Fungi are among the most essential organisms on Earth, quietly driving processes that keep ecosystems functioning. They decompose dead material and recycle nutrients back into soil, form underground partnerships with over 90% of land plants, connect trees through chemical signaling networks, regulate plant populations to maintain biodiversity, and sequester billions of tons of carbon each year. Without fungi, forests would be buried in dead wood, most plants would starve for nutrients, and the global carbon cycle would look radically different.
Breaking Down What Nothing Else Can
Dead trees, fallen branches, and leaf litter would pile up endlessly without fungi. The reason is simple: fungi are among the only organisms that can break down lignin, the tough structural compound that makes wood rigid. White-rot fungi produce specialized enzymes that oxidize and dismantle lignin molecule by molecule, while a separate set of enzymes chops cellulose into individual glucose units. No single bacterium can do both jobs this efficiently.
This decomposition process is the engine of nutrient cycling. As fungi digest dead plant material, they release carbon, nitrogen, and phosphorus back into the soil in forms that living plants can absorb. Without this recycling, the nutrients locked inside a fallen tree trunk would stay trapped for decades or longer. Fungi essentially convert death into fertility, making them the primary recyclers of terrestrial ecosystems.
Feeding Plants Through Underground Partnerships
More than 90% of terrestrial plant species form symbiotic relationships with mycorrhizal fungi. These fungi extend threadlike filaments called hyphae far beyond the reach of plant roots, dramatically expanding the area a plant can draw nutrients from. In exchange, the plant feeds the fungus sugars produced through photosynthesis.
The nutrient exchange happens at specialized structures inside root cells called arbuscules, which look like tiny branching trees. Phosphorus is the star of this exchange. Arbuscular mycorrhizal fungi can supply plants with up to 80% of their inorganic phosphorus through a dedicated uptake pathway. The fungi absorb phosphorus through transporters on their hyphae, shuttle it through their internal network, and deliver it directly into plant root cells through a separate transporter protein. They also secrete organic acids and enzymes that dissolve phosphorus compounds otherwise locked in the soil, making them available for uptake.
When soil phosphorus is scarce, fungal hyphae actually ramp up their production of these dissolving enzymes. Fungi can even stimulate phosphorus-solubilizing bacteria nearby, using chemical signals secreted by the mycelium to recruit bacterial help. The result is a multi-layered nutrient acquisition system that most crops and wild plants depend on to survive.
The Underground Network Connecting Trees
Mycorrhizal fungi don’t just connect one plant to one fungus. Their hyphae link multiple plants of different species into shared underground networks, sometimes called common mycelial networks. These networks transfer carbon, water, nitrogen, phosphorus, micronutrients, and even chemical warning signals between plants.
Carbon and nitrogen travel through these networks together as simple amino acids, moving from a donor plant into the fungal mycelium within one to two days and reaching the shoots of neighboring plants within three days. The flow follows a source-to-sink pattern: nutrients move from plants that have more toward plants that have less, creating a kind of resource-sharing system across a forest floor.
The signaling function is equally remarkable. When broad bean plants were attacked by aphids, they transferred defense signals through the fungal network to neighboring bean plants, which responded by producing chemicals that repelled aphids and attracted aphid predators. In a forest study, Douglas-fir trees infested with spruce budworm triggered defense enzyme production in neighboring ponderosa pines through the shared network within 24 hours. Stress responses in undamaged plants connected by these networks have been detected as quickly as six hours after an insect or fungal attack on a neighboring plant.
Maintaining Forest Diversity
Fungal pathogens play a counterintuitive but critical role in keeping ecosystems diverse. In tropical forests especially, soil fungi infect seeds and seedlings on the forest floor, and this infection follows a pattern: plants are far more likely to be attacked when they’re surrounded by others of their own species. This is called negative density dependence, and it works much like a human disease spreading faster in a crowded room.
