Fungal Diversity (Continued)

The Fungus Body: Mycelium and Yeasts

At one time or another, we have all come across food that has become contaminated in some forgotten corner of our refrigerator and observed the filamentous growth of fungi. The filamentous growth is called mycelium (pl.=mycelia) and represents the "body" of the fungus.   A fragment of mycelium is referred to as a hypha (pl.=hyphae). A less familiar fungus body, is the yeast. The yeast fungus is unicellular. Whether the fungus body is composed of mycelium or yeast, both will function in feeding and reproduction. Let us first discuss the more familiar filamentous appearance of the mycelium.

Mycelium

If we place a single fungal spore in the center of a petri plate, hyphae will germinate from the spore and grow radially, to form a circular growth of mycelium (Figure 1a). The radial growth of the mycelium can be small as shown in the petri plate below or may be extensive. Recall from the video "The Moldy World About Us" where a circle of mushrooms, called a "fairy ring", was formed . 

Figure 1a. Mycelium with approximately circular colony (left) and Figure 1b. Mycelium as seen through the microscope (right)

 In this case the mycelium that produced the mushrooms is hidden from our eyes in the soil beneath the grass (Figures 2a and b). In either case, if this growth is observed under a microscope, you will be able to see that it is composed of branched, filaments (Figure 1b). Each branch will form more branches at a given interval which will in turn branch further. Growth will occur only at the ends of the cells, not from the extension of all of the mycelial cells.

Fig. 2a. Chlorophyllum molybdites  mushrooms (left) and Fig. 2b. Fairy ring of same species (right): Note that the grass is greener inside the ring than out.

When mycelium form a continuous tubular growth it is said to be coenocytic (Figure 3a) or it can be divided into cells that are attached end to end and is said to be septate (Figure3b). The cell walls that divide the mycelium into cells are referred to as septa (sing.=septum):

Fig. 3a. Coenocytic hypha fragment (left) and Fig. 3b. Septate hypha fragment (right).

Yeast

A less familiar fungus body is the yeast. Under the microscope, you can observe that yeast is composed of single cells that continually divides, by budding or fission to form lots and lots of cells. To the naked eye, this growth will appear to be a thick, syrupy growth (Figure 4a). Fission is merely division of a yeast cell into two cells and will not be described here. The process of budding is illustrated in Figure 4b. A yeast cell that is about to bud has a predetermined area of the cell that becomes blown out forming a new cell, the so-called bud. The nucleus will divide by mitosis, with one nucleus migrating into the new cell. When the new cell is approximately the size of the original cell, the cells will seal off the opening and separate, giving rise to two yeast cells. Some species have both a yeast and mycelial stage (Figure 4c). Such species are said to be dimorphic.

Fig. 4a. Yeast colony of Rhodotorula (right), Fig. 4b. Yeast cell undergoing budding (middle) and Fig. 4c. Dimorphic species having both mycelium and yeast.

Spores

Fungi reproduce by spores. Spores are usually composed of one to a few cells. They may be sexual or asexual and are variable in shape, size and color (Figure 5):

Fig. 5. Spores of various shapes and sizes

Spores are often borne directly on modified mycelial structures. In the colony of Penicllium notatum pictured below (Figure 6a), the green, central portion of the mycelium is where the spores are borne on the modified mycelial structures called conidiophores (Figure 6b). The specific types of spores produced are asexual spores called conidia (sing.=conidium) (Figure 6b):

Fig. 6a. Colony of P. notatum (right), and Fig. 6b. Conidia and conidiophores from green portion of colony, as observed under the microscope.

An example of a mushroom that forms fairy rings that occurs in Hawai‘i is Chlorophyllum molybdities (Fig. 6a and 6b). The grass was greener inside the fairy ring because the mushroom mycelium having grown through the area, inside the circle, had decomposed the organic material, within. Provided with more minerals, the grass growth was greener and lusher, inside the circle: 

Instead of producing spores directly on the mycelium, the mushroom mycelium becomes very tightly interwoven, to produce the large mushrooms that you saw in Figs.3a and b, where the spores are borne. 

Cell Wall

The mycelium or yeast cell is surrounded by a cell wall that is typically composed of chitin, the same material that makes up an insect's exoskeleton. However, one group of fungi that we will be studying has cell wall composed of cellulose, which is  is the same material that is found in plant cells. The presence of a cell wall, regardless of its composition, was once used as evidence for fungi being closely related to plants, but presently the two groups are not thought to be closely related. During the early 1970's Solomon-Barnicki Garcia proposed that only those organisms that produce chitin in their cell wall should be defined as fungi. The characteristic of a chitinous cell wall is still one of the accepted criterion for defining a fungus. However, those "fungi" with cellulose cell walls are no longer believed to be closely related to the fungi and have even been classified in a kingdom of their own.

Eukaryotes

Fungi are eukaryotes. This term has been used to refer to organisms that have nuclei in their cells. For our purpose this definition will suffice. This characteristic separates them from bacteria, which are prokaryotes, i.e. they lack nuclei in their cells. 

Mode of Nutrition: Absorption

The mode of nutrition or the matter in which fungi "eat" is called absorption. Among eukaryotes, absorption is unique to the fungi. Fungi obtain their food by transporting it through their cell walls, but first, how does a fungus "find" its food since like plants, they are not mobile organisms and cannot seek out their food. The answer is fungi do not have to find their food. In order to eat, the spores that give rise to fungi must be dispersed to a location where there is food and after the spore germinates, the mycelium of the fungus must grow into its food. Another words, usually fungi must live in their food if they are to eat. If the food is composed of simple molecules such as glucose or sucrose, soluble food can be immediately transported through their cell walls. However, most food that a fungus might consume is composed of complex, organic compounds, e.g., cellulose, lignin, pectin, starch, etc., which is insoluble. In order for this food to be utilized by the fungus, it must be broken down into simpler molecules that can be transported through their cell walls. Another way in which you can think of this is that the cell wall is like a sieve that will allow only particles of a certain size to enter. The fungus breaks down the complex material by secreting digestive enzymes through their cell wall that will digest the complex organic compounds and convert them into simple molecules that can readily be transported through their cell walls. For example, If a fungus is growing in wood, digestive enzymes would be secreted from the fungus, into the wood, and break down the complex compounds of wood, e.g. cellulose and lignin into simpler materials, such as simple sugars, which then can be transported into the mycelium. 

Although this process may seem very different than our own means of obtaining food. It is not that different. The essential difference between fungi and animal digestive systems is that fungi digest their food first and then "eat" it, while animals eat their food before digesting it. The basic process of digestion is otherwise more or less the same. Our digestive system requires that our food is chewed by teeth, go through the esophagus, stomach, intestine and many associate organs. So there are a lot of things that can go wrong when we eat our food. The fungal digestive system is much more simplified and one which has been very successful for them.

It is important to understand here that different kinds of fungi will secrete only a specific number of different enzymes. This means that they can only "eat" certain materials. For example, a fungus that is usually found in your stored food, probably will not be able to "eat" wood because it does not have the enzymes that is needed to break down or "rot" the wood. Some fungi have a very broad range of enzymes. Species of Penicillium, for example, can be found on a number of different "food"; leather, cloth, paper, wood, manure, animal carcasses, ink, syrup, paint, glue, hair, literally thousands of products. A summary of absorption is illustrated in Figure 7, below:

hypha2.JPG (35566 bytes)

Fig. 7. Illustration of the process of absorption, the mechanism by which fungi consume their food.

Although yeasts are quite different in their appearance than mycelial fungi, their means of obtaining food is identical.

 

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