Bioreactor cultures
Strain GSU5 was grown in bioreactor batch cultures using glucose or
xylose as carbon sources in order to analyze solvent production in a
larger scale. Several tests were performed to determine the best pH
conditions for growth and solvent production, since previous studies had
indicated that solvent production was enhanced by low pH in otherT. thermosaccharolyticum strains. Preliminary tests in glucose
cultures with initial starting pH of 6.7 or 7.5 did not allow adequate
growth, so all cultures were performed with an initial pH of 7, and pH
was controlled to prevent values lower than 5. Sugars were added at an
initial concentration of 10 g/L, and replenished after 17 h by adding
half the initial amount to avoid sugar depletion.
Growth was more vigorous in glucose cultures, achieving 52% more
biomass than when using xylose (1.9 vs 1.0 g/L). Solvent production was
also much higher in glucose, while xylose cultures produced more acids
than solvents (Figure 4). Butyric acid remained the main product in
xylose cultures, but glucose cultures produced slightly more ethanol
than butyrate. The final butanol concentration obtained was 0.33 ± 0.01
g/L in glucose and 0.26 ± 0.02 g/L in xylose, while the final ethanol
concentration was 4.34 ± 0.06 g/L in glucose and 0.78 ± 0.49 g/L in
xylose (Table 3).
As expected, fermentor cultures corresponding to both sugars had a much
higher biomass and solvent production that 5 ml tube cultures. Although
the main products were the same observed in tube cultures, the relative
amounts of acids and alcohols varied. A higher relative amount of
butanol was observed with both substrates when all major metabolites
were considered, showing a slight increase in carbon fluxes towards
butanol in bioreactor cultures (Figure 5). When cultures grown using
glucose were compared to tube cultures, solvent production was observed
to increase more than biomass, with the most important increase observed
in butanol production. While an 8 fold change was observed in biomass
(1.78 vs 0.22 g/L), final concentrations of ethanol and butanol
increased 10 and 14 times respectively, so that the amount of alcohols
produced in the fermentor was similar to that of acids (Figure 5).
Results obtained with xylose were different, as a 6 fold increase in
biomass (1.04 vs 0.20 g/L) was accompanied by a similar increase
in butanol concentration, while ethanol concentration only increased 1.5
fold when fermentor cultures were compared with 5 ml cultures (Figure
5).