Introduction to Fungi
A forest full of fungi
Autumn is the best time for fungus lovers to walk through a native pinewood — you are surrounded with them. Not because there are more fungi, but because many of the fungi that are there all year round become more conspicuous, sending forth their familiar reproductive mushrooms and toadstools. Since they depend on moist conditions to feed and grow, autumn is an ideal time for reproduction. The familiar smell associated with autumn woodlands is all due to fungi working their way through the soil.
Perhaps because they are so hard to see, fungi have been largely overlooked in spite of their importance — without these strange and fascinating life forms, neither we, nor the inhabitants of our native forests, would survive for long.. About 69,000 species of fungi have been discovered worldwide, but it is thought that as many as 1.6 million actually exist. So in spite of the fact that fungi surround us, there could be many more to discover, even on your own doorstep.
Common questions about fungi
Q. Can I eat it?
A. Probably not. Of the 69,000 species of fungi about 250 species are considered good delicious edibles. Another 250 species can kill you– or at least make you wish you were dead. Everything else is something in between– from some that are “sort of ok tasting if there’s nothing else to eat and you’re starving in the woods” to some that are “just too bitter or taste too bad to eat,” or some that are too small or too tough to eat or that have something else wrong with them.
Q. I have lots of mushrooms on my property. What can I do with them?
A. Some people do not like mushrooms, and even want to get rid of them, but you will not hear that from anyone who knows about them. Most mushrooms are good for our soil, degrading waste products and returning them to the ground. Even more important are the mushrooms that are associated with trees as mycorrhizae. Without this mutualistic association most trees would not survive. Killing these fungi would kill the trees.
Q. How about the fungi growing in my bark mulch or wood chips? Are those important as well?
A. Most of these fungi are near impossible to get rid of without completely replacing the mulch and paving over the yard. Find a way to enjoy the fungi in your yard. Show them off to your friends or cultivate a fungus garden. Wouldn’t it be nice to be able to show off something unusual in your yard?
Q. Everywhere I go now I see mushrooms and other fungi. And I’ve been hearing a lot about fungi on the news. How can fungi be so prevalent in the environment?
A. Fungi are very successful organisms because of their genetic plasticity and physiological versatility. Many produce large numbers of spores that can be spread everywhere through the air. Fungi can degrade just about anything we humans can make, with the exception of some plastics and some pesticides. Fungi can penetrate even the toughest substrates with the exoenzymes produced by their hyphae. Exoenzymes are found in fungi and some bacteria. They are digestive enzymes that are secreted into the environment, where they digest the food into small molecules that can be absorbed and used by the fungus.
Q. I’ve heard about a large fungus growing underground in Michigan and a couple other large fungi in western states. Can you tell me more about this?
A. The fungi are all members of the genus Armillaria, which has become famous for its large clonal size. The original “humongus fungus” was a 37 acre underground mycelium of Armillaria gallica found in the Upper Peninsula of Michigan and was reported on the front page of the New York Times and most other papers in this country in April 1992. The furor was intensified when a few weeks later some other researchers claimed they had a 1500 acre mycelium of Armillaria ostoyae in Washington State. In summer of 2000 another set of researchers claimed they had a larger fungus (2500 acres) in the Malheur National Forest in eastern Oregon.
Q. While splitting some wood in the evening, a friend of mine found some bioluminescent fungi (of course, at the time he did not know what it was). We live on Long Island, NY, and would like to know more about this fungi, also known as ’foxfire’.
A. Most foxfire is caused by Armillaria species. Sometimes called ’fairy fire’, foxfire is the bioluminescence created by some species of Armillaria fungi present in decaying wood. The bluish-green glow is attributed to luciferase, an oxidative enzyme, which emits light as it reacts with the compound luciferin until it is fully oxidized. It is widely believed that the light attracts insects to spread spores, or acts as a warning to hungry animals, like the bright colors exhibited by some poisonous or unpalatable animal species. Although generally very dim, in some cases foxfire is bright enough to read by.
As recently as the 1960s, fungi were considered plants. In fact, at that time all organisms were classified into only two groups or kingdoms: plants and animals. In fact, however, fungi are more closely related to animals. But they have since been awarded a well-deserved kingdom of their own.
Unlike plants, fungi do not contain the green pigment chlorophyll and therefore are incapable of photosynthesis. That is, they cannot generate their own food — carbohydrates — by using energy from light. This makes them more like animals in terms of their food habits. Fungi need to absorb nutrition from organic substances: compounds that contain carbon, like carbohydrates, fats, or proteins.
In 1969 a new five-kingdom system of biological classification was proposed. The proposed kingdom of fungi included a vast array of species, among them mushrooms, yeast, molds, slime molds, water molds, puffballs and mildews. Since then, the system of classification and the fungal kingdom have been further refined, with slime molds and water molds shuttled off to a different kingdom. Today, the members of the kingdom Fungi are also known as the “true fungi.”
