A thallus-forming N-fixing fungus-cyanobacterium symbiosis from subtropical forests

Fungi engage in diverse symbiotic relationships with phototrophs. Lichens, symbiotic complexes involving fungi and either cyanobacteria, green algae, or both, have fungi forming the external layer and much of the interior. We found an erect thallus resembling a lichen yet with an unexpected thallus structure composed of interwoven cyanobacterial filaments with numerous fungal hyphae inserted within individual cyanobacterial sheaths, contrasting with typical lichen structure. Phylogenetics identified the fungus as a previously undescribed species, <i>Serendipita cyanobacteriicola</i>, closely related to endophytes, and the cyanobacterium belongs to the family Coleofasciculaceae, representing a genus and species not yet classified, <i>Symbiothallus taiwanensis</i>. These thalli exhibit nitrogen-fixing activity similar to mosses but lower than cyanolichens. Both symbiotic partners are distinct from known lichen-forming symbionts, uncovering a phylogenetically and morphologically unprecedented thallus-forming fungus-cyanobacterium symbiosis. We propose the name "phyllosymbia" for these thalli to underscore their unique symbiotic nature and leaf-like appearance. This finding marks a previously unknown instance of fungi solely residing within structures generated by cyanobacteria.