Chapter 10: Fungi in ecosystems

Fungi make crucial contributions to all ecosystems because of their abilities as decomposers. One of the most important Kingdom-specific characteristic of fungi is that they obtain their nutrients by external digestion of substrates. In the real world, though there is some digestion and chemical modification of inorganic substrates (see Chapter 1, CLICK HERE to view a reminder), the bulk of the substrates that fungi recycle are the remains of animals and, most particularly, plants. In this Chapter we give an account of the ways in which fungal hyphae obtain, absorb, metabolise, reprocess and redistribute nutrients.

In doing this, fungi obviously contribute to recycling and mineralisation of nutrients and in what follows we will describe the enzyme systems that enable this activity. Our description is relatively brief and more details can be found in the premier text on fungal physiology (Jennings, 2008). For clarity we have to describe separately the enzyme systems involved, and in an order that we have chosen for descriptive purposes. This introduction is intended to convey the impression that for most fungi in most circumstances the initial nutritional step is the excretion of enzymes able to convert polymers to the simple sugars, amino acids, carboxylic acids, purines, pyrimidines, etc., that the cell can absorb. Also, in most circumstances most fungal mycelia will be carrying out all of these biochemical changes simultaneously.

In this Chapter we will see how fungi contribute to ecosystems. How they breakdown the polysaccharides cellulose, hemicellulose, pectins, chitin, starch and glycogen. Their (unique) ability to degrade lignin, and the ways they digest protein, lipids, esters, phosphates and sulfates. The flow of nutrients is dealt with in terms of transport and translocation; and in the final two Sections we deal briefly, but comprehensively, with the main pathways of primary (intermediary) metabolism as well as secondary metabolites, including commercial products like statins and strobilurins.

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Updated July, 2019