Wallace's line structures seagrass microbiota and is a potential barrier to the dispersal of marine bacteria

Abstract

Background: The processes that shape microbial biogeography are not well understood, and concepts that apply to macroorganisms, like dispersal barriers, may not affect microorganisms in the same predictable ways. To better understand how known macro-scale biogeographic processes can be applied at micro-scales, we examined seagrass associated microbiota on either side of Wallace's line to determine the influence of this cryptic dispersal boundary on the community structure of microorganisms. Communities were examined from twelve locations throughout Indonesia on either side of this theoretical line.

Results: We found significant differences in microbial community structure on either side of this boundary (R² = 0.09; P = 0.001), and identified seven microbial genera as differentially abundant on either side of the line, six of these were more abundant in the West, with the other more strongly associated with the East. Genera found to be differentially abundant had significantly smaller minimum cell dimensions (GLM: t₉₂₃ = 59.50, P < 0.001) than the overall community.

Conclusion: Despite the assumed excellent dispersal ability of microbes, we were able to detect significant differences in community structure on either side of this cryptic biogeographic boundary. Samples from the two closest islands on opposite sides of the line, Bali and Komodo, were more different from each other than either was to its most distant island on the same side. We suggest that limited dispersal across this barrier coupled with habitat differences are primarily responsible for the patterns observed. The cryptic processes that drive macroorganism community divergence across this region may also play a role in the bigeographic patterns of microbiota.

Keywords: Marine biogeography; Metabarcoding; Microbial dispersal; Microbiome; Seagrass; Wallace’s line.

Fig. 1

Map of study locations. Map of sampling locations for this study. Colour indicates side of Wallace’s line. Dashed line indicates approximate position of Wallace’s line. Each location was subsampled 16 times. Exact GPS coordinates are available in Additional file 1: Appendix S6

Fig. 2

Alpha diversity measures. Observed ASV richness (left panel) and Shannon diversity (right panel) estimates. Locations arranged from West to East

Fig. 3

Ordination plots. Ordination plots for community structure of all samples using both Bray–Curtis (left panel) and UniFrac (right panel) dissimilarity indices. Samples are coloured by side of Wallace’s line (West = Purple; East = Orange)

Fig. 4

Differential abundance plots. Differential abundance and dispersion both East (Orange) and West (Purple) of Wallace’s Line. Points represent relative abundance of a given taxon in each sample. Bars represent dispersion estimates. Only taxa identified as significantly different between East and West by all four methods (corncob, indicspecies, random forest, and Sloan Neutral Model) are shown