Spore Dispersal in Fungi (continued)

Dispersal Mechanisms

The production of large number of spores is only partly responsible for the abundance of fungi around us, in order for them to become widespread, there must also be mechanisms by which they can disperse their spores, over long distances. There is also a practical reason why it is important for fungi to disperse their spores. If spores germinate and grow among the parent mycelium, food will become limiting since the new mycelium must now share the food and the probability of survival will not be as great. Some of the different mechanisms by which fungi disperse their spores are quite ingenious.

Air Borne Spores

The most common means, by far, that most fungi have of dispersing their spores is by riding air currents. The wind dispersed fungi often produce what are referred to as dry spores. These spores do not readily soak up water and when clusters of these spores are splattered by water, as may often occur in those fungi that produce their spores directly on their mycelium, rather than absorbing the water, the impact dislodges the spores and scatters them into the wind. Because these spores do not readily absorb water, they are said to be hydrophobic. Although this may not be very intuitive, the initial resistance of these spores to water makes a great deal of sense. The absorption of water by spores would give them extra weight, making it more difficult for them to stay afloat. The majority of the known species of fungi disperse their spores by wind.

The importance of airborne reproductive propagules, to which I am including all spore producers, e.g., algae, fungi and plants, as well as pollen and other air borne organisms, is such that there is a discipline, aerobiology, that is dedicated to their study. One aspect that has been much studied is the cause of allergies by organisms in the air. Fungal spores are the cause of a significant number of allergies each year. Unlike pollen, however, it was not until 1924 that fungal spores were thought to cause respiratory allergies. Specifically, Puccinia graminis, the wheat rust was identified to be the cause of allergies of three Canadian who were threshing wheat, but further studies indicated that the allergies were caused by mold spores of Cladosporium, Alternaria and Penicillium spores (Feinberg, 1946). Wherever spores have been monitored, an abundance of these genera may be observed. In 1937, in Minnesota and the Dakotas, it was estimated that thousands of tons of spores of Cladosporium and Penicillium were present in the air that blew eastward into the ocean and may possibly have blown across the Atlantic (Feinberg, 1946). Considering the size of spores and the fact that there was an estimated thousands of tons of spores, the number of spores present must have been astronomical.  It is not any wonder then that the above two genera of fungi are significant factors in the cause of allergies.

Although simple, the efficiency dispersal of air borne spores should not be underestimated since most fungi utilize this method to disseminate their spores. An air sample may contain as many as 200,000 spores/meter3, but just how effective is dispersal of airborne fungal spores? Since air-borne spores cannot be seen, it is difficult to appreciate the number of spores that are in the air. However, in order to give you an idea as to the number of spores that are in the air, let us make an indirect comparison with small air-borne objects that are visible to the naked eye. The ability of any small objects to stay afloat can be readily observed.  When we look at the morning or afternoon sun shining through a window, in a room, where the air is still, numerous small particulate pieces of "lint" or dusts, in the light beam can be observed to be kept afloat by the convection of heat generated by the light beam. So it should not be difficult to imagine that spores, which are far smaller and lighter, would and probably are also present in such a light beam. The extent that spores can travel indoors where the air is still was nicely demonstrated with an experiment carried out by Dr. Clyde Christensen (1975), at the University of Minnesota St. Paul Campus, in the plant pathology building.

The Preliminary Experiment

The experiment used Cladosporium resinae as a "marker fungus" whose spores are not usually found in the air. In nature this fungus is found only in resin permeated soil, and in wood that has been impregnated with coal tar creosote in order to protect them from decay, such as telephone poles and railroad ties.  Because of its requirement for creosote, a "selective medium" containing this compound is not only required for growth of C. resinae, but at the same time, will prevent the growth of other common air-borne or soil fungi.

Christensen demonstrated the selectiveness of this medium by inoculating decaying plant material with known fungi, and soil samples, on campus, infested with common and uncommon fungi into creosote agar plate medium (Figure 2). Neither the fungi known to be infecting the plant material and fungi present in the soil samples were able to grow on this selective medium, nor was C. resinae recovered, indicating that this species was absent in these substrates.

Fig. 2: Experiment demonstrating selectivity of Creosote Agar medium and absence of Cladosporium resinae in soil and on plants.

Tests were also carried out by exposure of blocks of creosote impregnated wood and agar plates to the air around and inside the plant pathology building. Again, C. resinae was not recovered from the substrate and agar, indicating that spores of this species were not air-borne in this area (Figure 3).

Fig. 3: Cladosporium resinae was not recovered from creosote agar plates and impregnated wood, from air and experiment sight, indicating its absence in experiment sight and area around sight.

