TCA Cycle Organic Acids Produced by Filamentous Fungi: The Building Blocks of the Future

Abstract

The use of filamentous fungi for manufacture of TCA cycle organic acids is at present a reality only for citric acid. Research efforts using filamentous fungi are under way for fumaric and malic acids. The prospect to use renewable feed stocks would allow for a cyclic economy contributing to the goals of sustainable development. Fermentation processes can be sustainable when performed with renewable feedstock under ambient conditions with reduction of greenhouse gases and wastes. Citric acid production exceeds 2 million tons year⁻¹ (Estimates since market data is not available in the public domain) by fungal fermentations, whereas fumaric (90,000 ton year⁻¹) and malic acid (70,000 ton year⁻¹) are made by chemical synthesis and succinic acid (50,000 ton year⁻¹) both by chemical synthesis and bacterial and yeast fermentations. The choice of feedstock and production mode and strain for each acid is of utmost importance to obtain an economic process. A combination of “omics” data and metabolic engineering is a novel technique in development for prospective production strains to be optimized for final product concentration (g L⁻¹), yield (g (g carbon substrate) ⁻¹), and productivity (g L⁻¹ h⁻¹). The intrinsic limitations of biological reactions have to be taken into account. The (re-)assimilation of CO₂ is important to allow for increase in the above parameters with concomitant decrease in release of CO₂. Currently the prospect that more sustainable production will take over is dependent on laws and regulations as well as economics. The capability to compete and substitute current chemical processes to produce organic acids as feedstocks will have a positive effect on the environment and help to achieve a more sustainable future.