OXYGEN DELIVERY REQUIREMENTS OF COLLETOTRICHUM-TRUNCATUM DURING GERMINATION, VEGETATIVE GROWTH, AND SPORULATION

Optimization of O2 delivery was the key to successful conidiation of Colletotrichum truncatum in submerged fermentor cultures supplied with 20 g carbon/l and C:N at the optimal 10:1 mass ratio for spore efficacy. Minimal mycelial fragmentation and maximal biomass and spore yields were provided by an O2 transfer program that called for gradual increases in stirring rate to compensate for rising cell concentration and viscosity. The utility of an event-based O2 transfer program was further supported by our observation of different O2 requirements for each phase of the life cycle. Spore germination did not occur in cultures sparged with N2. However, even low levels of O2 [10% dissolved O2 tension (DOT)] allowed 100% germination. The specific growth rate of the mycelia was a Monod-like function of DOT. The maximal growth rate was achieved when greater-than-or-equal-to 15% DOT was provided via O2 transfer at a specific rate of 5.4 x 10(-3) mol/g per hour. Sporulation had a strict O2 requirement, and its rate and yield were optimized by providing 55% DOT following the cessation of growth. The specific O2 demand of optimally sporulating mycelia was 4.9 x 10(-4) mol/g per hour, an order of magnitude less than that associated with growing mycelia. Behaving as a pseudoplastic fluid, the fermentation broth reached a maximum apparent viscosity of 70 P at the onset of sporulation when the O2 demand was low. However, the maximum power requirement approx. 7.9 W/l occurred during the last 36 h of growth when the O2 demand was highest.