Determination of the optimal formic acid:glucose ratio in glucose-limited chemostat cultures
Industrial fermentations are typically performed in carbon-limited fed-batch processes, the design parameters of which include the feed profile and composition. We performed a series of carbon limited chemostat cultures with various feed compositions to determine the optimal feed-ratio of formic acid to glucose (F:G) for the Y. lipolytica strain used (Figure 3). A dilution rate of 0.10 h-1 was chosen, well below the maximum specific growth rate of W29 determined from offgas CO2 of the batch phase preceding the chemostat phase (0.37 ± 0.01 h-1, see Supplementary Materials 2).
For F:G ratios between 0 and 5 mol/mol, a linear increase in the biomass yield on glucose was observed from 0.50 ± 0.02 to 0.60 ± 0.01 g biomass/g glucose, indicating that in this range formic acid dissimilation could effectively displace glucose dissimilation. In accordance with the 20% increased biomass yield, the biomass specific uptake rate of glucose decreased from 0.20 ± 0.01 to 0.16 ± 0.00 g glucose/g biomass/h (Supplementary Table S9). A further increase in the F:G ratio gave no increase in the biomass yield, even though up to the highest tested F:G ratio of 11.5 mol/mol >98% of the ingoing formic acid was consumed. Apparently, beyond a F:G ratio of 5, further consumption of formic acid is decoupled from additional ATP formation.