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.