Why do exopolysaccharide yields from the fungus Aureobasidium pullulans fall during batch culture fermentation?

When the fungus Aureobasidium pullulans ATCC 9348 was grown in a continuously stirred tank reactor (CSTR) with varying concentrations of NO3− (as NaNO3) as sole nitrogen source, the final yields of exopolysaccharide obtained fell as the initial NO3− concentration was increased, under otherwise identical fermentation conditions. Thus, at an initial NO3− concentration of 0.78 g l−1 total nitrogen (N), the final exopolysaccharide yield was substantially less compared to when lower initial NO3− levels of ≤0.13 g l−1 N were used. Examination of the fermentation profiles showed a drop in exopolysaccharide yield that coincided with glucose exhaustion from the medium after approximately 40 h incubation with high NO3− levels (≥0.26 g l−1 N). At initially lower NO3− levels (≤0.13 g l−1 N), glucose exhaustion did not occur as early, if at all, and no subsequent drop in exopolysaccharide yield was observed. Evidence presented here suggests one explanation for these falls in yields is a result of exocellular exopolysaccharide-degrading enzyme/s synthesised by A. pullulans during fermentation. No exocellular pullulanase activity could be detected. Exocellular amylolytic, and (1→3)-β-glucanase activities (not fully classified), the latter reported here for the first time in this fungus, were present in culture filtrates throughout the fermentation. Furthermore, (1→3)-β-glucanase and amylolytic activities continued to increase after glucose exhaustion and exopolysaccharide disappearance from the medium.