Abstract
BackgroundThe spore germination rate and growth characteristics were compared between the citric acid high-yield strain Aspergillus niger CGMCC 5751 and A. niger ATCC 1015 in media containing antimycin A or DNP. We inferred that differences in citric acid yield might be due to differences in energy metabolism between these strains. To explore the impact of energy metabolism on citric acid production, the changes in intracellular ATP, NADH and NADH/NAD+ were measured at various fermentation stages. In addition, the effects of antimycin A or DNP on energy metabolism and citric acid production was investigated by CGMCC 5751.ResultsBy comparing the spore germination rate and the extent of growth on PDA plates containing antimycin A or DNP, CGMCC 5751 was shown to be more sensitive to antimycin A than ATCC 1015. The substrate-level phosphorylation of CGMCC 5751 was greater than that of ATCC 1015 on PDA plates with DNP. DNP at tested concentrations had no apparent effect on the growth of CGMCC 5751. There were no apparent effects on the mycelial morphology, the growth of mycelial pellets or the dry cell mass when 0.2 mg L-1 antimycin A or 0.1 mg L-1 DNP was added to medium at the 24-h time point. The concentrations of intracellular ATP, NADH and NADH/NAD+ of CGMCC 5751 were notably lower than those of ATCC 1015 at several fermentation stages. Moreover, at 96 h of fermentation, the citric acid production of CGMCC 5751 reached up to 151.67 g L-1 and 135.78 g L-1 by adding 0.2 mg L-1 antimycin A or 0.1 mg L-1 DNP, respectively, at the 24-h time point of fermentation. Thus, the citric acid production of CGMCC 5751 was increased by 19.89% and 7.32%, respectively.ConclusionsThe concentrations of intracellular ATP, NADH and NADH/NAD+ of the citric acid high-yield strain CGMCC 5751 were notably lower than those of ATCC 1015. The excessive ATP has a strong inhibitory effect on citric acid accumulation by A. niger. Increasing NADH oxidation and appropriately reducing the concentration of intracellular ATP can accelerate glycolysis and the TCA cycle to enhance citric acid yield.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0190-z) contains supplementary material, which is available to authorized users.
Highlights
The spore germination rate and growth characteristics were compared between the citric acid high-yield strain Aspergillus niger CGMCC 5751 and A. niger ATCC 1015 in media containing antimycin A or DNP
When the concentration of antimycin A in Potato dextrose agar (PDA) plates was less than 0.7 mg L−1, the spore germination rate was over 60% for both strains, but by adding 0.7 mg L−1 antimycin A, the spore germination rate of CGMCC 5751 was only 12.39%, that of ATCC 1015 was 63.87%; the spores of CGMCC 5751 could hardly germinate when antimycin A was present at a concentration greater than 0.9 mg L−1 (Figure 1)
Comparing the growth extent of the two strains on PDA media containing antimycin A or DNP revealed that antimycin A significantly inhibited the spore germination of both strains, but CGMCC 5751 was more sensitive to antimycin A than was ATCC 1015
Summary
The spore germination rate and growth characteristics were compared between the citric acid high-yield strain Aspergillus niger CGMCC 5751 and A. niger ATCC 1015 in media containing antimycin A or DNP. We inferred that differences in citric acid yield might be due to differences in energy metabolism between these strains. The effects of antimycin A or DNP on energy metabolism and citric acid production was investigated by CGMCC 5751. The glucose uptake rate has been identified by Torres [8,9] as an important factor for the rate of citric acid production. They apply mathematical modelling to predict that the glycolytic reactions of A. niger are limited by the supply of the initial substrate and the removal of the final product. To promote glycolytic metabolism, the inhibitors of these three rate-limiting enzymes must be removed. Some studies revealed that single or joint over-expression of the enzymes could not significantly improve the rate of glycolytic flux in yeast [6] or bacteria [11]
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