Siderophore production by the lichen fungus Xanthoria parietina supports its algal symbiont

Lichens are symbiotic associations between a fungal mycobiont and a photosynthetic photobiont. They thrive in nutrient-poor environments; yet the mechanisms underlying their adaptation to iron limitation remained largely unknown. Here, we characterize the iron acquisition system of Xanthoria parietina , a globally distributed lichen-forming fungus associated with the microalgal photobiont Trebouxia decolorans . We demonstrate that the mycobiont produces the siderophore ferrichrome and possesses the full genetic repertoire not only for siderophore biosynthesis, but also reductive iron assimilation, iron detoxification, and regulation. The ferrichrome-synthesizing non-ribosomal peptides synthetase exhibits a lichen-specific compact architecture but retains functionality when heterologously expressed in a non-lichenized ascomycete. Transcriptomic analysis and ferrichrome quantification reveal substrate-dependent regulation of the siderophore system. Importantly, ferrichrome promotes photobiont growth independent of extracellular iron reduction, indicating direct utilization. These findings provide the functional evidence of siderophore-mediated iron acquisition in a lichen symbiosis and highlight ferrichrome as a key mediator of mutualistic nutrient exchange.