Introduction

Organic acids are important in fungal nutrition because:

1. They can be a source of combined carbon for growth.

2. They can contribute to the internal osmotic potential.

3. They can contribute to the internal charge balance.

4. They may be involved in the control of internal pH.

5. They can be involved in the loss of protons across the plasma membrane.

6. It has been proposed that the fixation of carbon dioxide into organic acids may be part of the battery of processes allowing a particular species to grow oligotrophically.

7. The production of oxalic acid appears to be important in assisting the decay of wood by fungi (see Chapter 5).

8. Oxalic acid may also be important in reducing the inhibitory effect of too much calcium in the external environment (see Chapter 11).

As well, production of various organic acids, particularly by Aspergillus and Candida spp., is commercially important (Miall, 1978; Mattey, 1992).

Transport

Table 12.1 gives details of those carboxylic acid transport systems that are known to be present in fungi. Except for succinate transport in Neurospora crassa, all the information is for yeasts. The interest in the ability of yeasts to transport carboxylic acids across the plasma membrane is two-fold. First, the ability to use, for growth, carboxylic acids of the citric acid cycle is used as a taxonomic discriminator.