Activity regeneration in continuous Clostridium acetobutylicum bioconversions of glucose

Abstract

The regeneration of product‐forming activity by periodic nutrient feeding was studied in a continuous immobilized Clostridium acetobutylicum reactor system. Activity levels were increased by feeding ammonia and vitamins during selected intervals of the experiments; these compounds were not otherwise present in the medium. In contrast with experiments utilizing rich, complex media, regeneration with a defined medium resulted in increased rates of product formation far in excess of rates of concomitant cell growth. During one nutrient feeding phase, the increase in the butyric acid production rate was five times that of the cell growth‐rate increase, while the analogous ratio for butanol was 18. A hypothesis explaining the patterns of substrate consumption and product formation during the regeneration and deactivation periods has been developed: decreasing levels of exogenous ammonia induce the high‐affinity ammonia uptake system, causing the adenylate energy charge of the cells to increase; this in turn leads to the repression of glucose carbon flow through the EMP pathway and to the use of the acid‐forming catabolic reactions for more efficient production of ATP. Pulsewise addition of three different vitamins during one regeneration phase yielded interesting results, including a 50 % increase in butyric acid production rate that appeared to be connected with the addition of biotin. Finally, enumeration of the different morphological types of immobilized cells provided the first measurements of population dynamics in an immobilized cell system. Changes in the numbers of each cell type are consistent with the sporulation and germination processes of C. acetobutylicum observed with suspended cells.