Beneath the surface of nearly every ecosystem lies a complex biological network of fungi. Within this kingdom, the terms mycelium and mycorrhizae are often used interchangeably, creating confusion. Although fundamentally linked, these two concepts describe entirely different aspects of fungal biology. Understanding the distinction is the first step toward appreciating the intricate roles fungi play in the health of our planet.
Defining Mycelium
Mycelium is the primary body of a fungus, its vegetative part. It consists of a vast, interconnected network of microscopic, thread-like filaments called hyphae. These hyphae branch out through soil, wood, or other organic matter, collectively forming the mycelium, which can range in size from microscopic to covering many acres. The largest known organism on Earth is a honey mushroom mycelium in Oregon, spanning over 2,400 acres.
A helpful analogy is to compare a fungus to a plant. If the mushroom we see above ground is the fruit, then the mycelium is the equivalent of the tree’s extensive root system, hidden from view but responsible for its growth. The mycelium’s main purpose is to seek out and absorb nutrients from its surroundings. It accomplishes this by releasing enzymes that break down complex organic material, which it then absorbs through its hyphal network.
This process of decomposition is a foundational function of many mycelial networks. Saprophytic fungi, for instance, specialize in decomposing dead organic matter, recycling nutrients back into the ecosystem. The mycelium, in this case, acts as a solitary agent, tunneling through leaf litter and fallen logs to fuel its own growth and the eventual production of a fruiting body, such as a mushroom.
Understanding Mycorrhizae
Mycorrhiza is not a physical object but a functional, symbiotic relationship. The term, derived from the Greek words for “fungus” (mykes) and “root” (rhiza), describes a mutualistic association between a fungus and the roots of a living plant. This is a classification for a type of interaction that many different fungal species can form. An estimated 80-90% of all land plant species engage in some form of this ancient and widespread partnership.
In this mutually beneficial arrangement, both partners exchange resources. The plant, through photosynthesis, produces sugars and shares them with the fungus, providing it with a consistent energy source. In return, the fungal mycelium extends far beyond the reach of the plant’s own roots, acting as an expanded root system. This network is more efficient at absorbing water and sourcing mineral nutrients like phosphorus and nitrogen from the soil, which it then delivers to the plant.
This exchange is highly regulated, almost like a negotiated trade. Research shows that a plant will provide more sugars to a fungal partner that supplies it with more nutrients. Conversely, a fungus will reduce nutrient delivery to a plant that provides less carbon. This dynamic allows plants to thrive in nutrient-poor conditions where they would otherwise struggle, while the fungus gains direct access to the energy it needs.
The Core Distinction and Relationship
The essential difference between the two terms comes down to structure versus function. Mycelium is the tangible, physical network of fungal hyphae—it is the organism itself. In contrast, mycorrhiza is the name given to the specialized, functional relationship that a mycelium establishes with the roots of a plant.
To clarify with an analogy, think of the mycelium as the physical infrastructure of the internet—the countless miles of fiber-optic cables laid underground. The mycorrhizal relationship is the actual data being exchanged through those cables, an active partnership between users. The cable (mycelium) can exist on its own, but the data exchange (mycorrhiza) requires both the cable and a connection to a user (the plant).
This highlights a key aspect of their relationship: not all mycelium forms mycorrhizal associations. Many fungi are saprophytic, meaning their mycelial networks are dedicated solely to decomposition without partnering with live plants. However, every mycorrhizal relationship, by definition, involves a mycelium. The plant cannot form this symbiotic bond without the fungal network to act as its partner.
There are different forms this interaction can take. Ectomycorrhizae, for example, form a sheath around the outside of a plant’s root tips and a network between the root cells. Endomycorrhizae, also known as arbuscular mycorrhizae, are distinct in that their hyphae penetrate into the plant’s root cells to facilitate nutrient exchange. Both of these relationships are powered by the mycelium.
Ecological Significance
The distinction between mycelium and mycorrhizae is significant because their interplay underpins the health of entire ecosystems. Mycorrhizal networks, formed by the mycelium of one or more fungi connecting the roots of many different plants, create a biological web often referred to as the “Wood Wide Web.” This network is not just a series of one-to-one trades between a single fungus and a single plant.
These interconnected mycelial pathways link numerous plants, even those of different species, allowing for the transfer of nutrients, water, and chemical signals. A healthy, well-established tree can share resources with a struggling seedling through this shared fungal partner. Plants can also use the network to send warning signals about insect attacks, allowing neighboring plants to mount their defenses.
In agriculture, fostering healthy mycorrhizal relationships can improve crop yields and resilience to drought, while reducing the need for chemical fertilizers. Because the fungal mycelium is so effective at sourcing phosphorus, it can unlock this nutrient from the soil. In reforestation projects, inoculating soil with mycorrhizal fungi can boost the survival and growth rates of newly planted trees. The stability and nutrient cycling in forests and fields are deeply connected to these fungal partnerships.