Characteristics of ‘true fungi’
Generalizations can be difficult. Nevertheless, there are a few key aspects common to all members of the fungal kingdom.
Cells: Fungi are eukaryotes, just like plants and animals. This means they have a well-organized cell, characteristic of all eukaryotes. Their DNA is encapsulated in a central structure called the nucleus (some cells can have multiple nuclei). They also have specialized cellular machinery called organelles that execute various dedicated functions such as energy production and protein transport.
Fungal cells are encased in two layers: an inner cell membrane and an outer cell wall. These two layers have more in common with animals than plants. Like animal cell membranes, those of fungi are made of proteins and fatty molecules called lipids.
Fungi can be made up of a single cell as in the case of yeasts, or multiple cells, as in the case of mushrooms. The cell walls of fungi are made out of a tough substance called chitin (pronounced ‘kytin’). While chitin is similar to cellulose, which helps form the cell walls of plants, it is a different substance and is actually the same material that makes up the hard external skeletons of insects.
Structure: With the exception of yeasts, the smallest units of fungi are tiny threads known as hyphae (singular ‘hypha’). Many of these can only be seen with a microscope. Most often, the individual cells in hyphae sit right next to each other in a continuous line but they can sometimes be separated into compartments by a cross wall (septate hyphae). Several hyphae mesh together to form the mycelium, which constitutes the fungal body.
A mycelium can be miniscule: spreading though the body of a dead fly; or it can rank among the largest, heaviest and oldest living things on the planet. Such is the one in Oregon’s Blue Mountains, between 2,400 and 8,650 years old!
It is said that “the fungi are the kings of surface area,” meaning that hyphae expand their surface area in order to take in food, facilitate digestion and also to reproduce. Astonishingly, while hyphae can be tiny, there can be 100 metres of them in a gram of soil, and in a hectare (2.5 acres) of British woodland, there may be well over three and a half tonnes of fungi! If you pick up a handful of leaf litter in the forest, you are likely to expose the slightly furry looking network of fungal mycelia. While the individual threads are microscopic, there are so many of them, often clustered together, they can become visible to the naked eye.
Feeding habits: Whereas plants get their energy directly from the sun and atmosphere using photosynthesis, fungi get theirs by digesting living or dead organic matter, as animals do. Fungi obviously have no mouths or stomachs and instead they work their way through or over their food, absorbing nutrients directly through their cell walls. Nutrients with simple molecules, such as sugars, can be absorbed fairly readily. Larger, more complex molecules, such as proteins, are harder to tackle, and the fungi must then make use of various enzymes so that they are easier to absorb.
They find their food, dump their enzymes out onto the food, and digestion takes place outside their body. These specialized digestive enzymes are known as exoenzymes, and are secreted from the tips of growing hyphae onto their surroundings. These enzymes are the primary reason why fungi are able to thrive in diverse environments from woody surfaces to insides of our body.
As a result of exoenzyme activity, large food molecules are broken down into smaller ones, which are brought into the hyphae. Cellular respiration then takes place inside fungal cells. That is to say, organic molecules such as carbohydrates and fatty acids are broken down to generate energy in the form of ATP.
Fungi have multiple sources of food. Fungi that feed on dead organisms — and help in decomposition — are called saprophytes. If a fungus derives sustenance from a live host without harming it, then it is called a symbiont or a mutualist. Lichens — fungi and algae together — are an example of a mutualistic relationship. If a fungus feeds on a live host while harming it, then it is a parasite.
Reproduction: Reproduction is a complex business for fungi. The various fungi are capable of reproducing asexually or sexually. Both processes can generate spores which, when released into a suitable environment, can give rise to a new fungal body.
Asexual reproduction occurs through mitosis, when a fungal cell divides and produces identical genetic copies of itself. In simpler, single-celled fungi like yeast, this process is known as budding. In this case, a small offshoot or bud emerges from the parent cell, slowly growing in size. The nucleus divides into two and the bud splits off once it is the same size as the parent cell. Multicellular fungi such as molds also reproduce through the formation of asexual spores. Many fungi can reproduce sexually and asexually. Under certain conditions they can send up a fruiting body without interacting with another fungus. This is asexual reproduction and has the advantage that it can happen quickly, to make the best of a small window of good conditions. The fruiting body will release millions of its microscopic spores, a tiny proportion of which will germinate.
