Physiology of Aspergillus niger in oxygen‐limited continuous cultures: Influence of aeration, carbon source concentration and dilution rate

Abstract

In industrial production of enzymes using the filamentous fungus Aspergillus niger supply of sufficient oxygen is often a limitation, resulting in the formation of by-products such as polyols. In order to identify the mechanisms behind formation of the different by-products we studied the effect of low oxygen availability, at different carbon source concentrations and at different specific growth rates, on the metabolism of A. niger, using continuous cultures. The results show that there is an increase in the production of tricarboxylic acid (TCA) cycle intermediates at low oxygen concentrations. Indeed, at these conditions, a decrease in the mitochondrial respiratory chain activity leads to an accumulation of NADH and to a decreased ATP production which uncouples catabolism and anabolism, influences the intracellular pH and leads to production and excretion of organic acids. Moreover, mannitol is being produced in order to ensure reoxidation of NADH, and this is the main cellular response to balance the ratio NADH/NAD at low oxygen availability. Mannitol production is also coupled to low specific growth rate, which suggests a control of carbon catabolite repression on the mannitol pathway. The roles of two other polyols, erythritol and glycerol, were also investigated. Both compounds are known to accumulate intracellularly, at high osmotic pressure, in order to restore the osmotic balance, but we show that the efficiency of this system is affected by a leakage of polyols through the membrane.

Global transcriptional response of Aspergillus niger in the process of glucoamylase fermentation

“…It is worth to pay attention that the expression levels of pyruvate kinase (An07g08990) and asparagine synthase (An04g01340) were very high, also showing the trend of up-regulation during the whole fermentation process. Although the expression level of pyruvate kinase was decreased at 24 h compared to that at 16 h, it then increased to the same level as 16 h. The high expression level and up-regulation of pyruvate kinase and asparagine synthase ensured the high concentration of pyruvate and oxaloacetic acid as the carbon skeleton of Thr, Ala, Leu, and Val, which is consistent with the previous literature that the intermediates of TCA cycle were accumulated under the oxygen limitation (Diano et al 2009). What's more, 3-phosphoglyceric acid as the carbon skeleton of Ser and Gly, the high expression level of its biosynthesis gene An11g01390 (glycerolate dehydrogenase) guaranteed the biosynthesis efficiency of serine and glycine.…”

“…At present, the reports on oxygen limitation in fermentation process of filamentous fungi are mainly about the physiological data and metabolomics to describe the parameter changes during the oxygen-limited stage (Pedersen et al 2012;Diano et al 2006Diano et al , 2009Lu et al 2015), while the global response at transcriptional level is not well studied. In this paper, we analyzed the transcriptomic data from samples of different stages in the process of glucoamylase fermentation using a strand-specific RNA-Seq approach.…”

“…Moreover, oxygen limitation may repress the synthesis of biomass, allowing remained precursors and energy flows to the synthesis of glucoamylase and by-products. In order to obtain a more distinct description about cell metabolism under oxygen-limited conditions, Diano et al (2009) explored the formation mechanism of different by-products in A. niger under oxygen limitation condition via chemostat culture. The results showed that the concentration of TCA cycle intermediates increased and the activity of respiratory chain decreased under low oxygen availability, resulting in the accumulation of NADH and the decrease of ATP, respectively.…”

“…Along with the oxidation of NADH and FADH2, electrons are transferred to oxygen through electron transfer chain, and the released energy leads to the formation of ATP, which provides energy for strain growth and metabolism (Diano et al 2009). Hence, the oxygen availability is generally one of the most important factors in the fermentation process of aerobic microorganisms.…”

Identification of Hypoxia-Inducible Target Genes of Aspergillus fumigatus by Transcriptome Analysis Reveals Cellular Respiration as an Important Contributor to Hypoxic Survival

“…Alternatively, the reduction of fumarate to succinate is linked with the regeneration of NAD ϩ and may function as an electron sink, as described for Mycobacterium tuberculosis (63). Also, the fungal species Aspergillus niger, A. oryzae, and S. cerevisae produce elevated amounts of succinate under oxygen-limiting conditions (18,52,64). Gene expression of the cytosolic fumarate reductase frdA was significantly induced in A. fumigatus during hypoxia, and succinate was detectable in the supernatant of hypoxic batch cultures.…”