Cultivation Strategies of Clostridium autoethanogenum on Xylose and Carbon Monoxide Combination

Clostridium autoethanogenum is capable of converting C1 carbon sources such as CO and CO2, as well as CS carbon sources such as xylose, into various products, rendering it suitable for syngas conversion. However, pure gas fermentations generally have low volumetric productivity caused by low cell mass concentrations, resulting from low specific growth rates and low gas-liquid mass transfer. The strong dependency on gas-liquid mass transfer causes data generated in serum bottles to often deviate from experiments in stirred tank reactors. This study therefore characterizes growth and product formation in both the serum bottle and stirred tank reactor, while investigating a sequential and simultaneous feeding of xylose and gaseous carbon substrates. The ratio of the product was shown to be independent of the initial xylose concentration in serum bottles, while in stirred tank reactor experiments the product ratio changed under xylose-limited conditions. The product ethanol caused inhibiting effects which could be quantified in a kinetic model. The comparison of feeding strategies showed clearly that a fed-batch process with simultaneous xylose and CO feeding led to higher CO conversion when compared to CO conversion in a sequential cultivation strategy. Carbon can be directed toward acetate formation via fed-batch fermentation under C-limited conditions. Moreover, the combined feed of xylose and CO is an advantageous method to significantly enhance gas conversion in comparison to sequential feeding.