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.