The Fifth Kingdom - Chapter 3a     

Eumycotan Fungi  -  The Mainstream

Phyla Zygomycota, Glomeromycota and the Dikarya

Kingdom Eumycota is made up of five phyla, Chytridiomycota, Zygomycota, Glomeromycota, Ascomycota and Basidiomycota. These, and particularly the last two, far outnumber  the chromistan fungi in species diversity. We already know about 100,000 eumycotan or true fungi, and it is obvious to those of us who work with them that these are just the tip of the iceberg. We estimate that there are well over a million species waiting to be found and described. Hundreds of new fungal taxa are described every year. For example, in 1990, Rafael Castañeda and I described 14 new genera and 40 new species of microscopic moulds from dead leaves of Cuban rainforest plants. This wealth of species is a measure of fungal success in evolutionary terms, just as the existence of millions of species of insects tells us that they, too, are winners (though their total biomass is far less than that of the fungi). Before we look at the eumycotan fungi in detail, it is worth enquiring into the reasons for their success.

Earlier, I introduced the idea that the number, kind and arrangement of motility organelles (flagella) found in the chromistan and some eumycotan fungi are very basic, highly conserved features. As a corollary of this, the absence of motile cells from the life cycle of most eumycotan fungi must also be considered important. This seems to reflect a radical shift in evolutionary direction. It shows very clearly that most true fungi are basically terrestrial (landlubbers), and must have been so for a long time. Many more ecological niches and substrates are available on land than in the water, and the challenges of survival and dispersal are very different.

Fungi are heterotrophic, which means that they depend on energy-rich carbon compounds manufactured by other organisms. But this doesn't seem to have been a serious disadvantage. Fungi have evolved enzymes that can digest some extremely tough substrates. Chitin (insect exoskeletons), keratin (skin, hair, horn, feathers), cellulose (most plant debris) and lignin (wood), nourish many fungi, though cellulose and lignin remain completely unavailable to almost all animals (except with the collaboration of microbial symbionts). Their unusual ability to exploit cellulose and lignin gives some saprobic fungi almost exclusive access to the massive quantities of plant debris produced every year, and may well make them the world's number one recyclers. Only man-made plastics are, perhaps unfortunately, immune to their attacks, which means that we, not the fungi, must take responsibility for recycling these substances.

There are two major physical reasons for the incredible success the fungi enjoy.  The first is the fungal spore, the second the fungal hypha. 

Spores permit rapid dispersal and a kind of scattershot saturation of the biosphere - fungal spores are everywhere. 

Hyphae permit the thorough and intimate exploration and exploitation of newly available substrates.

SPORES  -  The non-motile microscopic spores of eumycotan fungi, which come in a dazzling array of forms (below) to fit specific functions, are often produced very quickly (in a matter of days or even hours after the initial colonization of the substrate), and in enormous numbers.

 


Spores.gif (18254 bytes)

Spores
are dispersed by wind,  by water, or by animal
vectors, and they can often survive long periods, sometimes even years, of unfavourable conditions such as freezing, starvation or desiccation (which means drying out, and is spelled with one 's', two 'c's).
 

Like bacteria, fungal spores are everywhere, and especially in the air we breathe (sometimes up to 10,000 of them in a cubic metre - see Chapter 8 for a discussion of airborne spores).  If you are curious about the ways in which we describe and name these spores, zip off to Chapter 4 and find out.
 

HYPHAE - these are the vegetative, assimilative organs of most fungi.  When a spore germinates, what emerges is a hypha (sometimes more than one hypha), which grows at its tip, and explores the microscopic world in which it landed.  The picture below shows hyphae emerging from spores and looking for food.

 

In order to explore the territory properly, hyphae must  branch as they spread out.  This is shown in the drawing below.
A young fungal colony (left), arising from a single spore (the black dot in the middle).  Its strong, waterproof, chitinous hyphae, its richly branched growth pattern, the  digestive enzymes it secretes at its growing tips, and the hydrostatic pressures it can bring to bear - all these make it ideally suited for actively penetrating, exploring and exploiting solid substrates in a manner that the bacteria, chief competitors of the fungi in the recycling business, cannot match.   (How many hyphal tips do you think there are in this illustration of a very young colony? 176?  294?  388?  502? -- the answer is 388, and this number will double every hour or so.  
If a fungus is growing in liquid culture or in a solid substrate and producing a spherical colony, the rate of increase in the number of hyphal tips is much faster, and the final number in a mature colony, astronomical. 

The transmission electron micrograph (TEM) below (from Cole and Samson 1979) shows how vesicles (v) containing new wall material or enzymes concentrate at the hyphal tip, while the energy engines of the cell, the mitochondria (m), sit a little further back.  The many, many little round black dots in clusters are ribosomes, the sites of protein synthesis.  Make no mistake, it is at the hyphal tip that the fungus interacts with the world outside, and where all growth in length takes place.

Fungi have learned to cope with environmental extremes. They can grow at temperatures as low as -5o Celsius and as high as 60o Celsius. They include the most xerotolerant organisms known: some moulds will grow at the amazingly low water activity of 0.65 (most plants wilt permanently at a water activity of 0.98). Other moulds grow in oxygen concentrations as low as 0.2% (air contains 20% oxygen). Certain fungi can grow under extremely acid conditions (pH 1): others can tolerate alkalinity up to pH 9.  These topics are covered in more detail in Chapter 20 (Food Spoilage by Fungi and its Prevention).

As I have already noted, the saprobic fungi are recyclers par excellence, but they are also among the world's greatest opportunists, and don't restrict their attentions to naturally occurring dead wood and leaves. Where there is a trace of moisture, their omnipresent spores will germinate, and the hyphae arising from them will attack food and fabric, paper and paint, or almost any other kind of organic matter. Some of their metabolites (mycotoxins) are extremely dangerous - even carcinogenic - if they contaminate food (Chapter 21 - Mycotoxins in Food and Feed). And parasitic fungi cause the majority of serious plant diseases (Chapter 12 - Fungal Plant Pathology in Agriculture and Forestry), as well as some diseases of animals and people (Chapter 23 - Medical Mycology).

Fortunately, there is a brighter side to fungal intervention in human affairs: we have harnessed the biochemical virtuosity of the saprobic fungi in the production of beer, wine, bread, some gourmet cheeses, soy sauce, some antibiotics and immunosuppressants, organic acids, and many other useful chemicals. Fungi are even being used to convert plant waste into high-protein animal feed. We ourselves eat a number of the large, spore-producing structures developed by fungi:  mushrooms, chanterelles, morels and truffles are all familiar to devotees of French cuisine, who prize them for their unique flavours. Some of the parasitic forms are now being recruited to attack insects, weeds and other fungi which threaten our welfare (Chapter 14). And fungi in intimate, obligatory association with the roots of almost all higher plants (forming mycorrhizas), silently, invisibly and influentially perpetuate one of the world's oldest and most successful forms of mutualistic symbiosis (Chapter 17).

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© Mycologue Publications 2012