4.3 Distinct assembly processes underlie similar biogeography of
abundant and rare bacteria
Disentangling the biogeography and community assembly of abundant and
rare microbial taxa is essential for understanding microbe-driven
ecosystem processes and functions in the Yarlung Tsangpo River. We
observed that both abundant and rare bacteria exhibited significant
distance-decay relationships (Fig. 2), which was consistent with
previous studies (Ji et al., 2020; Jiao et al., 2017; Mo et al., 2018).
Although both subcommunities yielded similar biogeographic patterns, we
found that the relative contributions of ecological processes to the
assembly of abundant and rare subcommunities were different (Fig. 4b).
Null model showed that the assembly of abundant subcommunity was
dominated by dispersal limitation (84%), and is in line with previous
investigations on inland lakes (Liu et al., 2015), epipelagic waters (Wu
et al., 2017), and agricultural soils (Jiao and Lu, 2020). This result
may be due to the strong environmental adaptation of abundant taxa as
mentioned above, which make them less sensitive to environmental
filtering and thus being more influenced by dispersal limitation.
Additionally, the VPA results in our study indicated the pure effect of
spatial variables surpassed those of environmental factors and land use
types for abundant taxa, suggesting the greater significance of
dispersal limitation in the abundant subcommunity assembly (Fig. S4). By
contrast, assembly of the rare subcommunity was dominated by
heterogeneous selection (77%), which was contradictory to previous
findings that the rare subcommunity was primarily controlled by
homogeneous selection (Jiao and Lu, 2020; Zhang et al., 2021a).
Heterogeneous selection refers to selection under heterogeneous abiotic
and biotic environmental conditions leading to more dissimilar
structures among communities (Zhou and Ning, 2017). These discrepancies
may be ascribed to the environmental heterogeneity in fluvial sediments.
Due to the interplay of climate change, increasing anthropogenic, and
complex geomorphologies, the water quality and biogeochemical nutrients
in the Yarlung Tsangpo River were suspected to be highly variable over
time and space (Wang et al., 2017; Wang et al., 2020b; Zhang et al.,
2021c). Therefore, we would expect a strong heterogeneous selection on
rare taxa in the investigated fluvial sediments.