The Cladosporium resinae Experiment

The four storied, plant pathology building, in which the experiment was carried out, has stairways at each end, with a hallway in the middle of each floor and does not have a central ventilating system. In testing the extent to which the C. resinae spores could remain afloat in the still air of the plant pathology building, agar plates with coal tar creosote were exposed throughout the building. A culture of C. resinae was then placed on the first floor hallway and the spores of the fungus were then brushed off the agar surface into the air of the building. Remember that Christensen had earlier exposed plates of the creosote agar prior to dispersing the spores and had not recover C. resinae. Therefore, any plates that were now discovered to have this fungus growing on it would be due to the brush dispersal of C. resinae, by Christensen, to that part of the building.  Two of the several tests that were carried out are summarized in Tables 1 and 2. Table 1 summarizes the number of colonies recovered on creosote plates exposed at successive five minute intervals. In Table 2, seven sets of plates were exposed at each location for intervals of 0-5, 5-10, 10-20, 20-30, 30-60, 60-120 and 120-240 minutes. All plates were incubated and later examined for the number of colonies of the fungus formed on each plate. Colonies were recorded because it is assumed here that each colony was produced from a single spore. Plates with colonies of C. resinae were isolated throughout the buildings where the creosote plates were placed. As might be expected, there were generally more colonies on plates closest to the source, i.e. on the first floor, where the spores were dispersed and fewer occurred on those plates that were placed on the upper floors, more colonies were recovered in the hallway than in the rooms on the same floor and more colonies were recovered in those rooms with open doors than those with close doors.
 

Table 1. Number of colonies of Cladosporium resinae recovered on creosote plates in successive five-minute periods on the second, third and fourth floors after spores were liberated in the first-floor hallway.
 

Number of colonies/plate

Floor

First 5 Minutes Second 5 Minutes Third 5 Minutes Fourth 5 Minutes
2 85 70 27 20
3 20 33 9 16
4 4 17 10 7

 
 

Table 2. Number of colonies of Cladosporium resinae recovered on creosote agar plates exposed for different periods of time after liberating spores in room on first floor

Number of colonies/dish exposed for given period of times (in minutes)

Location of exposed plates 0-5 5-10 10-20 20-30 30-60 60-120 120-240 Total
First floor hall 228 49 43 31 26 15 5 397
Second floor room, door closed 0 0 1 1 3 10 2 17
Third floor greenhouse connected to the main building by a 30 foot passage 0 0 0 0 1 0 1 2
Third floor laboratory, door open 8 0 5 19 55 28 8 123
Third floor hall 0 8 70 35 44 12 0 169
Fourth floor hall 68 78 50 34 51 7 3 291
Fourth floor room, door open 0 0 2 3 7 14 5 31

Total

304 135 171 123 187 86 24 1,030

Christensen's demonstration not only showed us the remarkable ability of fungal spores to disperse themselves in the still air of a building, but also serves to remind us that if you hear someone sneezing in a building, on the floor below you, they are literally sneezing their germs right in your face.

Another experiment that you can carry out to demonstrate fungal spores ability to stay afloat can be done with a mature mushroom and an elongated cardboard box approximately 10" high and a yard long (Fig 4a). The basidiospores from the mushrooms are initially "shot off" from the basidia to the area between the gills (recall from the last lecture that these structures are characteristic of the Basidiomycota) (Fig 5a-b). Under these conditions it would be expected that all of the spores would drop directly below the cap of the mushroom. Although most spores will fall directly beneath the cap of the mushroom, some will manage to stay afloat and travel the length of the box, a yard away (Fig. 4b).
 

Experiment1.jpg (4206 bytes) Experiment1b_Top.GIF (7339 bytes)
Fig. 4a: Mushroom spore dispersal in a covered cardboard box without air circulation. The spores will land on the cardboard bottom where we can record the number of spores. Fig. 4b: Top view of spores on the bottom of the box. The greatest number of spores landed directly below the mushroom, as you might expect, but some spores stayed afloat until reaching the other end of the box.

 

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Fig. 5a: A section through a mushroom gill showing the path of two spores that have been ejected between the gills and away from the mushroom. Fig. 5b: A magnified animation showing the ejection of a single spore from the basidium to an area between the gills. 

The movement of spores indoors is of significance to the aerobiologist. Most of us may not suffer from any respiratory symptoms while outdoors, but once inside we may suffer from difficulty in breathing, have sniffles and perhaps even feel ill. This may occur at work in air-conditioned buildings or at home. Symptoms are more likely to occur at the latter when you are sweeping and vacuuming causing the dust in your house to shift around. It is commonly believed that it is the dust that is the sole cause of your misery, but instead it is more probable that the cause is due to fungal spores (more about this topic later).

There was one extreme case of fungi over running a house that occurred in Manoa. The owner of the house called me to ask my advise as to what he should do because of an apparent fungal allergy that had become progressively worst over time. The owner's brick planter was cracked and he was in the process of repairing it. However, each time he approached the planter his eyes became watery and he had difficulty breathing. Finally, he hired someone to demolish the old planter and rebuild it. Upon demolishing the old planter, he discovered that there was a large fungal infestation that was probably causing his illness. Having discovered this, he had the parts of the planter taken away and believed his troubles to be over. Unfortunately, they had just began. Shortly after the planter was demolished, his symptoms grew worst and soon he was unable to enter his house without having these symptoms. That was when he called me to inquire as to what happened. I explained to him that in demolishing the planter, the fungal spores became scattered throughout his house and that with the high humidity in Manoa, the fungus growth was probably now growing throughout his house instead of being restricted to his planter.

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