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Nematode Destroying Fungi

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Fig. 1. Nematode captured by the constricting rings of the predatory fungus Arthrobotrys anchonia.  Note that the ring cells "cushion" around the body of the victim but have not yet constricted the body.  This is a very early stage after capture.  See below for explanation. Scanning Electron Micrograph  N. Allin and G.L. Barron

The richness and variety of  life in soil and debris is remarkable and a few grams of soil may contain microscopic life forms numbered in the millions. These include fungi and bacteria as major components and also large numbers of  protozoans, tardigrades, rotifers, nematodes and other tiny things. Competition for nutrients and the close physical proximity amongst different groups of organisms in this environment has been conducive to the development of many predatory and parasitic associations. 

     The relationship between fungi and nematodes is not only one of the most fascinating but it has led us unexpectedly in a biological direction that is beginning to explain more clearly the complexities of  wood decay and the CARBON CYCLE.  You should understand right away that most introductory biology texts have a pretty poor version of the various biological components involved in the carbon cycle and the roles they play. 

    Fungi can capture nematodes in a variety of ways but the most sophisticated and perhaps the most dramatic is called the constricting ring.  An erect branch from a hypha curves round and fuses with itself to form a three-celled ring about 20-30 microns in diameter (30/1000th mm).  When a nematode "swims" into a ring it triggers a response in the fungus and the three cells expand rapidly inwards with such power that they constrict the body of the nematode victim and hold it securely with no chance to escape.   It takes only 1/10 th of a second for the ring cells to inflate to their maximum size. 

  Remarkably, the constricting ring cells go to three times their volume with a two times increase in surface area of wall and membrane in the blink of an eye.  At the same time the osmotic pressure (OP)of the cell is apparently maintained to the point where it can crush the nematode. 

  Transmission Electron Microscopy by others has shown that there is preformed reserve wall material and preformed reserve membrane material located at the  inside wall of each ring cell.  Others have also shown that you can "tickle" the inside walls of the ring with a needle and this mechanical stimulation can trigger the ring.  It has also been also shown that the rings could be artificially triggered by heat stimulation (e.g. hot scalpel in the vicinity of the ring).

      Our scanning electron microscopy studies, resulting in the picture shown above, showed that there is a rip in the outer wall of the ring cells, and that a new wall has expanded into the centre of the ring.  

    The above results suggested that there is a "hair trigger" line of weakness running along the inside wall of the constricting ring cells.   When this is touched by a nematode or any other mechanical force the cell is triggered with the following sequence of events.

1.  The line of weakness breaks.

2.  The osmotic pressure of the cell results in a rapid intake of water over the surface of the cell.

3.  The the cell expands in the direction of the break with a resulting 3X increase in cell volume.

4.  The reserve wall and membrane materials are located at the point of the break and reorganize to form new wall and membrane.

5.  The three times  increase in volume  of the ring cells must result in a three times decrease in osmotic pressure of the ring cells at the time of expansion.

6.  The ring cells  "cushion" around the body of the nematode and hold it fast but do not constrict it at that point in time. 

7.  There is a rapid conversion of  reserve materials in the rings cells to allow the OP of the cell to recover.  When the OP of the rings cells exceeds that of the nematode cells then the nematode wall will collapse and the body will be constricted.

8.  In its struggles to escape the nematode will often put its tail into a second ring and be held imobile.

9.  Once captured the rings cells will germinate  and invasive hyphae penetrate into the living body of the nematode, grow throughout the body and digest the contents.  This will be accomplished within 12-24 hours.  The food gained from this will be translocated elsewhere for further growth and production of more traps or for producing the reproductive bodies of the fungus which in this case is called Arthrobotrys anchonia.