Effects of melanin on the accumulation of exopolysaccharides by Aureobasidium pullulans grown on nitrate

Aureobasidium pullulans produced pullulan and melanin when grown in medium containing low nitrate levels. With high nitrate concentrations, however, this fungus produced a mixture of exopolysaccharides (EPS) without melanin synthesis. At 0.78 g l−1 N as nitrate, where no melanin synthesis occurred, maximum EPS yields reached 6.92 g l−1 and then decreased to the final yield of 2.36 g l−1. Following melanin addition (0.1 g l−1), yields reached 7.02 g l−1 at 48 h and fell to a final yield of 5.21 g l−1. The EPS produced in high nitrate medium contained both pullulan and (1 → 3)-β-glucan, but only pullulan was produced with melanin-supplementation. With melanin addition a doubling of (1 → 3)-β-glucanase activity was observed in high nitrate medium compared to that without supplementation. On the other hand amylolytic activities disappeared in medium with melanin production or addition. Culture filtrates sustained a higher reducing capacity (RC) when melanin was present. Low RC appeared to reduce (1 → 3)-β-glucanase activity and increase amylolytic activities. Thus, higher RC appears to inhibit production/activity of amylose-degrading enzymes capable of degrading pullulan, and stimulates (1 → 3)-β-glucanase synthesis/activity, leading to a preferential accumulation of pullulan.