Lichen is not a single organism but a partnership between a fungus and at least one photosynthetic partner, typically a green alga or a cyanobacterium. The fungus provides structure and shelter; the photosynthetic partner supplies sugars through photosynthesis. This arrangement has allowed lichens to colonize almost every terrestrial habitat on Earth, from tropical forests to polar ice fields to bare desert rock. But the classic two-partner definition is outdated, and the real biology of lichens turns out to be far more crowded and strange than textbooks long suggested.
More Than a Simple Partnership
For over a century, lichens were described as a fungus living with an alga. That tidy picture started unraveling in the 2010s when researchers discovered that many lichens harbor additional organisms woven into their structure. Bacteria form a surprisingly abundant and structurally integrated part of the lichen body, not just hitchhikers on the surface but functional participants in the symbiosis.1PubMed Central. The Lichens’ Microbiota, Still a Mystery? In one well-studied species, lung lichen, researchers identified not only the expected green alga and primary fungus but also a parasitic fungus and a complex bacterial community dominated by groups unrelated to the cyanobacteria typically associated with lichens.2PubMed. Symbiotic Interplay of Fungi, Algae, and Bacteria within the Lung Lichen Lobaria pulmonaria L. Hoffm. as Assessed by State-of-the-Art Metaproteomics
Yeasts add another layer of complexity. Cystobasidiomycete yeasts, a group only recently recognized as lichen inhabitants, turn up frequently within the lichen body but show much less specificity to the host fungus than the photosynthetic partner does.3PubMed. Contrasting co-occurrence patterns of photobiont and cystobasidiomycete yeast associated with common epiphytic lichen species So a lichen growing on a tree branch may contain a primary fungus, an alga, one or more secondary fungi, yeasts, and a bacterial community, all living in close physical contact. Calling it a “symbiotic organism” is fair shorthand, but the reality is closer to a tiny ecosystem wrapped inside a fungal scaffold.
The Photosynthetic Partners
The photosynthetic member of the partnership, called the photobiont, is usually either a green alga or a cyanobacterium, and which one a lichen contains affects its biology in meaningful ways. Most lichens partner with green algae, which provide carbon through photosynthesis much as plant leaves do. Cyanobacterial partners do the same but can also fix atmospheric nitrogen, making cyanolichens important contributors to nutrient cycling in forests and other ecosystems.
Some lichens manage to have it both ways. Certain species in the order Peltigerales host both a cyanobacterium and a green alga as co-primary photobionts within the same tissue layer, with both contributing to photosynthesis.4PubMed Central. Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis The outward appearance of the lichen does not change depending on whether one or two photobionts are present, which makes these arrangements easy to miss without microscopy.5Annals of Botany. Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis The fungal partner shows high flexibility in which photobiont it accepts, and researchers have found that compatibility may depend on the alga’s growth rate and its production of polysaccharides and pigments.6Cryptogamie, Algologie. Comparison of lichen-forming cyanobacterial and green algal photobionts with free-living algae
How Lichens Name Themselves (and Why It Gets Confusing)
When you see a lichen identified by a Latin name, that name refers to the fungal partner, not to the whole organism. The vernacular name, by contrast, refers to the whole lichen as you see it growing on a rock or tree. This creates an odd situation: the same fungal species can look dramatically different depending on which photobiont it pairs with. A fungus that forms a leafy green lichen with a green alga might produce a very different structure with a cyanobacterium. Taxonomists handle this by applying the rule “one fungus, one name” and using informal labels like “chloromorph” and “cyanomorph” for the different forms.7Fungal Diversity. Species in lichen-forming fungi: balancing between conceptual and practical considerations, and between phenotype and phylogenomics
How Lichens Reproduce
Lichens face a reproductive puzzle that most organisms do not: they need to pass along a partnership, not just their own genes. They solve this in two fundamentally different ways. The sexual route involves the fungal partner producing spores, which are released, germinate, and must then find a compatible photobiont in the environment to re-form the lichen from scratch. The asexual route bypasses that gamble by dispersing tiny fragments called vegetative diaspores that already contain both the fungus and the alga together.8Botanical Journal of the Linnean Society. Reproductive strategies in lichens
Vegetative reproduction gives a higher survival rate, which is why species that rely on it tend to be more abundant. But the sexual pathway allows new genetic combinations to arise, so many lichens keep both options open. Individual lichens often use a mixed strategy: smaller, younger thalli reproduce asexually first, while larger, older individuals are more likely to develop the structures needed for sexual reproduction.9PubMed. Multiple-scale environmental modulation of lichen reproduction Having both pathways available gives a species ecological versatility, because the asexual system can quickly spread a successful combination while the sexual system keeps generating genetic diversity over the long term.
