High-throughput screening of high-yield colonies of Rhizopus oryzae for enhanced production of fumaric acid

Fumaric acid is an important four-carbon dicarboxylic acid as a potential biorefinery target. A high-throughput screening method for fumaric acid overproduction strains was established. Nystatin (50 mg/L) was added into the production medium to restrict the spread of Rhizopus oryzae hyphae on agar plates. With bromocerol green as a pH indicator in the agar plates, the capability of fumaric acid biosynthesis by single colony was positively correlated with the diameter ratio of the colored ring and the colony. With this novel strategy, one high-yield mutant (Rhizopus oryzae ZJU11) was isolated from a large colony library of Rhizopus oryzae after UV irradiation. Starting with an optimized glucose concentration of 85 g/L, Rhizopus oryzae ZJU11 can produce 57.4 g/L fumaric acid in flask and 41.1 g/L in 5-L fermentor, which were 205% and 160% higher than that of the parent strain, respectively. Further studies showed that the production of fumaric acid by Rhizopus oryzae ZJU11 remained at the same level after three consecutive generations on the fermentation medium.