Novel Glomeromycotina–moss associations identified in California dryland biocrusts

Drylands, which comprise c. 45% of Earth's land area, host biological soil crusts (biocrusts): symbiotic communities of cyanobacteria, fungi, algae, lichen, and bryophytes that stabilize soil and support key ecosystem functions. Moss-dominated biocrusts are particularly interesting due to their potential to illuminate ancient bryophyte-fungal interactions. To test the hypothesis that mosses in biocrusts host endophytic Mucoromycota fungi and that local climate influences the composition of these fungal communities, we conducted amplicon metabarcoding and microscopic surveys employing fungal staining across sites with varying aridity. We identified novel associations between mosses and arbuscular mycorrhizal fungi (AMF), with phylogenetic analyses revealing distinct fungal communities in moss biocrusts compared with adjacent bare soil. Intracellular branching by fungi resembling Glomeromycotina was observed within healthy Trichostomopsis australasiae (Bryophyta) cells. Moreover, shifts in AMF community composition across different aridity levels within the same moss species highlight the variation in moss-associated Glomeromycotina diversity, composition, and relative abundance. These findings provide critical insights into ancient bryophyte-fungal symbioses, potentially analogous to those enabling early land plant colonization during the Ordovician (c. 470 million years ago). They also underscore the need to understand and protect biocrust microbial communities as aridity intensifies under climate change.