4S sample. Norank Calditrichaceae and UBA1819 only existed in the 5S sample.Granulicella and Candidatus_Riegeria only existed in the 6S sample. Additionally, Pseudorhodoplanes , norank JG30-KF-CM66 and Novosphingobium only existed in the 1S, 2S, 3S, and 4S samples.
In the subsurface samples, the dominated bacteria were unclassifiedMicrococcaceae (32.1%), Bacillus (28.8%),Sphingomonas (3.8%), unclassified Burkholderiaceae(3.6%), Escherichia-Shigella (2.6%), Acinetobacter(2.9%), Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium(2.0%), Microbacterium (1.7%), Methylobacterium (1.6%) and Streptomyces (1.4%) (Fig. 2B). Compared with the surface samples, the dominant communities in the 4Su sample were distinctly different from those in the 4S sample (Fig. 2B and Fig. S1B).Streptomyces , Brevundimonas , Pseudomonas , norankBeijerinckiaceae , Bacteroides , Aeromonas ,Stenotrophomonas , Lactobacillus , Fusobacterium, andBradyrhizobium abundances in the 4Su sample were significantly higher than those in the other samples. The abundance of unclassifiedMicrococcaceae was significantly lower than that in the 2Su (0.3%) and 4Sub (0.7%) samples than the 2S (27.2%) and 4S (25.2%) samples. The abundance ofAllorhizobium-Neorhizobium-Pararhizobium-Rhizobium (9.8%) was the highest in the 1Su sample.