When one tree species starts to dominate an area, its accumulated pathogens in the surrounding soil kill off a disproportionate number of its own seedlings. That creates openings for less common species to establish themselves. The net effect is that fungal disease prevents any single species from taking over, maintaining the extraordinary plant diversity found in tropical forests and other ecosystems. This mechanism, part of the Janzen-Connell hypothesis, positions fungi as invisible architects of biodiversity.
Storing Billions of Tons of Carbon
Fungi are a massive, underappreciated carbon sink. A 2023 study published in Current Biology estimated that terrestrial plants allocate roughly 13.12 gigatons of CO₂ equivalent per year to the underground mycelium of mycorrhizal fungi. That figure equals about 36% of current annual CO₂ emissions from fossil fuels. Ectomycorrhizal fungi, the type associated with many forest trees, account for the largest share at 9.07 gigatons, followed by arbuscular mycorrhizal fungi at 3.93 gigatons.
This carbon doesn’t just pass through. Fungal hyphae produce a glycoprotein often called “super glue” for its ability to bind soil particles together into stable clumps called aggregates. This protein, produced by arbuscular mycorrhizal fungi, cements soil particles into structures that physically protect organic carbon from being broken down and released as CO₂. Research has shown it has a stronger direct stabilizing effect on soil aggregates than the hyphae themselves, particularly in medium-sized soil clumps. The result is carbon that stays locked in the soil for extended periods rather than returning to the atmosphere.
Building and Stabilizing Soil
Beyond carbon storage, the soil-binding protein produced by mycorrhizal fungi is fundamental to soil health. It promotes the formation of macroaggregates, the larger soil clumps that give healthy soil its crumbly structure. Studies across diverse soil types have consistently found a positive correlation between the concentration of this protein and the stability of soil aggregates in water, meaning soils with more of it resist erosion and compaction better.
Healthy soil structure matters for water filtration, root penetration, and the survival of countless other soil organisms. Fungal hyphae themselves act as physical scaffolding, weaving through soil and holding particles in place, but the protein they release has an outsized effect. In combination, the physical network of hyphae and the chemical glue they produce create the structural foundation that topsoil depends on.
Feeding Animals and Spreading Spores
Fungi are a food source for a wide range of animals, from beetles and slugs to ants and springtails. In Southeast Asia, certain ant species actively harvest oyster mushrooms. Slugs and snails feed on various mushroom species and carry viable spores in their digestive systems, depositing them in new locations through their droppings. Beetles feeding on underground fungi have been found with viable spores in their fecal matter, and flies are particularly effective at transporting fungal spores in their intestines.
Some relationships are highly specialized. Certain termite species carry fungal spores stuck to their exoskeletons, and ambrosia beetles cultivate fungal gardens inside wood as their primary food source. One fly species acts as the primary pollinator for a specific grass fungus, fertilizing it through a behavior so targeted it resembles the relationship between flowers and their pollinators. These interactions mean fungi aren’t just decomposers or plant partners. They sit at the center of food webs, supporting animal populations while relying on those same animals to reproduce and spread.
Cleaning Up Pollution
Fungi can absorb and concentrate heavy metals from contaminated environments, a process known as mycoremediation. Oyster mushrooms in the genus Pleurotus have shown particularly strong biosorption potential for a range of heavy metals, outperforming several other fungal species in absorbing cadmium and chromium. Other genera studied for metal uptake include Agaricus, Boletus, and Russula. Because fungal mycelium spreads through soil so extensively, it can access and concentrate pollutants across a wide area, offering a biological approach to cleaning contaminated land.
A Kingdom We Barely Know
Scientists estimate there may be around 2.5 million fungal species worldwide, but only about 6% of them have been identified and described so far. Of those known species, just 818 appear on the IUCN Red List of Threatened Species, representing 0.5% of described fungi and a mere 0.03% of estimated total fungal diversity. This means the vast majority of fungal species have never been assessed for conservation status, even as the ecosystems they support face mounting pressure from land use change and climate disruption. Protecting fungi means protecting the invisible infrastructure that forests, grasslands, and agricultural systems depend on to function.