le culture conditions and physiological synchronization of replicate batch cultures. The use of minimal medium with maltose as the sole limiting nutri useful site ent, constant pH, su?cient aeration and homogeneously dispersed mycelial biomass reduced biological and tech nical variations to a minimum and allowed us to highlight those di?erences in gene expression, which were in direct relation to carbon starvation. Submerged growth is fundamentally di?erent from the natural fungal life style. Fungi experience spatio temporal gradients of various ambient factors such as nutrients, temperature and pH in their natural habitats. These gradi ents lead to heterogenity within the fungal colony. Several studies have investigated this heterogeneity during growth on agar plates and have characterized di?erential concen tric zones with respect to gene expression and protein secretion.
Recently, this heterogeneity has even been shown for microcolonies in liquid shaken cultures of A. niger. In an ideally mixed bioreactor, all dispersed hyphae experience identical environmental conditions and temporal pro?les can be monitored and controlled by process parameters. Accordingly, many evo lutionary acquired traits contributing to the natural fungal life style such as the formation of substrate exploring hyphae, secretion of certain hydrolases, cell death and conidiation are dispensable during industrial processes and might even negatively a?ect production yields. In this study A. niger showed general hallmarks of autol ysis during prolonged carbon starvation. However, in contrast to A. nidulans, A.
niger hyphae did not undergo substantial fragmentation. While an increasing number of hyphal compartments became empty after car bon depletion, microscopic analysis showed that hyphal cell wall skeletons remained mainly intact. Thus disinte gration of aging mycelia appears rather to be initiated by intracellular activities such as cell death and or endoge nous recycling of neighboring compartments leading to empty hyphal ghosts than by extracellular hydrolysis of fungal cell walls. This assumption is supported by studies in A. nidulans, where autolytic fragmentation of hyphae and cell death were described as simultaneous but independently regulated processes. While dele tion of the major carbon catabolite repressor CreA in A.
nidulans resulted in increased hydrolase activities and mycelial Drug_discovery fragmentation during carbon starvation, the via bility of selleck catalog A. nidulans was not a?ected. Consistently, we observed hyphal fragmentation and enhanced biomass decline in bioreactor cultures during the starvation phase only when the pH control was switched o? leading to an elevated pH of approximately 5. 8 towards the end of cultivation. We thus pro pose that hydrolytic weakening of the fungal cell wall and hyphal fragmentation is a secondary e?ect, which occurs after initial cell death events and only under favorable conditions. In ?ow chamber experiments with A. oryzae, Pollack et al. followed singl