Carbon monoxide conversion with Clostridium aceticum.

Carbon monoxide concentrations in syngas are often high, but tolerance toward CO varies a lot between homoacetogenic bacteria. Analysis of the autotrophic potential revealed that the first isolated acetogenic bacterium Clostridium aceticum was able to use CO as sole carbon and energy source for chemolithoautotrophic carbon fixation but simultaneously showed little tolerance to high CO concentrations. Not yet reported, autotrophic ethanol production by C. aceticum was discovered with CO as a substrate in batch processes. Growth rates estimated in batch processes at varying CO partial pressures were used to identify the CO inhibition kinetics of C. aceticum, using a substrate inhibition model. C. aceticum shows a strong CO inhibition with an optimum CO partial pressure of only 5.4 mbar in the gas phase at cell dry weight concentrations of up to 0.5 g·L -1 . At optimum conditions, growth and acetate formation rates were estimated to be 0.24 hr -1 and 0.52 g·g -1 ·hr -1 , respectively. Syngas fermentation at high partial pressures of up to 280 mbar CO in the inlet gas phase was enabled by applying a continuously operated stirred-tank bioreactor with submerged membranes with total cell retention. Around 70% CO conversion was achieved continuously in the membrane bioreactor with strongly CO inhibited C. aceticum resulting in space-time yields of up to 0.85 g·L -1 ·hr -1 acetate.