The symbiotic alga Trebouxia fuels a coherent soil ecosystem on the landscape scale in the Atacama Desert.

Biocrusts represent associations of lichens, green algae, cyanobacteria, fungi and other microorganisms, colonizing soils in varying proportions of principally arid biomes. The so-called grit crust represents a recently discovered type of biocrust situated in the Coastal Range of the Atacama Desert (Chile) made of microorganisms growing on and in granitoid pebbles, resulting in a checkerboard pattern visible to the naked eye on the landscape scale. This specific microbiome fulfills a broad range of ecosystem services, all probably driven by fog and dew-induced photosynthetic activity of mainly micro-lichens. To understand its biodiversity and impact, we applied a polyphasic approach on the phototrophic microbiome of this biocrust, combining isolation and characterization of the lichen photobionts, multi-gene phylogeny of the photobionts and mycobionts based on a direct sequencing and microphotography approach, metabarcoding and determination of chlorophyll_a+b contents. Metabarcoding showed that yet undescribed lichens within the Caliciaceae dominated the biocrust together with Trebouxia as the most abundant eukaryote in all plots. Together with high mean chlorophyll_a+b contents exceeding 410 mg m^-2, this distinguished the symbiotic algae Trebouxia as the main driver of the grit crust ecosystem. The trebouxioid photobionts could be assigned to the I (T. impressa/gelatinosa) and A (T. arboricola) clades and represented several lineages containing five potential species candidates, which were identified based on the unique phylogenetic position, morphological features, and developmental cycles of the corresponding isolates. These results designate the grit crust as the only known coherent soil layer with significant landscape covering impact of at least 440 km², predominantly ruled by a single symbiotic algal genus.