Scope and method of study. Previous results showed that biomass-generated syngas caused (1) cell-washout from a chemostat due to cell-dormancy, (2) cessation of hydrogen uptake due to hydrogenase inhibition and (3) product re-distribution in the microbial catalyst Clostridium carboxidivorans P7^T. The focus of this work was to investigate the factors responsible for these effects in order to overcome cell-dormancy and inhibition of hydrogen uptake for improving the fermentation process. Three goals were addressed in this work: (1) to overcome cell-dormancy due to biomass-syngas; (2) to eliminate cessation of hydrogen consumption due to hydrogenase inhibition and (3) to increase ethanol production by regulating the metabolic pathway of the microbial catalyst.

Findings and conclusions. The studies conducted resulted in the following conclusions: (1) Additional cleaning of biomass-syngas using a 0.025-µm filter prevented cell-washout. (2) Cell-dormancy was caused by particulate contaminants (likely tars) of the biomass-syngas. (3) Cells could adapt to tar after prolonged exposure and start growing. (4) Tars caused an increase in ethanol and decrease in acetic acid production. (5) Use of a cell-recycle system prevented cell-washout, and allowed the cells to adapt to and grow on biomass-syngas (without additional cleaning). (6) Hydrogenase inhibition was caused by nitric oxide. (7) A kinetic model was developed which indicated that nitric oxide inhibition was non-linear and non-competitive. (8) Nitric oxide also inhibited initial cell-growth and caused an increase in ethanol production. (9) Nitric oxide caused the alcohol dehydrogenase activity to increase. (10) Addition of neutral red, an artificial electron carrier, resulted in an increase in ethanol and decrease in acetic acid production. (11) Neutral red also caused an increase in alcohol dehydrogenase activity.