The fungus in a lichen helps its alga obtain iron

A lichen is not a single organism, though from a distance it looks like one thallus. In the common orange lichen Xanthoria parietina the fungal partner forms a structure in which an alga carrying out photosynthesis lives. New research shows one possible mechanism of exchange: the fungus produces ferrichrome, a siderophore that captures hard-to-reach iron, and the presence of the compound favours the growth of the algal partner Trebouxia decolorans under iron-limited conditions.

Iron is essential for many cellular reactions, but in the environment it is often poorly soluble. Microorganisms solve that problem with siderophores — small molecules that bind metal ions very effectively. The authors identified in the fungal partner the set of genes needed to produce ferrichrome, confirmed the presence of the product and tested its action in experiments culturing the photobiont.

Adding ferrichrome supported the growth of the alga when access to iron was limited. The result provides an experimental argument that the fungus may not only provide shelter and retain water but actively mediate the delivery of a micronutrient. In return the photosynthesising partner passes on carbon compounds. That picture is more accurate than the textbook shorthand in which one organism provides a "home" and the other simply produces sugar.

The mechanism should not, however, be transferred to all lichens. The study concerns a particular pairing of species and particular laboratory conditions. A thallus in nature also contains bacteria and other fungi, and the availability of iron changes with the substrate, humidity and pollution. The experiment shows a capacity and a probable function, but the scale of its significance in different habitats requires further measurement directly in thalli.

The discovery matters beyond lichenology as well. It shows that a stable symbiosis rests on the flow of many resources, not on a single universal "payment". Metabolites originally serving to acquire a metal may organise cooperation between partners. To an observer, Xanthoria remains a common yellow lichen on bark and rock. To a biologist it is a microecosystem in which the chemistry of a single molecule helps to explain how different organisms sustain a shared life in a poor environment.

Further experiments could establish who ultimately takes up the iron bound in ferrichrome, and how transport changes in a complete thallus. Comparing fungal strains that differ in siderophore production, and isotope measurements tracing the path of the metal, would help. Only then could a quantitative balance of exchange be drawn, rather than relying solely on improved growth of a partner in culture. The result opens precise questions about cooperation, but it does not close the whole biology of a lichen in one molecule or one experiment.

Evidence status — 29 August 2026

The article was published on 2 July 2026, with its version of record posted on 12 August. The authors identified the ferrichrome pathway genes and product in the fungal partner of Xanthoria parietina, then showed in culture that the compound supports growth of the alga Trebouxia decolorans when iron is limited. The study therefore confirms a capacity and a probable exchange mechanism, but it has not yet quantified iron flow inside a natural thallus. The result should not be generalized to every lichen. It matters because it replaces the broad “shelter for sugar” shorthand with specific, testable chemistry of cooperation.