0.01 0.1 1 Fdx measured
Figure 7. Comparison of measured and predicted biomass yield FDX (solid circles, fermentative; open circles, aerobic growth systems).
This gives a biomass yield on methanol of 1/12.564 = 0.08 C-mol biomass/C-mol methanol. The stoichiometric result also shows that the acetate production is 8.898/12.564 = 0.70 mol acetate/ mol methanol, showing a C yield of 1.4 acetate carbon/methanolcarbon. This is, of course, due to the CO2 fixation that occurs (6.232 mol HCO" per 12.564 mol methanol).
If maintenance is not allowed to be neglected mG must be taken into account. For example, the temperature is assumed to be 50 °C. Equation 2 then shows that mG = 38.8 kJ/C-mol biomass h. If the growth rate p = 0.03 h"1, then we can calculate, using equation 1c that 1/Fgx = 698 + 38.8/(0.03) = 1,991 kJ/C-mol biomass. Using this number one can solve the six equations to obtain the complete stoichiometry, which holds under these conditions.
Before ending this section, a final warning is relevant. The described thermodynamic method of predicting growth stoichiometry is based on a very wide database of experimentally measured growth systems. No detailed biochemical information is required, because intrinsically a kind of average biochemistry used by most organisms is assumed. This is an attractive feature, but in the end we should consider that, of course, the biochemistry used by microorganisms does have a significant influence. For example, for the anaerobic ethanol fermentation on glucose the mentioned method will give FDX = 0.15 C-mol biomass/C-mol glucose. This is indeed found for Saccha-romyces cerevisae. However, another organism, Zymo-monas mobilis, does the same glucose/ethanol process, but with Fdx = 0.07. The explanation is that Z. mobilis uses a completely different biochemical pathway for glucose ca-tabolism than S. cerevisae. From this example we can also
learn that if the predicted biomass yield differs very substantially from the actually measured yield, it might be possible that the microorganism being studied uses a novel pathway for catabolism or anabolism.
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