Mycorrhizas (3)
Why Are They so Successful?

© Steve Trudell
Ecosystem Science Division, College of Forest Resources
Box 352100, University of Washington
Seattle, Washington 98195-2100

It is thought that at least 80-90% of Earth's plant species can form mycorrhizas and that, at any given time, most plants living in natural environments are mycorrhizal. Mycorrhizas apparently have been around for over 450 million years and many believe that the colonization of land by plants could not have occurred without them. So, why is this symbiosis so successful?

Understanding the answer to this question requires that we recall some biology basics, review the nature of the soil environment, and consider the important differences in how plants and fungi make a living. It then should be easy to see why the fungus-root partnership has persisted so long and is now ubiquitous in terrestrial environments.

Some Necessities of Life

Depending on whom you ask, necessities of life could include books, the newest hot video game, a fine pinot noir, or a dependable morel patch. However, if we consider only the common needs of organisms of all types, the list becomes much shorter, including such things as water, raw materials, and energy.

Water is essential for life and all organisms are largely water. For instance, it makes up 60% of a human's body weight, 75-80% of most plants', and as much as 85 to 90% of a mushroom's (a big reason for the correlation between recent rainfall and good collecting conditions). Water is the medium in which all of the chemical reactions within an organism take place. It is also essential in the external environment for many reasons, including being the medium for most environmental chemistry.

To create its body an organism requires raw materials; for instance, plants require large amounts of carbon, hydrogen, and oxygen (C, H, and O form the basis of all biological, or "organic," chemicals; (see "A Chemistry Refesher"), small amounts of nitrogen, phosphorus, sulfur, potassium, and calcium, and very tiny amounts of a large number of other elements. Animals and many other organisms also require these but, in addition, must obtain a wide variety of other preconstructed organic materials.

Just as our homes and factories need energy, usually in the form of electricity or fossil fuels, to operate, the metabolic processing of raw materials into finished products requires energy. The ultimate source of energy for life on Earth is the Sun. Plants, algae, and some kinds of bacteria are phototrophic organisms, or phototrophs. Through the process of photosynthesis they obtain energy in the form of light from the Sun and store some of it in chemical bonds.

Phototrophs build the organic molecules of their own structure from carbon dioxide and other simple, mostly inorganic, materials. Energy -- here derived from light -- is needed to drive this synthesis, because the usually large product molecules contain more energy than the smaller raw material molecules. So, as they grow, phototrophs are storing up energy in their structures. These energy-rich materials then serve as food for most chemotrophic organisms, or chemotrophs, organisms such as animals and fungi whose energy source is chemical rather than light.

The phototroph/chemotroph distinction is based on an organism's energy source. Organisms are also distinguished by their source of carbon, a critical raw material because it is the central constituent of all organic molecules. Autotrophic organisms (autotrophs) can make their own organic compounds from carbon dioxide, water, and other inorganic materials, whereas heterotrophic organisms (heterotrophs) must rely on the organic compounds already made by other organisms. Combining the energy source with the carbon source creates four categories, the first and last of which contain the organisms that are most familiar to us:

Organisms must obtain supplies of these necessities ("resources") from their surroundings, that is, their environment. Different types of organisms acquire these resources in different ways; for instance, consider the differences between plants and fungi.

A self-reliant, non-mycorrhizal plant, such as a wild mustard growing along a disturbed roadside, gets its energy from sunlight absorbed by its leaves and other green parts, carbon from carbon dioxide in the atmosphere, water and dissolved nutrients from the soil via its root system, and oxygen from the soil air and atmosphere. Thus, both the above- and below-ground environments must be exploited by a single structure.

A non-mycorrhizal fungus gets everything it needs -- energy and carbon contained in organic compounds, water, nutrients, and oxygen -- from its immediate environment, usually soil or decomposing organic matter in or on the soil.

Despite some major differences between the plants and fungi, note the critical role that soil plays as the source of many or all of life's necessities for both of them. So let's take a look at how the special nature of soil influences how these organisms obtain the necessary resources and then we'll be nearly ready to see why the mycorrhizal life style has been so successful.

