Fungal community inside lichen: a curious case of sparse diversity and high modularity

Background: Lichens represent not only the mutualism of fungi and photosynthetic partners but also a compound microbial consortium harboring non-obligate fungi known as endolichenic fungi. While endolichenic fungi are known to exert a remarkable influence on lichen ecology through their crucial roles in nutrient cycling, bioprospecting and biodiversity, the enigmatic community structures of these fungal inhabitants remain shrouded in mystery, awaiting further exploration and discovery. To address this gap in knowledge, we conducted metabarcoding on two lichens, Dirinara applanta and Parmotrema tinctorum , and compared their microbial communities to those found in the pine bark to which the lichens were attached. Our hypothesis was that the endolichenic communities would exhibit distinct diversity patterns, community structures, network structures, and specialist composition compared to the surrounding epiphytic community. Results: Our investigation has shed light on the clear demarcation between the endolichenic and epiphytic fungal communities, as they exhibit markedly different characteristics that set them apart from each other. This research demonstrated that the endolichenic communities contained fewer diverse fungi compared to the epiphytic communities. Through community similarity analysis, we observed that the two endolichenic communities shared more commonalities with each other than with the adjacent epiphytic communities. Moreover, we unveiled a striking contrast in the network structures between the endolichenic and epiphytic communities, as the former displayed a more modular and less nested features that is evocative of a potent host-filtration mechanism. Our statistical investigation unveiled that endolichenic communities possessed a high abundance of plant-associated fungi in their specialist category, underscoring the similarity between endolichenic fungi and the phyllosphere mycobiome. Conclusions: Through our investigation, we have discovered that lichens exert a crucial "host-filtration" effect on the microorganisms within them, leading to less intricate and interconnected communities compared to the neighboring epiphytic environment. These observations provide valuable insights into the metagenomic architecture of lichens and offer a tantalizing glimpse into the unique mycobiome of these extraordinary life forms.