A Chemical Arsenal
Lichens are prolific chemists, producing over a thousand known secondary metabolites, many of which are unique to the lichen world. One of the most studied is usnic acid, a compound with a remarkably broad set of biological activities. It can inhibit bacterial growth, show antitumor properties, act as an antioxidant, and protect against UV radiation.10PubMed. Lichen secondary metabolites affect growth of Physcomitrella patens by allelopathy These chemicals serve the lichen in competitive situations: usnic acid and other metabolites suppress the growth of neighboring mosses, vascular plants, and even other lichen species through allelopathy, a form of chemical competition.11South African Journal of Botany. Lichens affect boreal forest ecology and plant metabolism
There is a practical side for humans, too. Oakmoss, extracted from the lichen Evernia prunastri, has been an important raw material in the fragrance industry for decades, lending a distinctive earthy, woody character to perfumes.12Flavour and Fragrance Journal. Lichen extracts as raw materials in perfumery. Part 1: oakmoss The same metabolites that provide ecological advantages in the wild have drawn interest from pharmaceutical researchers looking for new antimicrobial and anticancer compounds.
Breaking Down Rock and Building Soil
One of the more underappreciated roles of lichens is their ability to break down solid rock, a process that contributes directly to soil formation. They do this through both physical and chemical means. Physically, fungal filaments penetrate cracks and weak points in the rock surface, and the repeated swelling and contraction of the lichen body as it absorbs and loses water mechanically disrupts the mineral surface.13CATENA. Weathering of rocks induced by lichen colonization — a review Chemically, the fungal partner secretes organic acids, with oxalic acid being especially important. These acids dissolve minerals, chelate metallic ions, and create characteristic etching patterns on rock surfaces.14Geological Society, London, Special Publications. Lichen weathering of minerals: implications for pedogenesis
Over time, the combination of physical fragmentation and chemical dissolution converts bare rock into a thin layer of weathered material mixed with dead lichen tissue, and this becomes a substrate where mosses and eventually vascular plants can take hold. The process also forms new minerals: at the interface between rock and lichen, oxalates crystallize within and beneath the thallus, and poorly ordered weathering products accumulate.15Applied Clay Science. Weathering of rocks and neogenesis of minerals associated with lichen activity Lichens are, in a real sense, soil factories operating on geological timescales.
Nitrogen Fixation and Nutrient Cycling
In many forest ecosystems, cyanolichens (lichens that contain cyanobacterial partners) are among the most important sources of biologically available nitrogen. The cyanobacteria inside these lichens fix atmospheric nitrogen gas into forms that other organisms can use, and this nitrogen enters the ecosystem when the lichen dies, sheds fragments, or gets washed by rain. In a high-elevation cloud forest in Costa Rica, total nitrogen fixation from non-vascular organisms reached about 6 kg per hectare per year, with lichens contributing roughly a fifth of that total.16PubMed. Bryophyte and lichen biomass and nitrogen fixation in a high elevation cloud forest in Cerro de La Muerte, Costa Rica
The biochemistry of this process turns out to be more versatile than researchers initially assumed. In boreal forests, some cyanolichens use a vanadium-based nitrogenase rather than the standard molybdenum-dependent version, and this alternative enzyme makes a substantial contribution to nitrogen fixation, especially when molybdenum is scarce in the environment.17PubMed. Biological nitrogen fixation by alternative nitrogenases in boreal cyanolichens: importance of molybdenum availability and implications for current biological nitrogen fixation estimates This matters because it means current estimates of lichen nitrogen fixation, which assume only the molybdenum pathway is at work, may undercount the actual contribution. Climate change adds a wrinkle: dry-hot stress significantly reduces the nitrogen-fixing activity of epiphytic cyanolichens, raising concerns about nitrogen inputs to forests as temperatures rise and droughts intensify.18PubMed. Dry-hot stress significantly reduced the nitrogenase activity of epiphytic cyanolichen
Sentinels of Air Quality
Lichens have been used as biological indicators of air pollution since at least the mid-twentieth century, and for good reason. Because they absorb water and nutrients directly from the air rather than through roots, they accumulate airborne pollutants with unusual efficiency. Field and laboratory studies have demonstrated that lichens are extremely sensitive to sulfur dioxide and are also fairly sensitive to hydrogen fluoride and heavy metals.19PubMed. Lichens as indicators of air pollution In heavily polluted areas, sensitive lichen species disappear entirely, creating visible “lichen deserts” around industrial centers.
