Succinic acid is an important precursor for the synthesis of high- value-added products. Nowadays, succinic acid is primarily derived through petroleum-based processes, which leads to high prices and environmental problems. On the other hand, microbial fermentation is considered a green process that can produce succinate from renewable resources. The strong tolerance towards low pH and osmotic stress of yeast makes it suitable for succinic acid production. However, all studies to date focused on methods utilizing the TCA cycle in the oxidative direction or glyoxylate shunt to produce succinate. If a reductive TCA pathway is used, the maximum theoretical yield of succinate is 1.714 mol of per mol of glucose, superior to values achieved by other pathways. Moreover, this process results in net CO2 fixation instead of release which has benefits for reducing global warming.

In this study, enhanced production of succinate was achieved by activating the reductive pathway in engineered Saccharomyces cerevisiae. In addition, the succinate titer was further improved to 8.09±0.28 g/L by the deletion of GPD1 and even higher to 9.98±0.23 g/L with a yield of 0.32 mol/ mol glucose through regulation of biotin and urea levels. Under optimal supplemental CO2 conditions in a bioreactor, the engineered strain produced 12.97±0.42 g/L succinate with a yield of 0.21 mol/ mol glucose at pH 3.8. These results demonstrated that the proposed engineering strategy was efficient for succinic acid production at low pH value. Fermentation at low pH values markedly reduces the demand for alkaline neutralizers, prevents bacterial pollution and facilitates the downstream process.

This work was supported by National High Technology Research and Development Program of China (863 Project, No. 2012AA101807, 2011AA02A203 and 2012AA022301). The related research results were published on Bioresource technology (2014, 156: 232-239)