The Role of Fungi

Fungi are important components of our ecosystems, and are vital in maintaining the health and stability of this park by occupying three primary niches: as saprotrophs, as parasites, and as symbiotes.

 

Saprotrophs

The George Washington Memorial Parkway includes a diverse array of habitats, from rocky cliffs, to forests, to swamps, to grasslands. Every year, millions of trees shed their leaves, animals die, and organic sediments are deposited by storms. Nearly all of this material is eventually recycled back into new growth due to the activity of fungi in making these nutrients available.

Saprotrophs are organisms that participate in the decomposition and recycling of dead organic material at the cellular level.

A good example of saprotrophs are the fungi that are capable of breaking down lignin, the component in wood that presents the greatest obstacle to its decomposition. Lignin's strength as a molecule comes from its shape, which is composed of aromatic hydrocarbons. (Imagine six atoms of carbon attached to each other in the shape of a hexagon). Aromatic hydrocarbons are very stable, making them very difficult to break apart. In order to break apart these hexagons, lignin-decaying fungi secrete a molecule called laccase, which has two "sticky" hydroxyl ions that bind to two adjacent corners of the hexagon. As a result, the two corners of the hexagon want to stick to the hydroxyl groups more than they do to each other. Once one side of the hexagon is removed, the lignin is weak enough to allow the whole molecule to be broken down and recycled. Without this process and many others like it, dead wood would just continue to accumulate until the entire landscape was buried under a thick layer, suffocating plant life, and providing fuel for fires of cataclysmic proportions.

 

Parasites

Parasites are organisms that get their nutrients by continuously absorbing them from a living host organism, negatively affecting it in a measurable way.

Fungi are common parasites of plants, but they can also parasitize insects, other fungi, and even higher animals such as humans (for example, athlete's foot). Although they can cause great harm to the organism they infect, parasites are important in maintaining a balanced ecosystem by helping to control the population size of their hosts. However, exotic parasites can destroy entire populations of their hosts, since their hosts have not had enough time to evolve any resistance to the parasite. Every year the staff of the George Washington Memorial Parkway put tremendous effort into controlling exotic fungal infections in elms, chestnuts, and ashes.

Fungal Parasites can be subdivided into two categories: biotrophic, and necrotrophic parasites.

Biotrophic parasites grow into the tissues of their plant hosts, absorbing nutrients from the host's cells without killing their host. Several species of powdery mildews fall into this category.

Necrotrophic parasites, on the other hand, slowly kill their hosts; absorbing nutrients from the host's cells, pre and post mortem. American Chestnuts and American Elms have experienced severe losses from the invasive necrotrophic parasites Cryphonectria parasitica and Osphiostoma ulmni respectively.

 

Symbiotes

Mycorrhizas are a special type of symbiotic nutritional relationship shared between plants and fungi that confers advantages to both organisms.

All living things are composed of six primary elements: sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen. In most natural systems plants have access to an adequate supply of carbon and phosphorus. Nitrogen, on the other hand, is usually in short supply, thus becoming a limiting factor for plant growth. However, certain species of fungi are very good at obtaining nitrogen from the soil through their hyphal network. At some point early in their evolution, plants entered into a symbiotic relationship with these fungi. The fungus supplies the plant with nitrogen in exchange for some of the sugars that the plant produces through photosynthesis. Plants that possess this type of relationship tend to grow faster and are more tolerant of variable environmental conditions than plants which do not. This is why the vast majority of plants in natural communities possess mycorrhizas.

In forests such as those of the George Washington Memorial Parkway mycorrhizal fungi can grow and associate with several different trees at once, acting as a conduit for nutrients between trees. Seedlings that are able to tap into this fungal network can obtain nutrients from nearby mature trees, allowing them to grow in the shade of the forest floor until they are tall enough to reach the light. There are actually two different types of mycorrhizas that occur within our parks: ectomycorrhizas and arbuscular mycorrhizas.

Ectomycorrhizas are typical of large forest trees, such as tulip trees, pin oaks, sycamores, beeches, and others. These are the basidiomycete fungi that produce many of the familiar mushrooms such as amanitas, russales, chanterelles, and others. Ectomycorhizal fungi grow by forming a sheath around the growing tips of the roots of their plant hosts. Individual fungal hyphae grow inward from the sheath into the intercellular space between root cells, but do not actually penetrate the cell walls of the plant. (Hence the "ecto" in ectomycorrhiza.)

Arbuscular Mycorrhizas grow in association with herbaceous plants, grasses, some trees, and especially cacti. Unlike the ectomycorhizal fungi, arbuscular mycorhizal fungi do not form a sheath around the root tip. Instead, they grow directly into the host's tissue, penetrating the cell wall (but not the inner cell membrane) of the root cells and form a branching structure called an arbuscule, through which nutrients are exchanged.

Last updated: April 27, 2015

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