Semilichen, an unjustly neglected symbiotic system between green biofilms and true lichens

Symbiotic systems of photosynthetic microorganisms and fungi are widespread in terrestrial biomes and lichens are probably the most advanced and complex. Conversely, the least complex systems are "green biofilms" with a completely unexplored mycobiome. We describe here a new system intermediate between green biofilms and lichens-semilichens. Light and fluorescence microscopy, eDNA sequencing, molecular phylogeny, Chlorophyll a fluorescence and ¹³C labelling/metabolomics were used to study algal and fungal identity, morphology and physiology of the symbiosis. Tight contact between algae and a single predominant fungus (mycobiont) is revealed in semilichens. The algae are from the symbiotic lineages of Trebouxiophyceae and Ulvophyceae, the fungi belong to Arthoniomycetes, Dothideomycetes, Eurotiomycetes, Lecanoromycetes and Lichinomycetes. Algae are alive and perform substantial photosynthetic activity. ¹³C labelled photosynthates are partially converted into specific fungal polyols (arabitol, mannitol) demonstrating the C-flow from algae to fungi. The new symbiotic system was defined and compared with other terrestrial algal-fungal symbioses. It is characterized by minimalist environmental requirements and extremely low production of biomass. As a result, it also inhabits environments unfavourable for lichens. Our research supports the hypothesis that the long-term existence of algae and fungi in terrestrial conditions affected by frequent and repeated drying is likely dependent on their mutual coexistence.