The Algae and Fungi Symbiotic Relationship Explained

Lichens are the most familiar partnership between algae and fungi, and they are everywhere: crusting on tree bark, painting boulders orange, dangling from branches in old-growth forests. In a lichen, a fungus provides the physical structure while an alga (or a cyanobacterium) supplies sugars through photosynthesis. The arrangement is often called mutualism, but the real story is messier, involving controlled exploitation, third-party yeasts, hundreds of bacterial helpers, and a relationship so successful it has evolved independently many times over hundreds of millions of years.

What Each Partner Brings to the Table

The fungal partner, called the mycobiont, builds the body of the lichen. It forms a tough outer layer called the cortex, weaves a network of threadlike hyphae throughout the interior, and anchors the whole organism to rock, bark, or soil. Beneath the cortex, algal cells cluster in a well-lit zone where they can photosynthesize. In the foliose lichen Candelaria concolor, microscopy reveals that clusters of algal and fungal cells are interconnected by wall-to-wall junctions, forming stable units enveloped in a thin film of water-repelling hydrophobin proteins barely 10 nanometers thick.¹ That hydrophobin coating keeps the interior from flooding while still allowing gas exchange, a structural trick neither partner could pull off alone.

The algal partner, or photobiont, does what plants do: it captures light and converts carbon dioxide into sugars. Those sugars are the fungus’s primary energy source. In return, the fungus supplies water, mineral nutrients, and a physical scaffold that keeps the alga in a favorable position to receive light. When researchers separated the partners of the lichen Usnea hakonensis and then brought them back together, gene expression analyses showed two key changes: the fungus ramped up genes for building a hydrophobic sheath around the algal cells, and the alga activated genes for exporting photosynthetic sugars to the fungus.² The fungus also started supplying phosphorus and nitrogen compounds to the alga. In short, the partnership triggers a specific metabolic handshake that neither organism initiates on its own.

How They Find Each Other

Lichens do not reproduce as a single unit through sexual reproduction the way animals do. Fungal spores land on a surface and must find a compatible algal partner nearby, or the partnership never forms. Early contact appears to be mediated by chemical signals: fungal lectins (sticky proteins that bind to sugar molecules on cell surfaces) and algal cyclic peptides help the two organisms recognize each other. Once they make contact, phytohormone signaling and antioxidant exchanges sustain the developing relationship, while carbon begins flowing from alga to fungus through sugars and sugar alcohols.³

The earliest stages of lichen development have been studied by bringing isolated partners together in the lab. When the fungus Cladonia grayi was paired with the green alga Asterochloris, the fungus turned on genes related to self and non-self recognition, lipid metabolism, and a sugar-alcohol enzyme, while the alga activated a chitinase-like gene, suggesting it may be remodeling fungal cell walls to accommodate the new arrangement.⁴ The process is surprisingly selective: not every alga will do, and the wrong pairing produces nothing.

Which Algae Partner with Fungi

Roughly 50 algal genera include species that form lichens, but the workload is not evenly distributed. A handful of genera dominate: Trebouxia and Asterochloris are the green algae found in the vast majority of lichen species worldwide, while the cyanobacterium Nostoc is the leading partner in cyanobacterial lichens. A few other genera like Trentepohlia, Coccomyxa, and Symbiochloris each pair with dozens to hundreds of fungal species, but much of the remaining diversity participates in only a small number of known lichen associations.⁵ Cyanobacteria like Nostoc can also live independently, which raises a question researchers keep returning to: what makes one Nostoc cell amenable to lichenization while a genetically similar neighbor stays free-living?⁶

Mutualism, Parasitism, or Something in Between

Textbooks usually call lichens a mutualism, but that label has never sat comfortably with everyone who studies them. In artificial lab syntheses of the lichen Cladonia cristatella with different algae, the relationship looked more like controlled parasitism: the fungus grew at the alga’s expense, and the alga did not obviously benefit from the arrangement under those conditions.⁷ The fungus builds specialized contact structures that press tightly against algal cell walls, and a thin protein-and-fatty-acid layer produced by the fungus spreads over the algal surface, which some researchers interpret as a mechanism for extracting nutrients.⁸

The strongest argument for mutualism comes from what happens under stress. When the alga and fungus of the lichen Cladonia vulcani were separated, both suffered: the isolated alga could tolerate only very dim light and its photoprotection partly failed, while the isolated fungus had a sluggish, ineffective antioxidant system. Together in the lichen, each induced protective systems in the other, making the combined organism orders of magnitude more resistant to oxidative damage and desiccation than either partner alone.⁹ So the honest answer is probably “both”: the fungus exploits the alga for carbon, but it also provides protection the alga cannot produce on its own, and the combined tolerance allows both to occupy habitats neither could survive in alone.

