More random-walk than autotropism: A model-based study on how aerial hyphae of Rhizopus oligosporus grow in solid-state fermentation

During solid-state fermentation, aerial hyphae occupy the interparticle spaces, binding adjacent particles and decreasing the efficiency of mass and heat transfer within the bioreactor bed. The present work explores two questions related to this phenomenon: (i) what determines the direction of growth of vegetative aerial hyphae and (ii) what makes them stop growing? We developed a discrete mathematical model to explore the factors affecting the distribution of aerial hyphae with height above the surface and adjusted the model parameters by fitting the model to literature data of Rhizopus oligosporus growing on potato dextrose agar. The adjusted rules were simple and indicate that vegetative aerial hyphae grow in any direction, as though their tips moved in a random-walk fashion. Vegetative aerial hyphae stop extending when they enter crowded spaces, resulting in a maximum local biomass concentration. With these rules, the layer model was used to study the influence of increasing inoculum concentration and to simulate, for the first time, growth between the surfaces of two adjacent particles.