Influence of Aeration and of Pantothenate on Growth Yields of Zymomonas mobilis

Abstract

The growth yields and rates of Zymomonas mobilis were measured in aerobic and anaerobic cultures on glucose medium containing yeast extract, amino acids, or ammonium chloride as the nitrogen source. In the absence of yeast extract, pantothenate was required. The growth yield and rate of the cultures in synthetic (amino acids) or minimal (NH4Cl) medium supplemented with pantothenate corresponded only to about one-half the "normal" values obtained in the presence of yeast extract, suggesting a situation of energetically uncoupled growth. Attempts to restore normal growth by the addition of various compounds were unsuccessful. Aeration of the cultures resulted in a partial oxidation of ethyl alcohol to acetate, but did not modify the growth yield nor the division time. Both aerobic and anaerobic cells, however, contained cytochrome c and a cytochrome oxidase of the a2 type, which was completely inhibited by 10-4 M cyanide. In anaerobically grown cells, an additional cytochrome of the b type was present. The absence of a Pasteur effect suggests that the transfer of electrons by the respiratory chain of Z. mobilis may not be coupled with oxidative phosphorylation. Aeration had no effect on the catalase content of the cells. As shown by C14-glucose incorporation, 2 to 3% of the glucose metabolized was assimilated by the cells in both synthetic and rich complex medium. No intracellular glycogen nor poly-0-hydroxybutyrate was accumulated when growth was limited by nitrogen or by phosphate in the presence of excess glucose.

Metabolic shifts in Zymomonas mobilis in response to growth conditions

“…Aeration of cultures of Z. mobilis also leads to a diversion of reducing power and a consequent decrease in ethanol production [22,[43][44][45][46]. Furthermore, the ability of Z. mobilis to grow in the presence of oxygen is not correlated with higher cell yields or with higher growth rates as compared to anaerobic conditions [22,[43][44][45][46]. With increasing rates of aeration and concentrations of the carbon source, growth becomes strongly inhibited [44][45][46] and cell yields decrease [44,45].…”

“…The activity of NADH oxidation accounted for the oxygen consumption rate of cell suspensions in the presence of a carbon source [44]. The cytochrome content of the cell membrane [43,45] and the inhibition by cyanide of both intracellular oxygen reduction by the NADH-oxidase and oxygen consumption by whole Table 1 Influence of oxygen on growth and product formation of cells led to the suggestion that a respiratory chain is present in Z. mobilis [43,44]. Obviously, the transfer of electrons via this respiratory chain is not coupled with oxidative phosphorylation.…”

“…As a consequence, acetaldehyde, a toxic intermediate of sugar catabolism accumulates ( Table 1). In addition, acetic acid [43,44] and dihydroxyacetone [22] are formed as products during aerobic growth.…”

Membrane ?-lactate oxidase in Zymomonas mobilis: evidence for a branched respiratory chain

“…The aerobic growth of Zymomonas mobilis has received attention from several groups of investigators [1^3] due to major uncertainties in the understanding of the energy metabolism of this aerotolerant anaerobe. Long ago it was established that Z. mobilis possesses a respiratory chain [1,3]. However, under oxic conditions it grows with low biomass yields, and this ¢nding has served as a basis to conclude that oxidative phosphorylation either is not operating [2,3], that its impact is small in this bacterium [4], or that it may be operating to some extent only in fast-growing cells [5].…”

“…Little is known about the composition of the Z. mobilis respiratory chain. The presence of cytochromes b, c, and d [1,3], and of ubiquinone-10 [9] has been reported. NADH, NADPH, glucose, and ascorbate in the presence of redox mediators have been successfully used as electron donors in membrane preparations [2,7,9] but the electron transport pathways have not been analysed.…”