Effect of nutrient limitation on product formation during continuous fermentation of xylose with Thermoanaerobacter ethanolicus JW200 Fe(7)

Thermoanaerobacter ethanolicus JW200 Fe(7) was grown in continuous culture, using xylose as the primary carbon source, with progressively lower concentrations of supplementary yeast extract. This enabled the comparison of metabolic flux to fermentation end-products under carbon-limited and carbon-sufficient (yeast extract-limited) conditions and the determination of process data under fully mass-balanced conditions. Under carbon-limitation, the specific ethanol-formation rate was described by q_p=40.34 μ+3.74, the specific rate of substrate utilisation for maintenance was 0.31±0.02 g g^-1 h^-1 and the maximum cell yield on xylose, corrected for maintenance requirements, was 0.15±0.04 g g^-1. Based on the product profiles, these corresponded to a maintenance coefficient of m_ATP=4.1±0.5 mmol g^-1 h^-1 and a maximum cell yield of Y_x/ATP ^max = 14.7±0.8 mol^-1. Limitation by a component in yeast extract resulted in incomplete xylose utilisation, increased catabolic flux rates (primarily resulting in increased lactate production, due to limitations in the flux through the phosphoroclastic reaction), a reduction in cell yield Y_x/ATP ^max = 10.0±1.0 g mol^-1 and an increase in maintenance energy requirements of m_ATP=7.95±0.7 mmol g^-1. The latter was also reflected in a shift from ethanol to acetate production at lower growth rates. An analysis of ethanol and acetate tolerance indicated that any high-intensity process employing this strain would require a bioreactor design which incorporated continuous ethanol stripping.