Ubiquitous occurrence of the black fungus Melanina gundecimermaniae in the lichen Umbilicaria pustulata

Lichen symbioses frequently include additional fungal associates beyond the canonical mycobiont (fungus) and photobiont (alga/cyanobacterium). Despite the prevalence and diversity of these lichen cohabitants, their geographic distribution and role within the lichen consortium remain poorly understood. Combining genomics, metagenomics, and advanced microscopy, we identified the black fungus Melanina gundecimermaniae as a constant cohabitant in the lichen Umbilicaria pustulata. We analyzed metagenomes from 149 individuals across 15 populations, spanning the Europe-wide range of U. pustulata. Additionally, we screened pooled metagenomes of U. pustulata and Umbilicaria phaea along five elevation gradients (Europe and North America). Genome mapping, using a near-complete reference genome of M. gundecimermaniae, revealed that the black fungus was present in 100% of the screened lichen metagenomes, with 0.85%-3.78% of reads mapping against the reference. Among all lichen-associated fungi, it was one of the most common. These findings indicate that the black fungus is widely distributed and associated with different lichen species, underscoring its potential ecological significance. Using fluorescence in situ hybridization coupled with confocal laser scanning microscopy, we confirmed the presence of M. gundecimermaniae within various structures of U. pustulata, including vegetative symbiotic propagules involved in dispersal. Elucidating its widespread occurrence across continents, consistent presence in U. pustulata, and ability to be dispersed together with the lichens' canonical partners, our findings suggest a potential interaction of M. gundecimermaniae that extends beyond incidental colonization. Our study contributes to the growing body of evidence that organismal complexity within lichens is a prevalent and largely unexplored dimension of the lichen symbiosis.