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

Fungi engage in diverse symbiotic relationships with phototrophs. Lichens,
symbioticcomplexes involving fungi and either cyanobacteria, green algae,
or both, have fungi forming the external layer and much of the interior.
We discovered an erect thallus resembling a lichen, yet with a surprising
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, Serendipita cyanobacteriicola,
closely related to endophytes, and the cyanobacterium belongs to the
family Coleofasciculaceae, representing a genus and species not yet
classified, Symbiothallus taiwanensis. These thalli exhibit N-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.