Aspergillus niger citric acid accumulation: do we understand this well working black box?

Abstract

This Mini-Review summarizes the current knowledge on the biochemical and physiological events leading to massive citric acid accumulation by Aspergillus niger under industrially comparable conditions, thereby particularly emphasizing the roles of glycolytic flux and its control, excretion of citric acid from the mitochondria and the cytosol, and the critical fermentation variables. The potential of novel techniques for metabolic analysis and genomic approaches in understanding this fermentation is also discussed.

Plant- and microbial-based mechanisms to improve the agronomic effectiveness of phosphate rock: a review

“…The results of the above studies, with support from the available literature on citric acid production by A. niger (Karaffa and Kubicek 2003), suggest that through an aerobic fermentation process, A. niger converts SB residue, which is high in reducing sugars, to citric and oxalic acids. The organic acids produced by A. niger may then increase plant-available P from PR, effectively enhancing the agronomic effectiveness of the rock.…”

“…It is not surprising that the ratio of oxalic acid produced to citric acid produced was high, since the medium utilized did not have a high carbon concentration (∼ 1% glucose). By selectively altering the culture conditions, the ratio of citric acid to other acids should greatly increase (Karaffa and Kubicek 2003). As citrate is a strong chelator of Ca 2+ , higher citrate concentrations should have favorable effects on PR solubilization.…”

“…This method has the advantage of creating more optimal conditions for microbial organic acid production compared with conditions present in soil. For example, A. niger requires an excessive carbon source and suboptimal concentrations of nitrogen and many trace metals, to achieve maximal citric acid production (Grewal andKalra 1995, Karaffa andKubicek 2003). In a pot trial, using Grenada (Spain) soil, Vassilev et al (1996) studied the effect of adding sugar beet (SB) (Beta vulgaris) waste, pre-cultured with A. niger in the presence of Moroccan PR, on the growth of white clover (Trifolium repens).…”

Genome sequencing and analysis of the versatile cell factory Aspergillus niger CBS 513.88

“…Using the reconstructed metabolic network, we investigated why A. niger is an efficient organic acid producer 28 . Several enzymes involved in the formation of the citrate precursor oxaloacetate were found, for example, two pyruvate carboxylases (one cytoplasmic, one mitochondrial), four malate dehydrogenases (three cytoplasmic and one mitochondrial).…”

Organic acid production in Aspergillus niger and other filamentous fungi

“…Although technically not as mandatory a step as substrate import and conversion, citrate export is suggested to be the bottleneck of A. niger citrate production (Karaffa and Kubicek, 2003). Similarly, production of the C4-dicarboxylic acids fumarate, malate and succinate is suggested to be bottlenecked at the transport level across various cell-organelles and into the extracellular medium (Meussen et al, 2012).…”