Chromosome‐level genome assembly of the photobiont microalga <i>Trebouxia</i> sp. ‘ <scp>A48</scp> ’ from the lichen <i>Xanthoria parietina</i>

Lichens are symbiotic assemblies consisting of multiple organisms, chiefly a fungus and a photosynthetic microorganism, or photobiont. Among diverse photobionts, the most prevalent is the chlorophyte alga Trebouxia. We produced a chromosome-level assembly of Trebouxia sp. 'A48', a photobiont of Xanthoria parietina. The genome was assembled into 20 contigs, of which 16 had telomeric repeats at both ends and likely represent complete chromosomes. We compared this genome with those of other Trebouxia species and analyzed it to investigate adaptations to the lichen lifestyle. We then used the genome to profile gene expression in axenic culture and in lichen thalli. The predicted secretome is enriched in hydrolases and redox enzymes and contains carbohydrate-binding proteins potentially involved in cell-to-cell recognition and adhesion. We identified genes potentially involved in carbon concentrating and confirmed two instances of ancient horizontal gene transfer from fungi. The genome and the strain of Trebouxia sp. 'A48' provide a resource for the community to research algal evolution and lichen symbiosis.