Physiology 1031 Cultivation

The early workers reported that successful cultivation of thermophilic fungi requires supplementation of culture medium with a decoction of hay, casamino acids, peptone, yeast extract or a tricarboxylic acid cycle intermediate (for example, succinic acid), leading them to the view that thermal adaptation is associated with special nutritional requirements. However, some species were subsequently grown in synthetic minimal media (Gupta and Maheshwari, 1985) containing a carbon source (glucose), a nitrogen source (ammonium phosphate, urea or asparagine, a source of sulfur (magnesium sulfate), a source of phosphorus and buffering anions (phosphate) and trace elements. The pH optima of the thermophilic fungi studied are between 7.0 and 8.0, close to the pH level of composts. Control of the pH of growth media is an important factor in the cultivation of thermophilic organisms because at elevated temperatures carbon dioxide becomes unavailable due to its reduced solubility, more so if acidic conditions develop. If, for example, ammonium sulfate is used in culture media as a source of nitrogen, the absorption of ammonium ions and the counter transport of H+ from the cells results in the rapid acidification of the medium and the growth stops. The acidic conditions that develop reduce the solubility of CO2 required for anaplerotic reactions, for example, the reaction catalyzed by the enzyme pyruvate carbox-ylase:

This enzyme functions to replenish the intermediates of the tricarboxylic acid cycle as they are used up in generating energy and for biosynthesis. Thus, for growth to continue the medium needs to be supplemented with a tricarboxylic acid cycle intermediate such as succinic acid. Increasing the buffering capacity of the medium, or automatic pH control by the addition of alkali in an instrumented fermentor, are the methods commonly used for pH control. A convenient practice is to replace the inorganic nitrogen source with an organic nitrogen source, for example, L-asparagine.

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