A Visit to the Soil Environment

The soil is a fascinating place, yet few of us give it much thought as we tread over it. There's not time here to do justice to understanding the soil environment and all that happens there, but we do need to review a few of its more important features. Figure 1 shows a very tiny bit of soil greatly enlarged with the components drawn to scale.

cross-section of a non-mycorrhizal root
Figure 1: A tiny bit of soil shown at two enlargements. The background is enlarged approximately 30 times. At this scale, an earthworm would be about as wide as the entire drawing. The isolated portion is enlarged approximately 500 times. At this scale, that earthworm would be about 17 times wider than the pages of this magazine. Note the tremendous range of sizes, and how the sizes of pores and the "necks" that connect them constrain the movement of water, air, and organisms.
(Composited and redrawn from figures in Soil Science: Methods & Applications by D.L. Rowell.)

The first thing that should strike you is the huge range of sizes among the different types of mineral particles and organisms. In this and many other ways, the soil is extremely heterogeneous, that is, it differs greatly from place to place, often over what to us would be extremely short distances.

It also varies over time, especially with respect to its water content. Soil is a living entity, formed by the combined action of physical, chemical and, especially, biological processes. Let's look briefly at each of its main components -- mineral particles, water, air, organisms, and organic matter.

The Mineral Framework

Mineral particles of various size and composition comprise the physical framework of most soils. These particles are derived from physical and chemical breakdown of rocks. As the minerals that make up rocks are broken down, some of their elemental constituents, such as phosphorus, potassium, and calcium, are slowly released into the soil solution. This is the source of most of the nutrients needed by plants, fungi, and other organisms.

The spaces among the mineral particles are called pores and, depending on the sizes, relative numbers of different sizes, shapes, and arrangement of the mineral particles, a variety of pores can be formed. Those with a diameter greater than 50 micrometers are called macropores; those less than that are called micropores (1 micrometer equals one-millionth of a meter; for reference, spores of most mushroom fungi are between 5 and 15 micrometers long). The pores are critically important because that is where the soil water, air, and organisms reside.

Soil Water, Where the Action Is

The source of soil water usually is runoff from rainfall or snowmelt and, as it passes downward through the soil, gases, nutrient elements, and other substances become dissolved in it. As a result, it is usually called the soil solution rather than soil water.

The soil solution usually moves easily through the macropores and soils, such as sands, that have many large pores lose much of their water quickly. The water that remains in the soil is held in the micropores and as thin films that adhere tightly to the surfaces of particles. This water, and the nutrients it contains, are directly accessible only to very small organisms.

Nutrient elements in the soil can move from place to place in two ways, both requiring water. Either they move through the water by diffusion, similar to the movement of a perfume's odor through the still air of a quiet room, or along with the water by bulk transport, similar to the dissolved soap going down the drain with your dishwater. In the soil, lateral movement of water usually is limited and diffusion is a very slow process. Thus, the constituents of the soil solution don't have an easy time moving around.

Soil Air

Any soil pore, or part of a pore, that isn't occupied by water or an organism, is occupied by air. Near the soil surface, this air can be similar in composition to the typical air around us -- about 78% nitrogen, 21% oxygen, and 1% other gases, including less than 0.04% carbon dioxide. However, away from the surface, it tends to be much lower in oxygen and higher in carbon dioxide because of the uptake of oxygen and release of carbon dioxide by soil organisms. Unless the soil becomes saturated with water for extended periods, soil air usually creates few problems for plants and fungi.

Soil Organisms

Most soils contain a wide variety of organisms, from microscopic bacteria to tiny mites to much larger earthworms or even humongous moles. Excluding roots, microorganisms, especially fungi, dominate the biomass (the total weight of living things) of most soils. They are even more important in terms of their activity. Because the resources occur in heterogeneous ("patchy") fashion in soil, the organisms also occur in patchy patterns.

This can be seem most clearly in a comparison of the numbers of bacteria that live in the area adjacent to roots (the "rhizosphere" or "mycorrhizosphere") compared with those away from roots -- rhizosphere populations can be hundreds or thousands of times more abundant. "Oases of life in a biological desert" is one way in which this heterogeneity can be described.

Organic Matter

Organic matter is the remains of, and wastes from, organisms of all types. Thus, it comprises such things as leaves, twigs, slug corpses, rabbit pellets, deceased roots, rotting fungal hyphae, and all the chemical breakdown products derived from their decomposition. Organic matter occurs as recognizable large bits, unrecognizable tiny bits, part of the soil solution, and in the shapeless black moosh called humus. Organic matter forms an important food resource for decomposer and scavenger organisms and so its most easily available portions are rapidly consumed and thus represent a scarce commodity in most soils.

