Island biogeography on a micro-scale: Larger anammox granules not only harbor higher species diversity but also support more functional diversity

Abstract Background: According to the theory of island biogeography, there is a strong relationship between the species diversity and the isolated area. However, it is unclear whether the ecological distribution of microorganisms follows this island biogeography pattern at micro-scale. Here, we use microbial granules harvested from a partial nitritation and anammox (PN/A) system as a model to test if the microbial and functional diversity follow the island biogeography. We collected and divided these granules into five discrete size-fractions (<0.2, 0.2–0.5, 0.5–0.8, 0.8–1.0 and >1.0 mm).Results: By comparing the composition and functional attribute of five pools of the size-fractionated granules by 16S rRNA gene amplicon, metagenomic and metatranscriptomic sequencing, larger granules were shown not only to harbor higher microbial diversity, but also to support more diverse functions than smaller granules. De novo co-assembly and binning of metagenomic reads yielded 22 near-complete genomes of dominant microorganisms, which allowed us to infer an ecological model of the microbial ecosystem in anammox-based granules. This genome-based ecological model indicates that nitrifying organisms in smaller granules feed nitrite to anammox bacteria in larger granules.Conclusion: Our findings substantiate that microbial communities in PN/A granules follow a species-volume relationship, suggesting the generality of the theory of island biogeography on microscopic scale.