Australia: The Land Where Time Began

A biography of the Australian continent 

Saprotrophic fungi

Fungi do not have the ability to produce their own food directly as plants do; instead, fungi are recyclers of dead organic matter by the use of a full range of digestive enzymes they discharge into their surroundings. These enzymes degrade organic material externally after which it is ingested by the hyphae as soluble nutrients that result from the external digestion. Saprotrophs (‘decomposer’) are organisms such as these live by digesting debris left by other organisms. A saprotroph is a specific type of heterotroph, or consumer, which absorbs its nutrients ready-made from its surrounding environment. Moore says this distinction becomes important when the first organisms are considered. He suggests the literally ‘first’ must have obtained their nutrients from their surrounding environment, and by definition those nutrients could not have been the leftovers of other organisms, as no other organisms had yet evolved; so the first organisms were of necessity heterotrophs, consuming what was available, which include quite a wide range of organic compounds. The second generation of these organisms would have been able to evolve ways of using the debris of the dead first generation; from this point they could have been saprotrophs, recycling debris in the same manner as modern fungi do. Though they shared the characteristic lifestyle, they would not have been fungi in those ancient times.

At the present plant litter such as leaves, plant stems and woody branches comprise the bulk of the debris in moist soil environments. But as well as the plant matter debris, fungi are also capable of digesting the protein, cartilage and bone of dead animals, and the chitinous exoskeletons of insects. They also extract nutrients such as metal ions, phosphorous and sulphur, from inorganic minerals. The fungi demonstrate their unique abilities as recyclers by their ability to digest woody plant cell walls. Fungi are the only organisms which can degrade woody lignin. Without fungal wood decay dead timber would accumulate as it did 360 Ma in the Carboniferous, a time when the thickest coal seams were deposited as it seems prior to the Carboniferous fungi had not evolved the ability to recycle dead timber, which meant that wood accumulated until it fossilised into coal.

Cellulose, a molecule that is sugar-based, forms the first, or primary cell wall, and lignin, characteristic of the plant cell’s secondary cell wall, is composed on lignin, which is a phenol-based molecule, and together these make the cell walls of plants very strong. For the decay of dead wood in the environment fungi are very important as they are the only organism that can break down both cellulose and lignin. Cellulose is a glucose polymer forming fibres that are incredibly strong, Cellulose is broken down by brown rot fungi, the lignin that remains after they have broken down the cellulose give these their name from the colour of the lignin which remains intact. Brown rot fungi release enzymes that break the cellulose chains into single glucose molecules that are then absorbed to be used by the fungus. Lignin, the other strong polymer, is the second most abundant polymer on the Earth after cellulose. White rot fungi break down lignin, and as the lignin content is decreased the colour of the wood becomes lighter, hence their name. White rot fungi degrade lignin by producing oxidising enzymes from their hyphae and in a very real sense ‘burn’ the wood in a manner that is enzyme controlled. White rot fungi are the only organisms that can deal with the phenol in lignin, which they do by breaking the carbon rings to form open chain molecules that can then be metabolised. Without these 2 types of fungi dead plant material would not decay which would result in soil nutrients being locked in an accumulating mass of lignin-based biomass that was undegradable, which makes the fungi very important in the recycling of nutrients.

In most natural habitats fungi are a crucial part of the food web; lichens, mushrooms, toadstools and truffles form a major part of the diets of snails, slugs, mice, voles, squirrels and deer, and millipedes, insects, insect larvae and beetle grubs eat the hyphae of mycelia in the soil. Fungi have formed ‘alliances’ (symbiosis) with other organisms throughout their existence, which enable the co-operative organisms to thrive in conditions that could not be easily tolerated by either member of the alliance on its own. According to Moore symbiosis was the usual term for such alliances but more recently is has often been replaced by the term mutualism as this term immediately indicates that both organisms benefit from their association.

Sources & Further reading

  1. Moore, David, 2013, Fungal Biology in the Origin and Emergence of Life, Cambridge University Press.


Author: M. H. Monroe
Last updated:  05/12/2015 
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