Abstract
BackgroundCitric acid is typically produced industrially by Aspergillus niger-mediated fermentation of a sucrose-based feedstock, such as molasses. The fungus Aspergillus niger has the potential to utilise lignocellulosic biomass, such as bagasse, for industrial-scale citric acid production, but realising this potential requires strain optimisation. Systems biology can accelerate strain engineering by systematic target identification, facilitated by methods for the integration of omics data into a high-quality metabolic model. In this work, we perform transcriptomic analysis to determine the temporal expression changes during fermentation of bagasse hydrolysate and develop an evolutionary algorithm to integrate the transcriptomic data with the available metabolic model to identify potential targets for strain engineering.ResultsThe novel integrated procedure matures our understanding of suboptimal citric acid production and reveals potential targets for strain engineering, including targets consistent with the literature such as the up-regulation of citrate export and pyruvate carboxylase as well as novel targets such as the down-regulation of inorganic diphosphatase.ConclusionsIn this study, we demonstrate the production of citric acid from lignocellulosic hydrolysate and show how transcriptomic data across multiple timepoints can be coupled with evolutionary and metabolic modelling to identify potential targets for further engineering to maximise productivity from a chosen feedstock. The in silico strategies employed in this study can be applied to other biotechnological goals, assisting efforts to harness the potential of microorganisms for bio-based production of valuable chemicals.
Highlights
Citric acid is typically produced industrially by Aspergillus niger-mediated fermentation of a sucrosebased feedstock, such as molasses
Fermenting sugarcane bagasse hydrolysate to produce citric acid To evaluate the fermentation of sugarcane bagasse hydrolysate for the production of citric acid, we obtained fermentative time series data on citric and biomass output as well as glucose, xylose, and phosphate input
From a hydrolysate containing 120 g/L total sugars consisting of glucose (80 g/L) and xylose (40 g/L), 50 g/L citric acid was produced in 6 days (Fig. 1)
Summary
Citric acid is typically produced industrially by Aspergillus niger-mediated fermentation of a sucrosebased feedstock, such as molasses. The fungus Aspergillus niger has the potential to utilise lignocellulosic biomass, such as bagasse, for industrial-scale citric acid production, but realising this potential requires strain optimisation. The commercial production of citric acid by A. niger fermentation is dependent on sucrose-based feedstocks, primarily molasses [5]. There is an increasing need to unlock this metabolic potential, so that A. niger can play a key role in harnessing the value of underutilised second-generation feedstocks for the bioeconomy [7]. One such feedstock is sugarcane bagasse, the main by-product of sugarcane processing and a potential source of lignocellulosic sugars. To achieve cost-competitive citric acid production from bagasse hydrolysate requires the optimisation of strains away from sucrose-based fermentation to bagasse hydrolysate as the fermentation medium
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