4. Discussion
Revealing the underlying mechanisms driving the spatial scaling patterns
of the complex soil microbial communities is an essential issue in
microbial ecology and community
ecology(Jiang et al., 2018;
O’Brien et al., 2016). In this study, we
focused on the ecological mechanisms structuring the spatial scaling
patterns of abundant and rare subcommunities in an ocean sediment
ecosystem. Rare subcommunities were mainly responsible for the spatial
scaling patterns followed by microbes. Environmental heterogeneity was
significantly associated with the spatial scaling metrics of whole
community and rare subcommunities, but not abundant subcommunities.
Further experimental and statistical analysis suggested that distinct
ecological mechanisms underlay the spatial scaling patterns followed by
abundant and rare subcommunities.
We found that rare subcommunities were mainly responsible for the
spatial scaling patterns followed by microbes. The slope coefficients of
rare subcommunities even exceeded the values of the whole community. The
role of rare microbial subcommunities has been ambiguous and awkward in
microbial ecology, and were frequently ignored due to their low relative
abundance and frequency in microbial
profiles(Sogin et al., 2006). However,
recent studies demonstrated that rare microbial taxa execute important
ecosystem functions in various
ecosystems(Q.-L. Chen et al., 2020;
Xiong et al., 2021;
M. Xue et al., 2020). Recent studies in
macrobial ecology also show that common species contribute little to the
spatial scaling pattern of functional
diversity(van Schalkwyk, Pryke, &
Samways, 2019; White, Pakeman, &
Buckley, 2022). This suggests that the major contribution of rare
subcommunities to the spatial scaling patterns might be a common rule in
both macrobial and microbial community ecology.
Traditional microbial TAR and DDR analyses generally employ community
richness and Bray-Curtis similarity indices and do not distinguish
abundant and rare subcommunities(Barreto,
Conrad, Klose, Claus, & Enrich-Prast, 2014;
Feinstein & Blackwood, 2012;
M. C. Horner-Devine, M. Lage, J. B.
Hughes, & B. J. M. Bohannan, 2004). A recent study extended TAR to DAR
using Hill numbers, incorporating species abundance in spatial scaling
analysis(Ma, 2018). Consensus has been
achieved that Hill numbers, also knowns as the effective number of
species, are the best choice to quantify abundance-based species
diversity(Ellison, 2010). In this study,
Hill numbers were also employed to quantify both alpha- and
beta-diversity, showing decreasing spatial scaling patterns when lower
weight was given to rare taxa (i.e., increasing q value). The
employment of Hill numbers confirmed the major contribution of rare
subcommunities to microbial spatial scaling patterns, bypassing the
ambiguous definition of abundant and rare taxa.
Multiple ecological mechanisms may drive the plain and well recognized
spatial scaling patterns of biological communities. Environmental
heterogeneity (niche theory) and dispersal limitation (neutral theory)
are generally considered as the most important factors responsible for
the spatial scaling patterns(B. Gilbert &
Lechowicz, 2004; Stein, Gerstner, &
Kreft, 2014). The effects of environmental conditions on microbial
communities may differ for abundant and rare subcommunities, as revealed
by previous studies(Jiao et al., 2017;
Jiao & Lu, 2020;
Mo et al., 2018). In this study,
significant associations between environmental heterogeneity and the
spatial scaling metrics of whole and rare subcommunities were observed,
but not with that of abundant subcommunities. This suggested that
environmental heterogeneity was an important factor responsible for the
spatial scaling patterns of microbial communities, via affecting rare
subcommunities.
In addition to environmental heterogeneity, local community assembly
mechanisms may also contribute to the spatial scaling patterns of
biological communities, such as processes like dispersal
limitation(W. Chen, Jiao, Li, Du, & Yang,
2019). The distinct local community assembly mechanisms for abundant
and rare subcommunities well explained their differed spatial scaling
patterns. Specifically, high homogeneous selection result in highly
similar communities(Vellend, 2010;
Zhou & Ning, 2017), leading to flat
spatial scaling patterns for abundant subcommunities. In contrast,
dispersal limitation and drift result in highly dissimilar
communities(Vellend, 2010;
Zhou & Ning, 2017), leading to strong
spatial scaling patterns for rare subcommunities. Noteworthily, the deep
sequencing experiment, which was previously used to uncover the ultimate
microbial diversity in the
environment(Gibbons et al., 2013;
J. A. Gilbert et al., 2012), also
demonstrated the distinct ecological mechanisms underlying abundant and
rare subcommunities. A recent study suggests that the compositional
variations of different type of microbes were structured by different
mechanisms due to different organismal body
size(Luan et al., 2020). Here, the
differed spatial scaling patterns and community assembly mechanisms
between abundant and rare subcommunities should be due to their differed
life strategies (e.g., adaptability to the environment).
In conclusion, this study investigated the ecological mechanisms driving
spatial scaling patterns of sedimental microbial communities in a
coastal sediment. Rare subcommunities were mainly responsible for the
spatial scaling patterns followed by microbes. Distinct ecological
mechanisms shaped the spatial scaling patterns of abundant and rare
subcommunities. The results in this study are heuristic that different
mechanisms may underlie the spatial/temporal patterns of microbes with
different relative abundance. The study provided novel mechanistic
insights into the spatial scaling patterns followed by different
microbes.