Influence of metabolic end-products on the growth efficiency ofKlebsiella aerogenes in anaerobic chemostat culture

Progressively increasing the input concentration of growth-limiting nutrient (glucose, ammonia, K⁺) to anaerobic chemostat cultures ofKlebsiella aerogenes (D=0.38 h⁻¹; 35°C; pH 6.8) led to a non-linear increase in bacterial cell concentration. At modest population densities, residual growth-limiting substrate levels increased substantially, with increasing input concentration, and the culture bacterial dry weight tended to a constant value. With the glucose-limited culture, increasing the glucose input concentration above 20 g·1⁻¹ led to accumulation of unused glucose and a change in the fermentation pattern. There was a concomitant lowering of the yield value with respect to glucose consumption, and the calculated Y_ATP value similarly declined. Addition of extra essential (non-limiting) nutrients to the culture was without effect. Similarly, addition of individual fermentation products (acetate, ethanol,d-lactate, 2,3-butanediol, succinate) to the feed medium, in varying concentrations and in different combinations, failed to influence the fermentation pattern or the energetics of cell synthesis. However, a clear correlation was observed between the yield values (of both glucose- and K⁺-limited cultures) and the steady state concentration of CO₂ in the effluent gas. Increasing the concentration CO₂ either by increasing the population density or lowering the sparging rate of nitrogen gas through the culture, effected a lowering of the yield values. It is suggested that dissolved CO₂ exerts an effect on both metabolism and the energetics of cell synthesis. A possible mechanism of energy dissipation (i.e., a futile cycle) involving carboxylation and decarboxylation reactions is proposed.