The Fungus as Sunscreen and Shield

One of the clearest benefits the fungus provides is photoprotection. Algal cells are vulnerable to intense light, especially ultraviolet radiation, which can damage their photosynthetic machinery. The fungal cortex acts as a physical sunscreen. In the forest lichen Lobaria pulmonaria, the fungus deposits brown melanin-like pigments in its outer cortex layer in proportion to how much sunlight it receives in its habitat. These pigments reduce UV-B, UV-A, and even visible wavelengths reaching the algal cells below.¹⁰ In semi-arid grassland lichens, the fungal partner provides additional protection when the lichen dries out: it produces solar-screening metabolites and causes the thallus to curl inward, shading the algal layer from direct exposure.¹¹

The fungus also produces a rich array of secondary metabolites. These chemicals serve multiple ecological functions: some act as light screens, others help chemically weather rock surfaces to extract minerals, some are allelopathic (discouraging competitors), and others deter herbivores like mites and slugs.¹² Many of these compounds are unique to lichens and are not produced by the fungus or alga when grown separately.

Nitrogen and the Cyanobacterial Connection

Green algae supply carbon but cannot fix atmospheric nitrogen. That job falls to cyanobacteria. Lichens with cyanobacterial photobionts, often called cyanolichens, have a built-in nitrogen supply: the cyanobacterium converts atmospheric nitrogen gas into a usable form. Lichens that contain only green algae and lack cyanobacteria appear to compensate by hosting a more diverse community of nitrogen-fixing bacteria on or within their thalli.¹³

Genomic work has revealed something striking about how nitrogen moves from cyanobacterium to fungus. Lichenized Nostoc cells behave as though they are starved of nitrogen, even though they are actively fixing it. Genes for storing nitrogen internally are turned down, while genes for ammonium transport are turned up. Most telling, about 82 percent of lichenized Nostoc strains lack a key gene called amt1 that free-living Nostoc uses to recapture ammonia leaking from its cells. In free-living strains, that gene is present about 97 percent of the time. Losing it means the lichenized cyanobacterium is less able to hold onto its own nitrogen, and the fungus benefits from the leak.¹⁴ The parallel to nitrogen-fixing bacteria in legume root nodules, where the plant similarly suppresses the bacterium’s nitrogen recapture, is hard to ignore. Evolution seems to have landed on the same trick in very different kingdoms of life.

Lichens Are Not Just Two Partners

For decades, the textbook picture of a lichen was simple: one fungus, one alga, done. That picture has been upended. In 2016, researchers discovered that the outer cortex of many large lichens consistently harbors basidiomycete yeasts, an entirely separate group of fungi from the primary ascomycete partner. These yeasts are embedded in the cortex, and their abundance correlates with previously unexplained variations in lichen appearance, like differences in color or chemical profile between populations of what was thought to be the same species.¹⁵ More recent work has shown that these lichen-inhabiting basidiomycetes can exist as yeasts in the cortex and medulla during normal conditions and switch to a filamentous growth form inside galls, abnormal swellings on the lichen body.¹⁶

Beyond yeasts, lichens host thriving bacterial communities. One comparative study found that more than 800 bacterial species can contribute to the symbiotic system, filling roles that include nutrient supply (nitrogen, phosphorus, sulfur), pathogen defense, abiotic stress resistance, vitamin B12 provision to support photosynthesis, hormone production that aids fungal and algal growth, metabolite detoxification, and breakdown of aging lichen tissue.¹⁷ Particularly abundant bacterial groups include Sphingomonadales, Rhodospirillales, and Myxococcales, which contribute functions ranging from vitamin and cofactor synthesis to the degradation of phenolic compounds.¹⁸ The modern view treats a lichen not as a two-player partnership but as a miniature ecosystem, a holobiont.

A Partnership That Evolved Many Times

Lichenization is not a one-time evolutionary event. Phylogenetic analysis of fungal DNA suggests at least five independent origins of the lichen habit across disparate groups of Ascomycetes and Basidiomycetes.¹⁹ On the algal side, a 2024 phylogenomic study identified at least three independent gains of the ability to engage in lichen symbiosis among green algae, one in the class Trebouxiophyceae and two in Ulvophyceae.²⁰ In other words, unrelated fungi and unrelated algae have independently converged on the same lifestyle multiple times. That repeated convergence is strong evidence that the arrangement is highly advantageous, especially for colonizing harsh terrestrial habitats.

Fossils back up the deep antiquity of the partnership. The enigmatic Devonian fossil Spongiophyton from Brazil, long debated in paleontology, has recently been confirmed as one of the earliest and most widespread records of lichenized fungi, based on the presence of internal hyphal networks, algal cells, calcium oxalate pseudomorphs, and lipid composition consistent with a lichen.²¹ Even older specimens from the Lower Devonian of the Welsh Borderland preserve two internally stratified lichen thalli: one with a cyanobacterial photobiont and one with a green algal photobiont, representing the oldest known records of lichens with anatomy similar to modern species.²² Lichens, in other words, were present during the initial greening of the land.

