Fungi are heterotrophs, meaning they cannot make their own food from sunlight or inorganic chemicals the way plants and some bacteria do. Instead, every fungus on Earth gets its energy from organic carbon compounds produced by other organisms. The twist is in how different fungi acquire that organic matter: some dissolve dead wood, some tap into living plant roots, some trap and digest tiny animals, and a few may even harness radiation. What unites them is a shared dependence on pre-made organic molecules and a remarkably flexible toolkit for obtaining them.
Absorbing Rather Than Eating
Unlike animals, fungi do not ingest food and digest it internally. They secrete enzymes into their surroundings, break complex molecules down outside their bodies, and then absorb the smaller products through their cell walls. This strategy, called absorptive heterotrophy, is the defining nutritional feature of the kingdom Fungi. It explains why you find fungal threads (hyphae) weaving through soil, wood, and leaf litter rather than chasing prey. The enormous surface area of a hyphal network maximizes contact with whatever substrate the fungus is decomposing.
Once simple sugars and other small molecules cross the cell membrane, fungi store them mainly as trehalose and glycogen. These storage compounds act as internal energy reserves. Research on the mold Aspergillus fumigatus shows that protein kinases regulate the buildup and breakdown of trehalose and glycogen, and that disrupting those signaling pathways reduces glucose uptake and glycogen synthesis significantly.1PubMed Central. Protein Kinase A and High-Osmolarity Glycerol Response Pathways Cooperatively Control Cell Wall Carbohydrate Mobilization in Aspergillus fumigatus In other words, fungi do not just passively soak up sugar; they actively manage their energy reserves the way your body toggles between burning glucose and storing it as glycogen.
Saprotrophy and Wood Decay
The most familiar energy strategy among fungi is saprotrophy: feeding on dead organic matter. Mushrooms sprouting from a fallen log, mold spreading across stale bread, mildew creeping over damp drywall. All of these organisms are breaking down dead tissue and extracting the carbon and energy locked inside. Saprotrophic fungi are the planet’s primary recyclers, converting dead plant and animal material back into forms that other organisms can use.
Wood presents a particular challenge because its main structural material, lignocellulose, is one of the toughest biological polymers on Earth. It is also the largest pool of above-ground biological carbon.2PubMed Central. Metabolomics Highlights Different Life History Strategies of White and Brown Rot Wood-Degrading Fungi Two broad categories of wood-decay fungi have evolved distinct approaches to cracking it open. White-rot fungi break down both the lignin and the cellulose in wood, eventually bleaching the remaining material pale. Brown-rot fungi largely leave the lignin intact while targeting the cellulose and hemicellulose, leaving behind a crumbly brown residue. Both groups degrade hemicellulose during early stages of decay, though the precise role hemicellulose breakdown plays in the overall process is still being worked out.3ACS Sustainable Chemistry & Engineering. New Insights for Biomass Utilization by Brown-Rot and White-Rot Fungi: the Differing Role of Hemicellulose Degradation in the Incipient Decay Process
These decay fungi are not just ecologically important; they are a major reason forests do not simply pile up with dead trees forever. The energy these fungi extract from lignocellulose powers their own growth and reproduction while simultaneously cycling carbon and nutrients back into the soil.
Mycorrhizal Partnerships With Plants
A huge number of fungi skip dead matter entirely and instead get their carbon directly from living plant roots through mycorrhizal symbiosis. The deal is straightforward in broad strokes: the fungus extends its hyphal network far beyond the reach of the plant’s own roots, pulling in water and mineral nutrients (especially phosphorus and nitrogen) and delivering them to the plant. In return, the plant feeds the fungus carbon compounds produced through photosynthesis. Roughly 90 percent of plant species participate in some form of mycorrhizal relationship, making this one of the most widespread symbioses on Earth.