More recently, researchers have used lichens to detect subtler contamination. Even small variations in ambient heavy metal concentrations show up in the physiology of local lichens: arsenic degrades their chlorophyll, copper increases fatty acid oxidation, and different metals produce distinct stress signatures that can be read like a chemical report card for the surrounding environment.20PubMed Central. Lichen as Bioindicators: Assessing their Response to Heavy Metal Pollution in Their Native Ecosystem Their sensitivity to both climatic and air quality variations makes them broadly useful as indicators of environmental shifts, from local industrial pollution to regional climate trends.21Total Environment Advances. Lichens as effective bioindicators for monitoring environmental changes: A comprehensive review
Surviving Where Nothing Else Can
Lichens thrive in environments that would kill most other complex organisms. Their mutualistic symbiosis enables them to maintain a presence in polar, high-altitude, and desert regions where few other life forms persist.22MDPI (Plants). Fungal–Algal Association Drives Lichens’ Mutualistic Symbiosis: A Case Study with Trebouxia-Related Lichens The Antarctic lichen Umbilicaria antarctica provides a case study in extreme cold tolerance. Rather than simply freezing solid and surviving, this lichen avoids ice formation altogether. Water within its tissue transitions into a tightly bound state at temperatures between about −17°C and −24°C, a process driven by sub-micrometer compartments in the thallus that physically confine water and suppress ice crystal formation. All freezing events occur below −10°C, well below what unprotected water would tolerate.23Scientific Reports. Cryotolerance in the Antarctic lichen Umbilicaria antarctica is driven by supercooling and structural confinement
This structural confinement strategy is distinct from the antifreeze proteins used by some animals and plants. The lichen essentially builds a physical architecture of tiny compartments that prevent ice crystals from nucleating, keeping its internal water in a supercooled liquid state. It is an elegant solution that combines the structural engineering of the fungal partner with the chemistry of the whole system, and it helps explain why lichens can colonize rock faces in Antarctica where temperatures routinely plunge far below freezing.
An Ancient Lineage
The fossil record of lichens is sparse, partly because their soft bodies do not preserve well, but the evidence that does exist pushes their history deep into geological time. The oldest known lichens with internal anatomy resembling modern species come from Lower Devonian rocks in the Welsh Borderland, roughly 415 million years old. These fossils already show the layered structure typical of advanced lichens today.24PubMed. The earliest records of internally stratified cyanobacterial and algal lichens from the Lower Devonian of the Welsh Borderland
A 2025 study identified the widespread Devonian fossil Spongiophyton as one of the earliest and most ecologically prominent lichens, based on internal fungal networks, algal cells, calcium oxalate residues, and nitrogen-rich compounds preserved in the fossil. Its broad distribution across Devonian-age sediments suggests that lichens were already ecologically significant during the late stages of land colonization, just before complex forest ecosystems evolved.25PubMed Central. The rise of lichens during the colonization of terrestrial environments Whether lichens actually preceded vascular plants on land, as some researchers have proposed, remains contested. Phylogenetic analyses of fungal and algal lineages suggest that the origins of lichenization postdate the earliest vascular plants, and unambiguous fossil evidence for pre-vascular-plant lichens is lacking.26PubMed. No support for the emergence of lichens prior to the evolution of vascular plants
Dating Landscapes with Lichen Growth
Because many crustose lichens grow slowly and predictably on rock surfaces, geologists use them as a kind of natural clock. The technique, called lichenometry, works by measuring the diameter of the largest lichen thallus on a surface and comparing it against a growth curve established for that species and climate. It has been used to date glacier moraines, rockfall events, and flood deposits across the globe. In Alaska and Kamchatka, lichen-based chronologies of glacial advances over the past thousand years align strikingly well with more precisely dated tree-ring records.27Arctic, Antarctic, and Alpine Research. Lichenometry as Applied to Moraines in Alaska, U.S.A., and Kamchatka, Russia
Growth rates vary by species and climate. In Patagonia, the lichen Placopsis perrugosa grows at a maximum rate of about 4.7 millimeters per year, and colonization of newly exposed rock surfaces takes anywhere from roughly 2.5 to 13 years.28Geomorphology. Dendrochronology and lichenometry: colonization, growth rates and dating of geomorphological events on the east side of the North Patagonian Icefield, Chile The technique is most reliable for surfaces less than a few centuries old, where individual lichen colonies can still be measured and have not yet merged with their neighbors. For older surfaces the method grows less precise, but it fills a useful gap in landscapes where tree rings and radiocarbon dating are unavailable.