The portion of the organic matter that remains in soils over time is highly resistant to chemical breakdown and so can be accessed by only a few types of organisms, most of them fungi. This residual organic matter is important in soils as it contributes greatly to their water- and nutrient-holding capacity -- hence the use of organic composts in our gardens.

In a nutshell, soil is extremely heterogeneous. It is usually poor in easily available carbon and nutrients. Most of the action happens in the water. Readily obtainable resources become that way very slowly and get taken up very quickly by competing organisms. Most of the resources are stashed in tiny hard-to-reach places, often in hard-to-use forms.

Why Mycorrhizas Make Sense

So, now that we've waded through all that biology and soil science, let's compare how plants and fungi make a living in terrestrial environments and what difficulties they might encounter. As we saw earlier, plants simultaneously live in two very different environments -- the air and the soil. Most of them are marvelously well designed solar collectors. They also acquire carbon dioxide from the atmosphere very effectively through tiny conduits in their leaf surfaces without losing much water in the process. Thus, they can capture large amounts of solar energy, use it to manufacture food for themselves, and, in doing so, provide heterotrophs such as animals and fungi with a rich source of carbon compounds for energy and raw materials.

However, plants don't function nearly as well below ground. Although root systems serve well for anchoring plants, most of them are not terribly good at absorbing nutrients from soil when those nutrients are in scarce supply, which is the usual situation in natural environments.

Roots are relatively large, slow-growing, and have little or no ability to play an active part in releasing nutrients from rocks or organic matter into the soil solution. While they may be able to take up nutrients from soil solution in the macropores and largest micropores, they quickly absorb all that's there and nutrient depletion zones develop around the roots. Because nutrients are released only gradually and move through soil very slowly, these depletion zones normally take a long time to recover. The large portion of nutrients held in the micropores is not available to the plants directly because even their root hairs, which are not very abundant in natural soils, cannot fit into many of these tiny pores. Thus, plants work very well above ground, but not as well below.

Fungi, on the other hand, are beautifully adapted to life in the soil. Their microscopic hyphal structure allows them to grow rapidly, find their way into places that larger organisms can't reach, and traverse inhospitable areas such as air-filled pores where bacteria cannot venture.

They have well developed ability to absorb resources from their surroundings and, perhaps equally importantly, the ability to increase the availability of nutrients through the release of a variety of organic acids and enzymes (enzymes are large complex proteins that hasten chemical reactions such as those involved in decomposition of organic matter).

The much greater ability of fungi compared to that of plants to scour the soil in search of nutrients is striking. Ectomycorrhizal tree seedlings grown in see-through containers (Figure 2) show quite clearly that the fungal mycelium reaches far more of the soil than do the roots. When you consider the much higher surface area-to-volume ratio (see "Surface Area to Volume Ratio") of fungal hyphae compared to even very fine roots, it is easy to understand how a given mass of mycelium can contain as much as 50 or more times the absorbing surface of an equal mass of plant roots. But, fungi cannot make their own food.

cross-section of a non-mycorrhizal root
Figure 2: The bible for mycorrhiza enthusiasts. The cover photo illustrates the spread of fungal hyphae in soil. In David Read's laboratory at the University of Sheffield, UK, researchers have conducted a large number of experiments using plants grown in containers with see-through walls. Here, three pine seedlings have been grown (the central one is older than the others) together with a species of Suillus. Note that the "root" system consists mostly of fans of fungal rhizomorphs (formed from very many hyphae somewhat like forming a braid from lots of individual hairs). Only the largest central strands are plant roots. When you consider that there are also many other hyphae invisible to us, the degree to which the fungus can exploit the whole volume of soil is impressive. It is also interesting to note that all three seedlings are connected to a single mycelium. This has some very interesting ecological implications that we will explore in a later article.

So the secret of the mycorrhiza's success becomes clear. Plants, through their well designed stem and leaf structure can manufacture an abundance of food; however, they aren't able to exploit the soil environment anywhere near as efficiently. On the other hand, fungi are ideally designed to explore the soil, exploit rich resource patches, and extract scarce resources from areas outside the rich patches.