Sexual reproduction enables the next generation to benefit from the genetic material of both parents, allowing it to develop new adaptations. The duration and timing of certain steps of sexual reproduction vary quite a bit between fungal species. In general, sexual reproduction in fungi produces spores through meio-sis. Hyphae from two different fungi of the same species intertwine and then send up a fruiting body or mushroom, again releasing spores, but containing genes from both parents. As a result, these spores contain half the number of parental chromosomes. Once released, the spores germinate into tree-like mycelia and are ready to “mate.” In the case of mushrooms, puffballs and toadstools, the branched mycelium (also called primary mycelium) is divided into segments containing a single nucleus. Mating takes place when two primary mycelia come into contact with one another and form a secondary mycelium. Each segment of the secondary mycelium has two nuclei: one from each original segment. The individual nuclei still have half the number of chromosomes as the parent cell. In the course of several steps nuclei fuse, giving rise to cells with the original number of chromosomes.
The fruiting body is actually made up of a collection of the same hyphae that form the mycelium, just more densely-packed. Mushrooms have a seemingly miraculous ability to appear over night as they use hydraulics to ‘inflate’ the hyphae with fluid so that they grow rapidly, and can push their way up through some surprisingly hard surfaces. Inkcaps, for example can even push their way up through tarmac! Mushrooms and toadstools, as well as bracket fungi, can protect their spores from the elements with their waterproof caps.
Whether sexual or asexual, the result of fungal reproduction is that innumerable spores are released into the air. Some species can release tens of millions of spores in a single hour. Spores from the mushroom are then carried on the breeze, often many miles from their source.
Other kinds of dispersal are also found: puffballs eject a puff of spores when a drop of rain hits the surface of their fruiting bodies, while the stinkhorn smells of rotting flesh, attracting flies which unwittingly disperse the spores on their feet.
Especially intriguing are the mycorrhizal fungi. Mycorrhizal relationships are fascinating partnerships that take place when the hyphae of certain fungi wrap around, or penetrate the roots of a plant, whereupon a mutually beneficial exchange takes place. The fungus, which cannot obtain energy directly from the sun itself (as it lacks the chlorophyll found in plants), is able to obtain sugars that the plant produces using photosynthesis. In return the fungus provides the plant with vital nutrients that it extracts and transports from the soil, and that would otherwise be unavailable to the plant. Surprisingly, most of plants in virtually all of the world’s terrestrial ecosystems rely on these relationships for their healthy growth. It gives some perspective on the importance of fungi when we consider that without them the world’s forest ecosystems would collapse. Among the mycorrhizal fungi in native pinewoods are species such as the chanterelle and various Boletus species, as well as the familiar red-with-white-spots fly agaric, that grows in mycorrhizal association with birch trees.
Ants are known to live symbiotically with fungi, particularly in the tropics, with leafcutter ants farming fungi for their own consumption. Recent observations in Glen Affric suggest that wood ants possibly have similar interactions with fungi, and further investigations are required to reveal the nature of this relationship.
Fungi that feed on living things are parasites. Some parasitic fungi simply weaken their hosts, while others kill them. Examples include the aspen bracket fungus, and honey fungus, with its thick, black, bootlace-like rhizomorphs, which are effectively giant hyphae. Many of these kinds of fungi dwell within their hosts for some time before attacking and killing them.
This in itself creates superb deadwood habitat for a host of other species, from beetles and flies (the larvae of many species feed on dead wood) to crested tits and ospreys, which nest in dead trees. When a tree is killed, it also provides an opening for young trees and other plants to grow, thus enriching the structural diversity of the forest.
Without fungi, the forest would pile up with layer upon layer of needles, leaves and other dead matter. The fungi that feed on dead organic matter are called saprophytes. The key role of these forest recyclers is to break down dead matter and return the nutrients to the soil to become available to plants once again. Leaf litter, dead animals, dead wood – in fact, anything that dies in the forest will be colonized by fungi (along with other decomposers) and eventually reduced to soil.
The role of fungi in breaking down dead wood is especially crucial. Lignin is the substance that makes wood stiff, and it is so tough that animals cannot digest it. However, certain fungi are able to biodegrade this substance using particular enzymes, thus allowing the vast amounts of dead wood in a natural forest to be broken down.
A forest feast
The fruiting bodies of fungi provide an abundance of food for the wildlife of the forest. Squirrels store fungi in the tops of trees to eat through the winter. Voles and other rodents also gnaw on this welcome feast: their teeth marks, and the marks of the rasping mouthparts of slugs, can often be seen on the caps of mushrooms.
Fungi can be filled with life. As any fungal forager will tell you, if you pick a fungus that’s past its best, the chances are it will soon be riddled with maggots. These are the larvae of various fungal gnats and other insects. Bracket fungi such as the hoof fungus are a haven for insects and can support a mini-ecosystem in their own right. A study of the hoof fungus in Swedish forests revealed 27 insect species that live within the brackets, including various fungivorous beetles and moths.