Lichens in Food Webs
For some animals, lichens are not just scenery but a critical food source. Caribou provide the best-known example. A review of the literature on caribou diet found that in winter, caribou primarily consume lichen, switching to graminoids and other vascular plants during warmer seasons when those are available. In more productive environments where caribou face more competitors and predators, lichen consumption actually increases, suggesting that lichen serves as a fallback resource when competition for richer food is intense.29Canadian Journal of Zoology. What do caribou eat? A review of the literature on caribou diet Caribou can access ground lichens by pawing through snow, a feeding strategy that ties their winter survival directly to lichen abundance.
Climate Change and the Future of Lichens
Lichens face a complicated future under climate change. As cold-adapted species, many arctic and alpine lichens are projected to lose habitat as temperatures rise, and modeling studies predict substantial losses of habitat suitability, particularly for branching (fruticose) lichens and cold-loving species. Contrary to the assumption that alpine species would simply shift upward in elevation, many lichen species showed no clear relationship between altitude and increased habitat suitability, suggesting their redistribution patterns are less predictable than a simple “move higher” model would suggest.30Diversity and Distributions. Range Shift and Climatic Refugia for Alpine Lichens Under Climate Change
Warming also affects lichens indirectly through changes to their host trees. In Italy, the lung lichen faces a double threat: its climate envelope is shrinking, and the native tree species it depends on for substrate are being displaced by the invasive black locust, which is a poor host. The result is both a direct loss of suitable climate and an indirect loss of suitable substrate happening simultaneously.31PubMed. Range shifts of native and invasive trees exacerbate the impact of climate change on epiphyte distribution: The case of lung lichen and black locust in Italy Warm-temperate lichen species may expand their ranges as new thermal habitat opens up, but cold-temperate and polar species are expected to contract toward higher latitudes and elevations.32PubMed Central. Lichens in times of climate change – impacts and responses especially in boreal and polar ecosystems
Why Scientists Still Cannot Grow Lichens in the Lab
Given how widespread and successful lichens are in nature, you might assume it would be straightforward to grow one in a laboratory dish. It is not. Despite about 150 years of attempts, no reliable lab model system for lichen synthesis exists. A review of the literature found that the term “resynthesis” has been applied loosely to many types of fungal-algal cocultures that do not actually resemble a lichen. The few studies that reported producing natural-looking lichens from pure, sterile cultures appear to have been isolated successes against a background of extensive contamination, and none proved reproducible enough to become a standard research tool.33PubMed. Making Fungal-Photobiont Symbioses in the Lab: Past, Present, and Future of the Elusive In Vitro Lichen
Most species that have been coaxed into forming something resembling a thallus in the lab are simple crustose forms with just two partners. Only one three-component lichen, Stereocaulon vulcani, has been resynthesized, probably reflecting the difficulty of coordinating multiple symbionts under artificial conditions.34IntechOpen. Experimental Lichenology – Section: Lichen resynthesis and producing artificial (model) associations The inability to recreate lichens reliably in controlled settings is both a frustration and a clue: it suggests that the signals and conditions needed to trigger true lichen formation are far more complex than simply putting the right partners in a dish together. Until that puzzle is solved, much of lichen biology will remain something researchers can observe in nature but not manipulate experimentally.