A kinetic model for quantitative evaluation of the effect of hydrogen and osmolarity on hydrogen production by Caldicellulosiruptor saccharolyticus

Background: Caldicellulosiruptor saccharolyticus has attracted increased interest as an industrial hydrogen (H-2) producer. The aim of the present study was to develop a kinetic growth model for this extreme thermophile. The model is based on Monod kinetics supplemented with the inhibitory effects of H2 and osmotic pressure, as well as the liquid-to-gas mass transfer of H-2. Results: Mathematical expressions were developed to enable the simulation of microbial growth, substrate consumption and product formation. The model parameters were determined by fitting them to experimental data. The derived model corresponded well with experimental data from batch fermentations in which the stripping rates and substrate concentrations were varied. The model was used to simulate the inhibition of growth by H-2 and solute concentrations, giving a critical dissolved H-2 concentration of 2.2 mmol/L and an osmolarity of 0.27 to 29 mol/L. The inhibition by H-2, being a function of the dissolved H-2 concentration, was demonstrated to be mainly dependent on H-2 productivity and mass transfer rate. The latter can be improved by increasing the stripping rate, thereby allowing higher H-2 productivity. The experimentally determined degree of oversaturation of dissolved H-2 was 12 to 34 times the equilibrium concentration and was comparable to the values given by the model. Conclusions: The derived model is the first mechanistically based model for fermentative H-2 production and provides useful information to improve the understanding of the growth behavior of C. saccharolyticus. The model can be used to determine optimal operating conditions for H-2 production regarding the substrate concentration and the stripping rate.