Fungi are a diverse group of organisms that cannot produce their own food through photosynthesis, unlike plants. Instead, fungi are heterotrophs, meaning they must obtain their nutrients from external sources. They rely on complex organic compounds found in their environment.
How Fungi Obtain Nutrients
Fungi employ a distinctive method of nutrient acquisition: external digestion. They secrete specialized digestive enzymes, called exoenzymes, directly into their surroundings. These enzymes break down complex organic molecules, such as proteins, carbohydrates, and fats, into smaller, simpler compounds. This process occurs outside the fungal body.
Once broken down, these smaller, soluble nutrient molecules are absorbed through the extensive network of fungal filaments known as hyphae, which collectively form the mycelium. This absorption occurs across the mycelium’s large surface area, allowing for efficient uptake. Unlike animals that ingest food before digesting it internally, fungi perform digestion prior to absorption.
Diverse Fungal Feeding Strategies
Fungi exhibit a variety of feeding strategies based on their nutritional sources. Saprophytic fungi are decomposers, obtaining nutrients from dead organic matter such as decaying plants, animals, and waste. They are crucial for nutrient cycling, as their enzymes break down complex materials like cellulose and lignin, returning simpler compounds to the environment. Common examples include molds like Penicillium and Mucor, as well as many types of mushrooms.
Parasitic fungi derive their nutrition from living hosts, often causing diseases. These fungi can infect a wide range of organisms, including plants, animals, and even other fungi. Examples include plant pathogens like rusts and smuts, or fungi causing human infections such as athlete’s foot and yeast infections. Some parasitic fungi, like Cordyceps, target insects. Many develop specialized structures, such as haustoria, to penetrate host cells and absorb nutrients directly.
Mutualistic fungi form beneficial symbiotic relationships where both the fungus and another organism benefit. Mycorrhizal fungi, for instance, associate with plant roots, helping plants absorb water and essential nutrients like phosphorus and nitrogen from the soil. In return, the plants provide the fungi with sugars produced through photosynthesis. Approximately 80-90% of vascular plants form such associations.
Lichens represent another prominent mutualism, involving a fungus and a photosynthetic partner, typically green algae or cyanobacteria. The photosynthetic partner produces food, while the fungus offers protection, water, and minerals.