Control of Fungal Morphology in Continuous Culture

Abstract

The aim of this project was the development of a system for the continuous production of fungal spores by the control of mycelial morphology and the optimisation of environmental factors stimulating sporulation. Spores, considered a dormant stage in the life cycle of fungi, have been shown to contain enzymes unrelated to those required for germination, but which perform a variety of commercially important metabolic activities. The production of spores in large numbers as stable enzyme packages therefore has considerable commercial potential. Sporulation of Aspergillus niger and Aspergillus ochraceus was induced by growth restriction using nutrient and environmental limitations. Citrate supported poor sporulation while ammonium nitrogen inhibited sporulation at all concentrations. Examination of the pH, temperature, carbon dioxide and ferrocyanide ion tolerance of both organisms indicated that no physical factor alone induced sporulation. However, a transient sporulation phase occurred in response to gradual carbohydrate and to shock nitrate limitations. To increase spore productivity, the transient sporulation phases were induced semi-continuously by cycling nutrient supplies. Continuous production was then achieved in a two-stage fermentation system. For these systems to operate successfully, continuous growth of the fungus during sporulation was essential, and there was no upper limit of growth rate which prevented sporulation. Spore production occurred at specific carbohydrate supply rates above the calculated maintenance requirements for fungal survival, but values could not be calculated for nitrogen maintenance because growth continued without an external nitrogen supply. Both organisms displayed variable and much-simplified sporulation apparatus, indicating that complex structures were not required in a submerged environment. Sporulation was controlled by a miltivariate and delicate interaction between growth rate and the nutritional and physiochemical environment. The continuous tower fermenter proved ideal for controlling fungal morphology and enabled a system for the controlled continuous induction of sporulation to be developed.