Ectomycorrhizal fungi, the type commonly associated with forest trees like pines and oaks, form a sheath around root tips and receive photosynthetically fixed carbon from their hosts. Culture studies suggest that plants allocate anywhere from about 1 to 21 percent of their total net primary production to their ectomycorrhizal partners.4PubMed. Carbon allocation to ectomycorrhizal fungi correlates with belowground allocation in culture studies That allocation is not constant; season and soil nitrogen availability cause dramatic swings. In boreal pine forests, below-ground transfer of recent plant carbon was roughly five times higher in late summer compared to early summer, and the carbon ended up primarily in fungal tissue rather than in soil bacteria.5PubMed. Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest Isotope-tracing work confirms that ectomycorrhizal structures consistently become more carbon-enriched than the lateral roots they are attached to, with the enrichment tracking linearly between root and fungus.6Communications Biology. Mycorrhizal C/N ratio determines plant-derived carbon and nitrogen allocation to symbiosis
Arbuscular mycorrhizal fungi, which colonize the roots of most herbaceous plants, have an even more intimate relationship. For decades, scientists assumed these fungi received sugars from their hosts and that was the whole story. Newer research overturned that picture. It turns out that arbuscular mycorrhizal fungi like Rhizophagus irregularis cannot synthesize their own fatty acids at all. They are fatty-acid auxotrophs, fully dependent on lipids made by the host plant and transferred to them across the root interface.7PubMed. Plants transfer lipids to sustain colonization by mutualistic mycorrhizal and parasitic fungi Isotope-tracing confirmed that the carbon patterns in fungal fatty acids mirror those of the host plant, showing unambiguous cross-kingdom lipid transfer.8PubMed Central. Lipid transfer from plants to arbuscular mycorrhiza fungi So the main energy source for these fungi is not just sugar but also fats, hand-delivered by the plant.
Parasites and Pathogens That Feed on Living Hosts
Mycorrhizal fungi give something back. Parasitic and pathogenic fungi take without returning the favor. Biotrophic plant pathogens, the rusts and powdery mildews that plague crops, are entirely dependent on a living host for nutrition. Their signature tool is the haustorium, a specialized structure that penetrates a plant cell without killing it, siphoning off sugars and other nutrients while simultaneously injecting molecules that suppress the plant’s immune defenses.9PubMed Central. The haustorium: The root of biotrophic fungal pathogens
The flow of sugar from plant to fungus through haustoria has been directly demonstrated. In the rust fungus Uromyces fabae infecting broad bean, researchers showed that hexose sugars move primarily through the haustorial complex, and that one of the most abundantly expressed genes in rust haustoria encodes a hexose transporter.10PubMed. The role of haustoria in sugar supply during infection of broad bean by the rust fungus Uromyces fabae The fungus essentially hijacks the plant’s own sugar economy, redirecting metabolic flow toward itself. Necrotrophic pathogens take a blunter approach: they kill host tissue outright and then feed on the remains, combining pathogenesis with saprotrophy.
Predatory Fungi That Trap Animals
A small but fascinating group of fungi supplements its diet by capturing and digesting microscopic animals, especially nematodes (roundworms). Species like Arthrobotrys oligospora form adhesive traps or constricting rings along their hyphae. When a nematode blunders into a trap, the fungus immobilizes it and sends hyphae into its body to digest it from the inside.
This is not a simple mechanical process. When A. oligospora detects nematodes, it ramps up expression of secreted proteins thought to function as effectors, some of which appear to help build the trapping structures and subdue the prey.11PLOS Biology. Key processes required for the different stages of fungal carnivory by a nematode-trapping fungus Most nematode-trapping fungi can also live as ordinary saprotrophs, decomposing soil organic matter. The carnivory appears to be a supplemental nitrogen source more than a primary carbon source, activated when soil nitrogen is scarce. Even so, the ability to capture and digest animal tissue is a genuine energy-acquisition strategy, and it blurs the line between fungus and predator.
Lichens and the Algal Subsidy
Lichens look like a single organism but are actually a partnership between a fungus (the mycobiont) and a photosynthetic partner, usually a green alga or a cyanobacterium (the photobiont). The photobiont does the photosynthesis, and the fungus wraps around it, providing structure and protection while siphoning off sugars and sugar alcohols. Carbon exchange through these compounds is important throughout the life of the lichen, from initial contact between partners through mature growth.12PubMed Central. How to build a lichen: from metabolite release to symbiotic interplay
The fungal partner in a lichen is still a heterotroph. It has no chlorophyll and cannot photosynthesize. But by housing an alga and receiving a steady stream of photosynthetically produced sugars, it achieves something close to self-sufficiency. This arrangement lets lichens colonize bare rock, desert crusts, and arctic tundra where neither partner could survive alone.