Surviving the Vacuum of Space

The mutual protection that algae and fungi provide each other reaches remarkable extremes. Lichens have been sent into actual outer space and survived. In the 2005 LICHENS experiment aboard a European Space Agency satellite, lichen samples were exposed to the full vacuum of space, cosmic radiation, and unfiltered solar UV for about two weeks. After the flight, photosynthetic activity was nearly identical to pre-flight levels, and most algal and fungal cells showed no detectable structural damage. The lichens also recovered full metabolic activity within 24 hours of rehydration after extreme dehydration from space vacuum.²³

Further experiments on the International Space Station and during Mars simulation tests have confirmed this resilience for several lichen species.²⁴ The key appears to be the cortex: when researchers removed the upper cortex, sensitivity to UV and vacuum increased dramatically, with vitality dropping to about 50 percent after modest UV exposure. Isolated algal photobionts could tolerate some UV on their own, but saturated at about five times lower doses than intact lichens.²⁵ The fungal cortex, combined with secondary metabolites and the alga’s own repair mechanisms, creates a survival package far more robust than either partner alone. This is why astrobiologists take lichens seriously as models for life that could persist on Mars-like surfaces.

Algae-Fungi Partnerships Beyond Lichens

Lichens dominate the conversation, but they are not the only form of algae-fungi symbiosis. One of the most unusual is Geosiphon pyriformis, a fungus in the group that includes arbuscular mycorrhizal fungi (the fungi that partner with plant roots). Geosiphon is the only known fungus that takes cyanobacterial cells inside its own cells, forming bladder-like structures where the cyanobacterium Nostoc punctiforme photosynthesizes, divides, and lives alongside bacterial endosymbionts.²⁶ This is a genuine endosymbiosis: the alga lives inside the fungal cell rather than being pressed against it from outside as in lichens. Beyond Geosiphon, chytrid fungi and Cryptomycota are known to parasitize microalgae, but the full diversity of these interactions remains undercharacterized.

Practical Applications and Biotechnology

The algae-fungi relationship has caught the attention of engineers trying to solve problems in biofuel production and wastewater treatment. Growing microalgae for biodiesel sounds great in theory, but harvesting the tiny cells from water is expensive and energy-intensive. Researchers have found that co-cultivating microalgae with filamentous fungi like Aspergillus fumigatus can solve this: the fungus forms pellets that trap algal cells, making them easy to collect by simple filtration. In one study, the fungus efficiently harvested 10 out of 11 tested microalgal strains, including marine species. The co-cultures also showed increased total biomass and lipid production compared to algae grown alone, and the fatty acid profiles could be tailored by choosing different fungal-algal combinations without genetic modification.²⁷ These co-cultures can also run on waste nutrients, pulling carbon, nitrogen, and phosphorus from wastewater while cleaning it.²⁸

Making Soil from Rock

Lichens play a role in creating soil that is easy to overlook but hard to overstate. Their secondary metabolites chemically weather rock surfaces, dissolving minerals and gradually breaking solid stone into smaller particles. In the Atacama Desert, one of the driest places on Earth, researchers have documented lichens and their microbial communities driving a full weathering sequence: large rocks become smaller rocks, then grit, and eventually a primitive soil called a protopedon. Because the Atacama’s climate has been stable for an extraordinarily long time, this process appears to operate over geological timescales, steadily transforming barren rock into a substrate where other organisms can eventually gain a foothold.²⁹

This soil-building function connects to a broader ecological role. Lichens are among the first colonizers of bare surfaces after disturbances like volcanic eruptions or glacial retreat. By weathering rock and trapping dust, they create thin soil layers that mosses and eventually vascular plants can colonize. They also fix nitrogen (when cyanobacterial partners are present) and contribute organic matter when they die and decompose. In biological soil crusts, common across arid and semi-arid ecosystems worldwide, lichens are often the most visible component, stabilizing the soil surface against erosion.

Lichens as Pollution Detectors

The same sensitivity that makes the algae-fungi symbiosis fragile outside the partnership makes it a useful environmental alarm. Lichens absorb water and dissolved nutrients directly from the air rather than through roots, which means they also absorb airborne pollutants with no way to filter them out. Their sensitivity to contaminants has made them widely used bioindicators: the presence or absence of certain lichen species in a given area, or the concentrations of nitrogen, sulfur, and metals in their tissues, can reveal local air quality better than some instrumental monitors.³⁰ In the eastern United States, specific lichen species are officially recommended as element bioindicators for nitrogen, sulfur, and metal pollution across defined ecological regions.³¹ If you live in a city and rarely see lichens on trees, that itself is data about what you are breathing.