Citric acid (2-hydroxy-propane-1,2,3-tricarboxylic acid) is a well-known example of fungal “overflow metabolism”, a term used to describe the apparently wasteful strategy of certain fungi to incompletely oxidize their carbon source. The estimated 2.1 million tons of citric acid manufactured annually is mostly performed by fermentation using the filamentous fungus Aspergillus niger. One critical prerequisite for high-yield citric acid overflow is to adjust the concentration of Mn(II) ions in the medium. Mn(II) concentrations >4 ppb reduce citric acid accumulation by about 20%. Understanding manganese metabolism in A. niger is critical to improve citric acid production. The A. niger gene NRRL3_07789 is predicted to encode a manganese transporter. We constructed a ?NRRL3_07789 strain as well as a strain overexpressing NRRL3_07789. Mn(II) uptake rate of the deletion strain dropped to zero, while that of the overexpression strain doubled. Under citric acid producing conditions both the reference and the deletion mutant strains displayed molar yields over 80%, but maximal biomass of the mutant decreased by a third. In contrast, molar yield of the OE cultures dropped to 17% of that of the reference strain, while biomass formation doubled. Notably, the overexpression strain displayed an overwhelmingly filamentous morphology characterized by branched mycelial clumps as opposed to the pellet-dominated cultures of the reference and deletion strains. Our results demonstrate that NRRL3_07789 is involved in the transport, and hence metabolism of Mn(II) ions; and is highly relevant to citric acid overflow A. niger. Research was supported by the EU and co-financed by the European Regional Development Fund under the project GINOP-2.3.2-15-2016-00008; by the EFOP3.6.1-16-2016-00022 project co-financed by the European Union and the European Social Fund; and the Industrial Biocatalysis Strategic Network of the Natural and Engineering Research Council of Canada. EF is supported by the ÚNKP-18-4 New National Excellence Program of the Ministry of Human Capacities.
Full conference title:
- Fungal Genetics Conference 30th (2019)