The patterns of biogeographic distribution and assembly processes of microbiota are of vital importance for understanding ecological adaptation and functioning maintenance. However, the role of morphological characteristics in microbial assembly is still poorly ascertained. Here, by integrating high-throughput sequencing and robust extrapolation of traits, we investigated taxonomic and phylogenetic turnovers of various cyanobacterial morphotypes in biocrusts to evaluate the contributions of deterministic and stochastic processes across a large scale of drylands in northwestern China. The results showed that the non-heterocystous filamentous category dominated biocrusts in the arid ecosystem and exhibited strong tolerance against environmental fluctuations. Despite the significant distance-decay relationship of β-diversity detected in all categories, both species composition and phylogenetic turnover rates of coccoid cyanobacteria were higher than non-heterocystous filamentous and heterocystous morphotypes. Moreover, the assembly of cyanobacteria was driven by different ecological processes that the entire community and non-heterocystous filamentous morphotype were governed by deterministic processes, while stochasticity prevailed in heterocystous and coccoid cyanobacteria. Nonetheless, aridity can modulate the balance between determinism and stochasticity and prompt a shifting threshold among morphotypes. Our findings provide a unique perspective to understanding the critical role of microbial morphology in community assembly and facilitate the prediction of biodiversity loss under climate change.

The patterns of biogeographical distribution and assembly processes of microbiota are of vital importance for understanding ecological adaptation and functioning maintenance that microorganisms provide. Although the assembly of microbial communities was increasingly inspected, the role of their morphological characteristics is still poorly ascertained. Here, by integrating high-throughput sequencing and traditional trait-based classification, we investigated taxonomic and phylogenetic turnovers of different morphotypes of terrestrial cyanobacteria in biocrusts, as a model system, to evaluate the contributions of deterministic and stochastic processes across a large scale in drylands. The results showed that the bundle-forming category dominated in arid ecosystems with high tolerance against environmental fluctuations. Despite strong distance-decay relationships of β-diversity in all categories, both spatial composition and phylogenetic turnover rates of bundle-forming cyanobacteria were significantly lower than unicellular/colonial, heterocystous, and other non-heterocyst filamentous cyanobacteria. Null model analysis of phylogenetic signals and abundance-based neutral model found that stochastic processes prevailed in the assembly, while aridity mediated the balance between determinism and stochasticity and prompted a shifting threshold among morphotypes. Our findings provide a unique perspective to understand the role of microbial morphology, highlighting the differentiation of biogeographic distribution, environmental response, and species preference between morphotypes with consideration of potential functional consequences, and therefore facilitated the prediction of biodiversity loss under climate change.