Discussion
This study was designed to reveal the variations in bacterial community structure and environmental factors and their correlations in the Taklimakan Desert. To date, the bacterial communities in about nine deserts in the world have been explored using 16S rRNA amplicon sequencing, and the results indicate the similar dominant bacterial phyla, i.e., Actinobacteria, Proteobacteria, Bacteroidetes, Firmicutes, Chloroflexi, and Cyanobacteria, though the proportion of each phylum displayed great difference among the deserts (Table 3). Actinobacteria have been documented to play an important role in soil development and the geochemical cycle (Goodfellow et al., 1983; Hill et al., 2011). The Actinobacteria bacteria can form spores to resist extreme conditions and can produce extracellular hydrolytic enzymes to decompose plant and animal residues and complex organic compounds in soils (El-Tarabily et al., 2006; Eisenlord et al., 2010; Miao et al., 2010). Firmicutes also is a representative bacterial phylum of desert microbes (Chanal et al., 2006; Lester et al., 2007; Gommeaux et al., 2010). The Firmicutes bacteria can produce endospores, which contribute to the colonization in arid environment. The abundance of Firmicutes in the Taklimakan Desert was much higher than that in the other deserts (Crits-Christoph et al., 2013; Gunnigle et al., 2017; Sun et al., 2018; Zhang et al., 2019). The dominated Cyanobacteria isolated from deserts such as Mojave Desert, Sonoran Desert, and Tengger Desert (Nagy et al., 2005; Mogul et al., 2017; Sun et al., 2018), which were covered by biocrusts in surface soil. However, the highest abundance of Cyanobacteria was 2% in the Taklimakan Desert. The dominated phyla in the Taklimakan Desert were similar to the Eastern Desert (Köberl et al., 2011). Moreover, although the Chloroflexi and Acidobacteria were the predominated phyla in some deserts, they were the low abundance phyla in the Taklimakan Desert. These results suggested that the common and unique characteristics of the bacterial community in deserts.
Several previous researchers investigated the key factors that shape the desert microbial communities. For example, Zhang et al. (2019) showed that salinity is the key factor to drive microbial community structure and assembly in the Gurbantunggut Desert. Crits-Christoph et al. (2013) suggested that water content and salinity are the key factor in shaping the microbiome in the Atacama Desert, the driest desert in the world. Scola et al. (2018) proposed that the soil chemistry and stochasticity influenced the bacterial community assembly in the Namib Desert, the oldest desert on the planet. The above results imply the differences in the soil physicochemical properties in different deserts, which in turn drive the succession and adaptation of microbes in the deserts. In the present study, RDA analysis showed that altitude and pH were strong significantly correlated with the bacterial community (Fig. 4). The VPA result indicated that altitude and pH were the main factors shaping bacterial community assembly in the Taklimakan Desert (Fig. S2A).
Many studies showed that the altitude gradient has the effect on soil microbial community (Körner, 2007; Manzoni et al., 2012; Serna-Chavez et al., 2013; Siles and Margesin, 2017). Many environmental factors, such as nitrogen loading, temperature, and UV radiation, vary with altitude elevation, resulting in the variation of the microbial community (Sundqvist et al., 2013; Hayden and Beman, 2016). Our results confirmed that the alpha diversity indices of bacterial communities were significantly different along the altitude gradient in the Taklimakan Desert (Table 2). Similar results were proved by the previous studies (Meng et al., 2013; Ren et al., 2018).
Altitude has a great influence on the distribution of microbial communities. Concerning bacterial community compositions,Actinobacteria displayed the highest relative abundance in neutral sands and was significantly negatively correlated with pH (Fig. 5). This result was consistent with the previous studies (Lauber, 2009; Yun et al., 2016). Many studies indicated that pH was the main driver to the Actinobacteria community and diversity in different ecological environments. For example, pH significantly influences the variation of the Actinobacteria community in the Palace Leas hay meadow (Jenkins et al., 2009). The Actinobacteria community in the Tianshan Glacier forelands was also affected by the pH (Zhang et al., 2016b). Our study showed that pH was a crucial environmental factor that affected the Actinobacteria community in the desert ecosystem. Additionally, pH was significantly positively correlated with Firmicute, Nitrospirae, Latescibacteria, Thermotogae, and Synergistetes. This could be explained by the fact that pH can directly affect the diversity of the microbial community by inducing the availability of substrates in soils (Kemmitt et al., 2006). Zhang (2019) and Feeser (2018) reported the EC was the key factor in structuring microbial communities for desert soil. In the present study, EC was nonsignificantly correlated with the bacterial community. This was inconsistent with the result that water content and conductivity had a great effect on bacterial diversity and community than that of temperature in desert stream sediments (Zeglin et al., 2011).
Zhao et al. (2019) reported that Chlorofexi was significantly positively associated with P. Li et al. (2014) showed that the abundance of Chlorofexi was closely related to EC. However, our result indicated that Chlorofexi was not associated with EC or P, but significantly negatively correlated with Ca2+ (Fig. 5). The effect of Ca2+ on desert microbial community assembly has been reported (Rao et al., 2016), which probably be another key factor for soil bacterial community structure composition (Xia et al., 2016).
The present study further investigated the difference of bacterial communities between the surface and subsurface sand (Fig. 6).Lachnospiraceae and Actinobacillus were the indicator bacterial taxa in the surface sand. The Lachnospiraceae is a common taxon observed in host-associated and sewage effluent samples (Meehan & Beiko, 2014). The Actinobacillus usually inhabited in the mucous membranes of humans and animals (Olsen et al., 2005). Obviously, the appearance of these taxa in the surface sand is related to human activity. Nitrosomonadaceae , Ellin 6067,Entotheonellaeota ,Paenibacillaceae ,Dialister, and TK10 were significantly enriched in the subsurface sand. The limitation of oxygen availability in the subsurface sand selected some anaerobic and microaerobic bacterial taxa, such as the anaerobicNitrosomonadaceae , Paenibacillaceae, andEntotheonella, which were dominated in the subsurface samples (Fig. 6). Nitrosomonadaceae has been reported to be ammonia-oxidizing bacteria (Xia et al., 2005). Studies have investigated theEntotheonellabacteria which can produce diverse natural active products (Bhushan et al., 2017). Moreover, the genomic analysis ofEntotheonella revealed thatEntotheonella can utilize a variety of carbon sources, fixation of CO2, anaerobic respiration, and denitrification (Liu et al., 2016). The order Ellin6067 was recognized as playing a vital role in the degradation of xenobiotic and other complex organic (Lezcano et al., 2017). The familyPaenibacillaceae not only secreted chitinase to degrade chitin (Tran et al., 2018) but also can utilize nitrates under anoxic condition through denitrification (Konishi et al., 2017) and degrade the explosives residues in anaerobic treatment (Indest et al., 2017). All these studies showed that the obvious difference of indictor bacteria in the surface and subsurface samples. These results implied that the strong adaptability of bacterial communities to different extreme environments.
In conclusion, our results showed that the abundance of bacterial communities varied significantly along an altitude gradient in Taklimakan Desert. Altitude and pH were the primary factors that determine the structure of bacterial communities. Besides, Ca2+ and P were also significantly correlated with the distribution of bacterial communities. At the abundance of Actinobacteria increased gradually while that of Firmicutes decreased gradually with the decrease in altitude gradient. The indicator groups were significant differences in the surface and subsurface sand. Studying bacterial diversity and community assembly processes along an altitude gradient are necessary for deeply understanding the fundamental ecological processes in desert ecosystems.