This is the third of a series of articles by the author with the theme that
'life is a continuous interaction between organisms'. (Read part one and two.)

When sitting to write this essay, I suddenly remembered the famous book by John Steinbeck, Of mice and men. At first glance, a story of an unnatural and rare kind of relationship, between men and mice...  But how unusual is such a relationship? Was I not thinking and writing in the same sense  'of mites, nematodes and fungi?'

As a rule, it is not possible to conceive of life in our biosphere without interdependence. With this in mind I've replaced 'mice and men' with other living organisms to create my new tale.

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Of mites ...

 

Pygmephorus sellnicki, the so-called red pepper mite, lives on the growing and mature fruit bodies of the cultivated white button mushroom, Agaricus bisporus. Pygmephori are small organisms (ca. 0.1 to 0.2 mm long) but can easily be observed with a magnifying glass or a stereomicroscope. Pygmephorus sellnicki is generally harmless to the host mushroom but its presence indicates the co-existence of other, frequently pathogenic, organisms like Trichoderma spp. Once the phenomenon of parasitism starts, other pathogens and/or less harmful parasites, nematodes for example, may easily flourish in due course.

Red pepper mite (Pygmephorus sellnicki) infestation on the growing, young fruit bodies of Agaricus bisporus. Note the readily visible, yellow-brown to reddish tinted areas on the caps which correspond to the agglomeration of hundreds of parasites. They are very loosely attached, so they easily fall off and spread if the fruit bodies are picked up. Air currents may also cause their dissemination. The mites can provoke skin irritation, itching and transitory allergic reactions .

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Pygmephorus sellnicki. Two LM images of the same mite. The second image was obtained using slight polarization. Dorsal view.

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Pygmephorus sellnicki, ventral view.

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Pygmephori sellnicki collected on a wet glass slide. First image: low magnification and slight polarization. Second image: high magnification and complete polarization.

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Pygmephorus sellnicki. Cover slip applied. LM, polarized light; oil immersion objective, 100x.

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nematodes ...

 

During red pepper mite infestation of the mushroom, the mass of nematodes can be seen as a waving structures on the soil. The closely packed nematodes are shown below:

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LM image, slightly polarized light. The nematode is an elongated organism with one end (lower left) blunter than the other. Nematodes possess a limited number of cells and show a relatively simple anatomy. This is why the nematode Caenorhabditis elegans became a preferred model organism among several others for molecular biological investigations.

LM image, using almost completely polarized light. Note that the cuticle of the nematode is birefringent, observed as a bright surface structure.

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and fungi ...

The white button mushroom Agaricus bisporus

Hyphae of Agaricus bisporus (dark blue stained, tubular structures) feeding on and growing towards the central part of a wheat grain. Inoculated grains prepared under sterile conditions are called spawn and used to cultivate mushrooms.

A detailed view of the hyphae of Agaricus bisporus growing inside the grain. The 'foamy' background is the starchy endosperm.

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The microscopic fungus Arthrobotrys

On the same cultivation bed for the mushrooms; as well as a mite infestation and swarming nematodes, there is another organism namely a microscopic fungus called Arthrobotrys. The presence of this heavily spore-forming microscopic fungus is visible as light blue to gray patches on the soil surface. This fungus usually feeds on trapped nematodes.

A hypha and a newly formed spore of
Arthrobotrys oligospora

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Some nematodes feed on mycelium, hence the name myceliophagous nematodes. Other nematodes feed on bacteria. Soil is a rich and diverse source of uni- and multicellular organisms in various stages of their life cycle which continuously compete for survival.

The defense mechanisms of the cultivated mushroom's fruit bodies steadily become exhausted when they are attacked by mites, nematodes and pathogenic fungi like Trichoderma spp. and bacteria like Pseudomonas spp. Such severely parasitized mushrooms will eventually disintegrate.

Arthrobotrys oligospora trapping a nematode.

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Massive, interhyphal, bacterial growth and cell wall degradation of tissue-forming hyphae. This infected fruit body of Agaricus bisporus is disintegrating and will soon decay as a result of bacterial invasion. Plastic embedding; 2 µm thick section; crystal violet stain.

 

In this brief account, fruit bodies of the cultivated mushroom Agaricus bisporus played the role of the host organism for the red pepper mite. The delicate balance of the mushroom's growing environment was upset by an excess of competing organisms. The mites, nematodes, bacteria and fungi, albeit very different organisms in almost every aspects, depend on each other for their existence and with this symbiosis each organism can survive for relatively long periods.

Comments to the author M. Halit Umar are welcomed.

 

Footnotes and Web Sites

Symbiosis means 'a living together'. Coincidentally, this term was coined by a famous mycologist, Heinrich Anton de Bary in 1879.  In a very broad sense, Encyclopaedia Britannica defines symbiosis as any association between two species that live together, whether the species benefit, harm, or have no effect on one another.

Among nematodes, Caenorhabditis elegans possesses a very special place in developmental and molecular biology. An exhaustive list of nematodes arranged by common and scientific names also exists on the Web.

Nematophagous fungus is in French and explains the mechanisms of fungal nematophagy in short.

Arthrobotrys oligospora is in German and diagrammatically illustrates nematophagous fungi.

The Web Site of Mushroom Experimental Station in Horst, The Netherlands, describes Agaricus bisporus and gives a fair amount of information about this cultivated mushroom. You can find there also the abstracts of recently published papers in general and a section about fungal morphology.

White button mushroom, Agaricus bisporus is the link to the scientific, oral presentation by the author during the 15th International Congress on the Science and Cultivation of Edible Fungi, Maastricht /The Netherlands / 15-19 May 2000, containing outstanding macroscopic images, plus light and electron microscope images.  

 

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Published in the July 2000 edition of Micscape Magazine.

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