A lichen on bare rock obtains much of its carbon from the air. Photosynthetic partners use light to build carbon compounds, and a fungus uses some of that production while forming much of the visible body. Rock provides a surface and can contribute mineral resources; it is not a substitute for the sugar the fungus cannot make itself.
That explains the food supply, but leaves another problem. A thin body exposed on stone cannot keep drawing water from roots. Many lichens instead tolerate drying, reduce activity, and resume when suitable moisture returns. Their survival depends on both a partnership and the intervals when that partnership can work.
The visible patch contains several kinds of life
The Australian lichen guide defines a lichen as an association involving a fungus and an alga or cyanobacterium. These photosynthetic partners are called photobionts. Some lichens contain more than one kind, so the familiar phrase “a fungus and an alga” is useful but incomplete.
The body is a thallus. Fungal filaments, or hyphae, supply much of its structure. Photosynthetic cells live within that structure while retaining their own cellular identity. A visible lichen is not one fungal cell fused with one algal cell.
The thumbnail’s cutaway is an explanatory illustration, not a microscope image. Its enlarged green cells and pale filaments make the relationship visible, but should not be used to identify a species or infer exact tissue proportions. Different lichens organize their bodies differently.
The central distinction is functional: the photobiont can capture carbon through photosynthesis, while the fungus depends on organic carbon obtained from elsewhere. Living together changes how those needs can be met on an exposed surface.
Sharing carbon involves specific cellular machinery
A 2020 study of Usnea hakonensis compared symbiotic material with cultured partners and reconstructed lichenization in the laboratory. The researchers investigated changes in gene activity associated with the relationship, rather than merely observing that fungus and alga were present together.
Their proposed nutrient-exchange model includes ribitol, a sugar alcohol released by the Trebouxia photobiont and taken up through fungal transporters. Genes resembling known ribitol transporters showed increased activity in the symbiotic state. The authors also discuss possible nutrient flows in the other direction, including phosphorus.
This is evidence about one experimental system, with some mechanisms inferred from gene expression. It does not establish that every lichen transfers identical compounds in identical quantities. It does make “sharing food” more concrete: production, release, transport, and use all need to connect.
The relationship also involves conditions within the thallus. Light reaching the photosynthetic cells and nutrients available to them can influence the exchange. The partners are responding organisms, not a miniature factory with an unchanging output.
Drying creates a light problem as well as a water problem
When photosynthesis slows during drying, light can still be absorbed. Energy that cannot be used productively can damage the photosynthetic apparatus. Desiccation tolerance therefore requires more than simply waiting for rain.
A 2008 photoprotection study examined the chlorolichen Cladonia rangiformis and the cyanolichen Peltigera neckeri. It investigated changes during desiccation and mechanisms that dissipate excitation energy as heat. The work connects survival in a dry state to protection of equipment needed after rehydration.
That does not mean sunlight warms a lichen into renewed activity or that every dry lichen is invulnerable. The relevant process disposes of excess absorbed energy. Its importance depends on species and conditions, and recovery is not a promise that any specimen will revive after any exposure.
Surviving and growing are separate outcomes. Dissipating excess energy can protect a dry lichen until moisture returns, but a protected photosynthetic system is not necessarily fixing carbon. A patch can remain alive through dry intervals while having only limited periods in which it gains enough carbon to grow.
More water does not always improve the carbon budget
A 2024 study of physiological responses makes the balance especially clear. In Evernia mesomorpha, researchers measured carbon assimilation and respiration after hydration with water vapor or liquid water across temperatures from 5 to 30°C.
Photosynthetic carbon uptake could be similar under the two hydration treatments, while respiratory losses were substantially greater after liquid-water hydration. At high experimental temperatures, some liquid-water-hydrated thalli could not achieve a positive carbon balance. The source of hydration changed the costs as well as the opportunity to become active.
The researchers also examined diverse lichen associations, finding that differences between the partners’ physiological responses were widespread, with exceptions. This is not a rule that rain harms all lichens. It is evidence against treating water availability as a single switch that makes every member of the association equally productive.
For a patch on rock, useful conditions involve timing and balance: enough moisture for carbon uptake, light that can be used, and respiratory demands that leave something for growth.
Some partners contribute nitrogen, others add further functions
In Peltigera aphthosa, a Planta study describes a fungus and green alga in the main thallus, with Nostoc cyanobacteria in specialized superficial structures called cephalodia. The study measured nitrogenase activity, connecting these structures to nitrogen fixation.
Carbon is only one requirement for growth. Nitrogen fixation converts atmospheric nitrogen into biologically usable forms. A lichen with specialized cyanobacterial partners therefore has an additional route to an essential resource. That arrangement belongs to particular associations, not every crust growing on a stone.
The community can be broader still. A 2017 meta-omics study of Lobaria pulmonaria identified bacterial functions involving vitamins, cofactors, and the degradation of complex compounds. Those findings extend the picture beyond the principal fungus and photobiont without proving that every detected resident is indispensable.
The resources in the opening question come through different routes. Light powers carbon capture from air; moisture permits activity without a root supply; the fungal structure organizes the partners; and particular associations add routes to nitrogen or other useful compounds. Bare rock removes the familiar soil-and-root arrangement. It does not remove the need for a positive carbon balance during the intervals when the lichen is active.