However, finding sufficient food to support these activities is a problem in most natural soils. Tapping into the rich carbon source provided by photosynthesizing plants, a mycorrhizal fungus's seek-and-acquire activities can be conducted much more vigorously than if it had to find its own high-energy carbon compounds. Although it has been said that allowing the fungus to access this food resource represents a "drain" or "cost" to the plant, it actually is a wise investment when you consider the huge increase in nutrient gathering capacity obtained by devoting the raw materials to construction of fungal hyphae rather than plant roots.

Thus, the basic arrangement in a mycorrhiza provides for a win-win situation between the two partners. Such an arrangement is termed a mutualistic symbiosis and it is an important element in the definition of mycorrhiza with which we began this series of articles. It is usually assumed that both partners receive significant benefit on a continual basis, although it is very difficult to determine if that is really so. For now, however, let's assume that it is, for it leaves us with a nice clean picture of things. But, remember, there seems to be more to many mycorrhizal associations than this sugar-for-fertilizer exchange. We'll touch on that topic in a later installment.

Some Useful Roots

Most scientific terms are derived from Latin or Greek. Knowing what the various roots mean can greatly simplify understanding of seemingly complicated terms, including the scientific names of organisms. Here are some roots used in terms from this article.

A Chemistry Refresher
  • Elements are substances that cannot be broken down to simpler substances by ordinary chemical means. Some examples are hydrogen, helium, carbon, oxygen, nitrogen, iron, and gold.
  • Atoms are the fundamental units of elements; a visible quantity of an element contains billions of atoms of that element. Atoms are composed of a nucleus surrounded by a cloud of electrically negative particles called electrons. The nucleus, in turn, is composed of two kinds of particle -- positively charged protons and neutral neutrons.
  • Molecules are groups of two or more atoms held in a specific arrangement by energy links called chemical bonds. They are the fundamental units of compounds such as water, sugars, and carbon dioxide.
  • Organic compounds or chemicals are those made up predominantly of carbon, hydrogen, and oxygen. They are created by biological processes and their basic structures are determined by the arrangement of carbon atoms in a so-called skeleton.
  • Inorganic compounds or chemicals are those that aren't organic.

Mini-Review of "The Bible"

For those of you wishing to know a whole lot more about mycorrhizas than I can cover in these articles, the best single source of information is the mycorrhizists' bible, Mycorrhizal Symbiosis, by Sally Smith and David Read (2nd edition, 1997, Academic Press, ISBN 0-12-652840-3, about $75-100; featured in Figure 2). It covers the entire subject in an authoritative, but very readable, fashion. Getting the most out of it, however, requires that you have a college-level understanding of plant and fungus biology and chemistry.

Surface Area to Volume Ratio

Consider the case of the lazy baker:

A very important concept in biology is the ratio between the surface area of an object and its volume. It is a fundamental determinant of how that structure interacts with its surroundings.

To understand the concept, consider the baker who resorts to a store-bought roll of dough for making chocolate chip cookies. All he needs to do is remove the wrapper, slice the prepared dough into rounds of the desired thickness, place them on a cookie sheet, and bake.

The roll of dough as it comes from the wrapper has a particular volume (occupies a certain amount of space) and a particular surface area (which could be ascertained by laying out the empty wrapper, measuring its length and width, and multiplying these two quantities together). Now, if the baker desired to make only one cookie, he could stand the 12-inch roll of dough on its end on a rather small cookie sheet (we'll ignore the fact that the dough would spread out as it baked).

If he instead preferred to have a dozen smaller cookies, he would slice the roll into 1-inch-thick segments and then would need a larger cookie sheet to hold them all. If he wanted a gross of cookies (a dozen dozen), he would make each slice 1/12th inch thick, and he would need a still larger cookie sheet, even though the amount of dough to be baked still is the same.

The take-home image is this: as a given volume of a substance is broken up into separate smaller bits, the amount of surface it exposes in relation to its volume, that is its surface area to volume ratio, increases. Note that this ratio has consequences -- in our cookie example, think about how long it would take to bake the different batches of cookies -- the single thick one would take a considerable length of time, whereas the very thin ones would burn quickly if not watched closely because the heat rapidly reaches the centers of the thinner cookies. To extend this analogy to roots and hyphae, just imagine making spaghetti, rather than cookies, from a roll of dough.

Copyright Notice

This material appeared in slightly modified form in Mushroom: the Journal of Wild Mushrooming, Issue 71, Spring 2001. It can be used freely for not-for-profit personal and educational purposes provided that its source is clearly credited.