On-line study of fungal morphology during submerged growth in a small flow-through cell

Abstract

A flow-through cell is designed to measure the growth kinetics of hyphae of Aspergillus oryzae grown submerged in a well controlled environment. The different stages of the growth process are characterized, from the spore to the fully developed hyphal element with up to 60 branches and a total length lt up to 10,000 micrometer. Spore swelling is found to occur without change in the form of the spore (circularity index constant at about 1.06) and the spore volume probably increases exponentially. The germ tube appears after about 4 h. The branching frequency and the rate of germ tube extension is determined. After about 10 h growth at a glucose concentration of 250 mg L-1, 6-7 branches have been set, and both the total hyphal length lt and the number of tips increase exponentially with time. The specific growth rate of the hyphae is 0. 33 h-1 while the average rate of the extension of the growing tips approaches 55 micrometer h-1. The growth kinetics for all the branches on the main hypha have also been found. The main hypha and all the branches grow at a rate which can be modeled by saturation kinetics with respect to the branch length and with nearly equal final tip speeds (160 micrometer h-1). Branches set near the apical tip of the main hypha attain their final tip speed in the shortest time, i.e., the value of the saturation parameter is small. Finally, the influence of substrate (glucose) concentration cs on the values of the morphological parameters has been determined. It is found that saturation type kinetics can be used to describe the influence of cs on the growth. Experiments with recirculation of effluent from the cell back to the inlet strongly suggest that the fungus secretes an inducer for growth and branching.