Fog, Symbiosis, and Survival: The Ecological Architecture of the Grit Crust From the Atacama Desert Represents a Lichen Holobiome Rather Than a Soil Microbiome

ABSTRACT

Biological soil crusts (biocrusts) fulfil key ecological functions in arid ecosystems, yet their microbiome composition remains insufficiently resolved. Here, we characterise the microbial communities of the fog‐dependent grit crust in the Pan de Azúcar National Park (Atacama Desert, Chile) using multi‐marker metabarcoding (16S rRNA, 18S rRNA, ITS2) across 11 coastal–inland sites. Chlorophyll _a+b concentrations reached up to 900 mg m ⁻² , ranking among the highest reported for arid biocrusts and reflecting exceptional fog‐sustained productivity. Bacterial assemblages were dominated by Proteobacteria and Actinobacteria, fungal communities by lichenized Ascomycota (Caliciaceae), and eukaryotic diversity by the green algal photobiont genus Trebouxia . Black‐pigmented crusts with dense colonisation exhibited higher biomass but lower taxonomic richness, consistent with later‐successional, lichen‐dominated stages, whereas lighter, less colonised crusts were taxonomically richer yet functionally less integrated, indicative of earlier succession. The prevalence of Trebouxia , lichenized fungi, and lichen‐associated bacterial taxa demonstrates that the grit crust microbiome is structured around symbiotic photobiont–mycobiont interactions rather than typical edaphic microbial assemblages. These findings redefine biocrust paradigms by documenting a fog‐driven, chlorolichen‐based system that bridges the ecological spectrum between lithic lichen communities and conventional soil crusts, establishing a critical baseline for assessing dryland microbial resilience under climate change.