1. Introduction
Microbes are ubiquitous in the Earth’s biosphere, executing essential ecosystem functions and maintaining ecosystem stability(Bardgett & van der Putten, 2014; Fuhrman, Cram, & Needham, 2015; B. Gilbert & Lechowicz, 2004). The diversity of soil microbial communities is in general positively associated with ecosystem multifunctioning (Bradford et al., 2014; Delgado-Baquerizo et al., 2016; Lefcheck et al., 2015). As a major component of microbial communities, revealing the diversity patterns across space and through time is of essential importance for better understanding the underlying ecological mechanisms governing the distribution and assembly of microbial communities. However, the tiny size of microorganisms and immense complexity of microbial communities make this issue more challenging than macrobial communities.
Taxa-area relationship (TAR) and distance-decay relationship (DDR) are two typical and perhaps universal spatial scaling patterns followed by both macrobial and microbial communities(Green et al., 2004; M. C. Horner-Devine, M. Lage, J. B. Hughes, & B. J. Bohannan, 2004; Tu et al., 2016; Zinger, Boetius, & Ramette, 2014). Of these, TAR describes the pattern of continuously increasing species richness with increasing sampling area(Connor & Mccoy, 1979; Rosenzweig, 1995), whereas DDR describes the pattern that the composition of biological communities becomes more dissimilar with increasing geographic distance(Nekola & White, 1999). Although different in concept, both TAR and DDR are assumed to be the result of a set of common processes, including environmental heterogeneity and local community assembly processes (e.g., speciation, drift, and dispersal limitation) across sampling area and distance(Connor & Mccoy, 1979; Hubbell, 2001). Specifically, higher environmental heterogeneity is associated with more ecological niche space and habitat types, allowing more microbial taxa to coexist(Allouche, Kalyuzhny, Moreno-Rueda, Pizarro, & Kadmon, 2012; Huber et al., 2020; Yang et al., 2015). The larger sampling area it is, the higher environmental heterogeneity and more coexisted microbial taxa are expected, resulting in TAR patterns. Environmental heterogeneity also contributes to DDR patterns for its being strongly correlated with geographic distance(Tilman, 1983). Local community assembly processes may also result in differed community structure and composition(Stegen et al., 2013; X. Zhang et al., 2020). leading to TAR and DDR patterns. However, TAR and DDR may not be directly derived from each other, and may be subjected to influences by different ecological factors(Zinger et al., 2014).
Microbial communities in natural ecosystems are typically composed by a small number of abundant taxa and an extremely long tail of rare taxa(M. D. Lynch & Neufeld, 2015; Sogin et al., 2006). The abundant taxa usually occupy < 20% of the total richness, but > 80% in relative abundance(Sogin et al., 2006). Although low in relative abundance, recent studies suggest that the rare microbial taxa execute nonnegligible ecosystem functions in the environment(Q.-L. Chen et al., 2020; Lyons & Schwartz, 2001; Mouillot et al., 2013; Xiong et al., 2021). Recent studies suggested that the abundant and rare subcommunities are structured by different community assembly mechanisms and environmental parameters(Jiao & Lu, 2020; Mo et al., 2018; W. Zhang et al., 2018). However, it remains not clear whether and how abundant and rare subcommunities differ in the spatial scaling patterns they may follow and how such patterns are linked to environmental heterogeneity and local community assembly processes.
In this study, we investigated the spatial scaling patterns followed by abundant and rare subcommunities of microbes in an ocean sediment ecosystem, aiming to address the following ecological questions: (1) Do abundant and rare subcommunities differ in following spatial scaling patterns? (2) How do environmental heterogeneity and local community assembly mechanisms respectively contribute to the spatial scaling patterns? We expected that abundant and rare subcommunities may differ in the spatial scaling patterns they follow, mainly due to their different life strategies (e.g., different adaptability to environmental conditions)(He et al., 2022; Wan et al., 2021). Specifically, abundant subcommunities may follow weak spatial scaling patterns, especially TAR, as they are more broadly distributed across the sampling space. As previously reported, abundant and rare subcommunities differ dramatically in local community assembly mechanisms(Jiao & Lu, 2020; Mo et al., 2018; W. Zhang et al., 2018). We therefore expected strong links between local community assembly and spatial scaling patterns. The results confirmed our expectation that spatial scaling patterns were rarely observed for abundant subcommunities, whereas rare subcommunities were mainly responsible for the observed microbial spatial scaling patterns. Distinct ecological mechanisms underlay the spatial scaling patterns followed by abundant and rare subcommunities. The study provided novel mechanistic insights into the spatial scaling patterns followed by different types of microbes.