Spatial turnover of composition and interaction networks in dark biocrust communities of the Mexican Chihuahuan Desert

Background and aims: Dark biocrusts—characterized by brownish-black cyanobacteria, microfungi, and lichens—are widespread in drylands. However, their spatial distribution and community composition remain poorly understood. We examined how biocrust microbial diversity and ecological networks of biocrust microbial communities differed between two microhabitats—beneath the canopy and in open interspace—across ecosystems in the Chihuahuan Desert, Mexico. Two of the studied ecosystems were dominated by shrubs (Larrea tridentata and Prosopis glandulosa), while the other two were dominated by grasses (Pleuraphis mutica and Sporobolus airoides). Methods: We collected 32 dark biocrust and soil samples from the Mapimí Biosphere Reserve (MBR). Using amplicon-based metabarcoding and soil physicochemical data, we compared bacterial and fungal communities between microhabitats and applied Lotka-Volterra interaction network analysis to characterize the microbial interactions. Results: We identified 15 bacterial phyla, with Cyanobacteria as the most abundant and Mastigocladopsis sp. as the dominant genus. Bacterial composition differed between microhabitats only in S. airoides ecosystems. Fungal communities were dominated by Ascomycota, primarily Dothideomycetes and Pleosporales, and differed between microhabitats only in the P. mutica ecosystem. Cross-domain network analyses revealed distinct interaction patterns, with interspaces exhibiting predominantly positive interactions, highlighting the importance of facilitative processes for microbial stability under extremely arid conditions. Conclusion: Despite their homogeneous appearance, dark biocrusts at the MBR harbor complex and spatially structured microbial networks that are shaped by both ecosystem type and microhabitat. Our study thus uncovered a hidden layer of taxonomic and interaction diversity, with significant implications for understanding the stability and functioning of the Chihuahuan Desert.