Fungi are a diverse kingdom of life, distinct from plants and animals, primarily due to their unique strategies for acquiring nutrients. Unlike plants, fungi do not perform photosynthesis; they are heterotrophs, meaning they obtain food from external sources. Unlike animals, fungi do not typically ingest their food before digesting it. Their specialized method of “eating” allows them to thrive in various environments and underpins their ecological roles.
How Fungi Absorb Nutrients
Fungi utilize extracellular digestion to acquire nutrients. They release digestive enzymes, called exoenzymes, directly into their surroundings, such as soil, decaying plants, or living hosts. These enzymes break down complex organic materials like cellulose, lignin, proteins, and lipids into simpler, soluble molecules. These smaller molecules can then pass through fungal cell walls and membranes.
Once broken down, these smaller nutrient molecules, like sugars and amino acids, are absorbed directly into the fungal cells. The main body of a fungus, often hidden, consists of a network of thread-like structures called hyphae. These hyphae branch and intertwine to form a mycelium. This extensive network increases the surface area for absorption, allowing fungi to efficiently take up nutrients.
Fungi as Decomposers
Many fungi function as decomposers, obtaining nutrients by breaking down dead organic matter. These saprophytic fungi secrete enzymes onto dead plants, animals, and other organic debris like rotting wood or fallen leaves. This process transforms complex organic materials into simpler substances, making nutrients available for reuse in ecosystems.
Fungi play a role in nutrient recycling. They release elements like nitrogen and phosphorus from decaying matter, making them available to other living organisms. Common examples include mushrooms on rotting logs, which break down wood, or molds on decaying food, returning its components to the environment.
Fungi in Mutualistic Relationships
Fungi also acquire nutrients through mutualistic relationships, exchanging resources with other organisms for mutual benefit. One example is mycorrhizae, associations between fungi and plant roots.
Fungal hyphae extend into the soil, acting as an extension of the plant’s root system. This enhances the plant’s ability to absorb water and mineral nutrients like phosphorus and nitrogen. In return, the plant provides the fungus with carbohydrates produced through photosynthesis.
Another example is lichens, composite organisms formed from a symbiotic relationship between a fungus and a photosynthetic partner, usually algae or cyanobacteria. The fungus provides a protective structure and absorbs water and minerals. The photosynthetic partner produces sugars through photosynthesis, which the fungus uses as a carbon source. Some lichens with cyanobacteria can also fix atmospheric nitrogen, providing this nutrient to the partnership.
Fungi as Parasites
Some fungi obtain nutrients by living on or within a living host. These parasitic fungi penetrate the host’s tissues and absorb nutrients directly from the host’s cells. This can reduce the host’s health or even cause its death.
Examples include parasitic fungi that cause plant diseases, such as rusts and smuts, impacting crops by drawing nutrients. Fungi can also parasitise animals and humans, leading to conditions like athlete’s foot or ringworm, where the fungus consumes nutrients from skin, hair, or nails. Specialized structures, such as haustoria, can be formed to facilitate direct nutrient absorption from host cells.