Carbon is the fuel, not sunlight
Unlike plants, mushrooms do not photosynthesize. They must obtain carbon and energy from external sources. For many fungi, this means breaking down organic matter in their environment. A 1988 study found that fungi, predominantly Aspergillus species, can utilize fatty acids with 14, 16, or 18 carbon atoms as their sole source of carbon and energy [3]. This shows that fatty acids can serve as a complete fuel for fungal growth.
More recent work on arbuscular mycorrhizal fungi, which normally live in symbiosis with plants, found that one specific fatty acid—myristate (a 14-carbon saturated fat)—can support their growth even without a plant host. The fungi took up myristate and used it as both a carbon and energy source, producing extensive hyphal growth and new spores [2][4]. This suggests that fatty acids can be a direct energy currency for some fungi.
Some mushrooms rely on plant partners for energy
Arbuscular mycorrhizal fungi are obligate symbionts, meaning they cannot complete their natural life cycle without a host plant. The plants supply them with lipids, including fatty acids, which the fungi use for energy [2]. This is a specialized case: the fungus depends on its plant partner for fuel, and in return the plant gains nutrients like phosphorus.
The fact that myristate alone can support growth outside a host [2][4] does not mean mushrooms in the wild typically feed on pure fatty acids. It does show that fatty acids are a usable energy source when available, and that these fungi have the metabolic machinery to extract energy from them.
Radiation and stress can also play a role
Some fungi with melanin pigments appear able to respond to electromagnetic radiation. A 2017 review noted that melanotic fungi showed enhanced growth when exposed to high-energy electromagnetic radiation, suggesting they might transduce that radiation into usable energy [5]. This is not the same as photosynthesis, and the evidence is based on growth responses rather than a fully mapped energy pathway.
Separately, fungi can accumulate lipids under metabolic stress. A 2022 review on biodiesel production noted that fungi can store more than 70% of their biomass as intracellular lipids during stress periods [1]. While this is about lipid accumulation rather than direct energy acquisition, it shows that fungi manage energy reserves in ways that can be harnessed industrially.
Sources used in this answer
Utilization of Fungal Bioprocess for Biodiesel Production as a Green Energy Source
Fungi can accumulate over 70% of their biomass as intracellular lipids during metabolic stress, which is relevant for biodiesel production.
Faculty Opinions recommendation of Myristate can be used as a carbon and energy source for the asymbiotic growth of arbuscular mycorrhizal fungi.
Myristate, a 14-carbon fatty acid, can serve as a carbon and energy source for the asymbiotic growth of arbuscular mycorrhizal fungi, promoting hyphal growth and spore formation.
Arachidonic acid from fungi utilizing fatty acids with shorter chains as sole sources of carbon and energy
Fungi, predominantly Aspergillus species, can utilize C14, C16, and C18 fatty acids as sole sources of carbon and energy.
Myristate can be used as a carbon and energy source for the asymbiotic growth of arbuscular mycorrhizal fungi
Myristate supports the asymbiotic growth of arbuscular mycorrhizal fungi by acting as a carbon and energy source, and mixtures with palmitate can accelerate growth.
Melanin, radiation, and energy transduction in fungi
Melanotic fungi have been reported to show enhanced growth in response to high-energy electromagnetic radiation, suggesting a possible energy transduction role for melanin.