How Fungi Turn Food Into Usable Energy
Once a fungus has absorbed organic molecules, it needs to convert them into ATP, the universal cellular energy currency. Most fungi do this through aerobic respiration, using mitochondria much like animal cells do. But fungal respiratory chains are more flexible than ours. Many fungi have branched electron-transport pathways that include components absent from mammalian cells.13Biochimica et Biophysica Acta (BBA) – Bioenergetics. Fungal respiration: a fusion of standard and alternative components
One notable feature is the alternative oxidase, an enzyme that branches off the main respiratory chain and provides a bypass route for electrons when the standard pathway is inhibited or overloaded. This enzyme does not pump protons and therefore does not contribute directly to ATP production in most species, though there are exceptions: in the gray mold Botrytis cinerea, for example, the alternative oxidase pathway does appear to contribute to ATP generation.14Biochimica et Biophysica Acta (BBA) – Bioenergetics. Alternative Oxidase – Aid or obstacle to combat the rise of fungal pathogens? The practical benefit of this bypass is resilience. Even when standard respiration is compromised, for instance under low-oxygen conditions inside infected tissue, fungi can keep electrons flowing and avoid the toxic buildup of reactive oxygen species. Work on Aspergillus fumigatus showed that respiratory chain activity continues during low-oxygen growth, though at lower efficiency and with greater reliance on the alternative oxidase.15PubMed Central. Aspergillus fumigatus mitochondrial electron transport chain mediates oxidative stress homeostasis, hypoxia responses, and fungal pathogenesis
A small group of fungi has abandoned aerobic respiration altogether. Anaerobic fungi in the gut of ruminant animals, like cattle and sheep, lack mitochondria entirely. Instead they use hydrogenosomes, organelles that generate ATP from malate, releasing hydrogen gas, acetate, and carbon dioxide as byproducts.16Mycological Research. Metabolic energy generation in hydrogenosomes of the anaerobic fungus Neocallimastix: evidence for a functional relationship with mitochondria These fungi thrive in the oxygen-free environment of the rumen, where they break down plant fiber that the animal cannot digest on its own. The hydrogen they produce does not just dissipate; it fuels hydrogen-consuming microbes living alongside them. If hydrogen accumulates without being removed, it actually inhibits the hydrogenosome’s own enzymatic reactions and slows the fungus down, so the partnership with hydrogen-scavenging bacteria and archaea is essential.17PubMed Central. Hydrogenosome, Pairing Anaerobic Fungi and Hâ‚‚-Utilizing Microorganisms Based on Metabolic Ties to Facilitate Biomass Utilization
Radiotrophic Fungi and the Melanin Question
Perhaps the most exotic energy-related claim in mycology involves fungi that appear to benefit from ionizing radiation. Melanin-rich fungi were found thriving inside the destroyed Chernobyl reactor, in spacecraft, and at Antarctic high altitudes, all places with elevated radiation.18PubMed Central. Ionizing radiation: how fungi cope, adapt, and exploit with the help of melanin Laboratory experiments showed that melanized fungi grew faster when exposed to ionizing radiation compared to non-melanized strains, and that radiation altered the electronic properties of melanin itself.19PLoS ONE. Ionizing Radiation Changes the Electronic Properties of Melanin and Enhances the Growth of Melanized Fungi
This led researchers to propose, cautiously, that melanin might function somewhat like chlorophyll: capturing electromagnetic energy and converting it into a form the cell can use. The idea is tantalizing but far from proven. Melanin absorbs across a broad spectrum and has strong oxidation-reduction properties, which theoretically could let it transduce radiation into chemical energy.20PubMed Central. Melanin, Radiation, and Energy Transduction in Fungi But “enhanced growth under radiation” is not the same as “powered by radiation.” These fungi still need organic carbon; melanin-mediated energy capture, if it exists, would supplement rather than replace heterotrophy. The mechanism remains speculative, and no one has demonstrated a clear biochemical pathway linking melanin’s radiation absorption to ATP synthesis in the way we understand photosynthesis in plants.
Evolutionary Roots of Fungal Feeding Strategies
All modern fungi descend from aquatic ancestors that were already heterotrophic. The earliest fungi were likely zoosporic (producing swimming spores) and fed on organic matter in water.21PubMed Central. Fungal evolution: major ecological adaptations and evolutionary transitions As fungi moved onto land and diversified, their nutritional strategies branched dramatically. Saprotrophy came first in many lineages, with mycorrhizal mutualism evolving later, often more than once independently.
Genomic comparisons reveal what happened at the molecular level during these transitions. When saprotrophic lineages evolved into mycorrhizal mutualists, they lost many of the genes encoding enzymes for breaking down lignocellulose. At the same time, they gained new genes that facilitated integration with plant root tissue. These transitions from decomposer to symbiont involved convergent evolution across distantly related lineages, suggesting that the shift from dead-matter feeding to living-host feeding follows predictable genetic paths.22Annual Review of Ecology, Evolution, and Systematics. Evolution of the Mode of Nutrition in Symbiotic and Saprotrophic Fungi in Forest Ecosystems
When Fungi Become the Food Source
The carbon that fungi receive from plants does not always stay with the fungus. In a phenomenon called mycoheterotrophy, certain plants have given up photosynthesis and instead steal carbon from mycorrhizal fungi. Plants like the ghost orchid or Indian pipe are pale, leafless, and entirely dependent on fungal networks for all their carbon and energy. These mycoheterotrophic plants tend to target fungi that are well-connected to many photosynthetic plants, effectively tapping into a wide underground network of carbon.23PubMed Central. Mycoheterotrophic plants preferentially target arbuscular mycorrhizal fungi that are highly connected to autotrophic plants The flow of energy goes from photosynthetic plant to fungus to parasitic plant, a three-step chain that uses the fungal network as a pipeline.24PubMed. Mycoheterotrophy
This reversal highlights something important about fungal energy dynamics: the carbon pool that fungi manage is not a dead end. Ectomycorrhizal fungi play a role in whether soil carbon accumulates or gets released. Their ability to access organic nitrogen bound in soil organic matter varies widely among species, and this variation can either promote or reduce soil carbon storage.25New Phytologist. Exploring the role of ectomycorrhizal fungi in soil carbon dynamics In a world increasingly concerned about carbon cycling and climate, the way fungi move energy and carbon through ecosystems is far more than an academic curiosity.
Fungi as Industrial Energy Converters
Humans figured out how to exploit fungal energy metabolism long before anyone understood the biochemistry behind it. Bread rising in the oven and beer fermenting in a vat both depend on Saccharomyces cerevisiae, baker’s yeast, converting sugars into ethanol and carbon dioxide under low-oxygen conditions. This is anaerobic fermentation, the same basic metabolic pathway yeast uses in the wild to survive when oxygen is scarce.
Today, yeast-based ethanol production is a global industry. Metabolic engineering has pushed S. cerevisiae well beyond its natural capabilities, fine-tuning the genes involved in ethanol metabolism to increase yields and broaden the range of sugars the yeast can ferment.26PubMed Central. From Saccharomyces cerevisiae to Ethanol: Unlocking the Power of Evolutionary Engineering in Metabolic Engineering Applications Efforts to use other fungi for industrial biomass conversion lean on the same decomposition machinery that white-rot and brown-rot fungi use in forests. If researchers can harness those enzymes efficiently, agricultural waste and wood residues could be converted into biofuels at scale. The energy source is the same one fungi have always used: organic carbon. The difference is that we are now directing the output.
Fungal Bioluminescence
About 100 fungal species glow in the dark, producing a faint greenish light visible on the forest floor at night. Unlike fireflies, which use a luciferin-luciferase reaction that evolved independently, fungi run their own distinct bioluminescence pathway. A recent study confirmed that the fungal bioluminescence cycle is fully self-sustaining: the enzyme caffeylpyruvate hydrolase breaks down oxyluciferin into caffeic acid and pyruvic acid, which can be recycled back into the pathway to produce more light.27PubMed. Caffeylpyruvate hydrolase from the bioluminescent fungus Neonothopanus gardneri is the key recycling enzyme in the fungal bioluminescence pathway
Bioluminescence costs energy rather than generating it. The light comes from chemical reactions that consume oxygen and organic substrates, so it represents a metabolic expense. The leading hypothesis for why some fungi bother glowing is that the light attracts insects, which then spread the fungus’s spores. In energy terms, the fungus is spending a small fraction of its heterotrophically acquired carbon budget to improve its reproductive odds: a marketing expense paid in photons.