Growth yields, polysaccharide production and energy conservation in chemostat cultures of Rhizobium trifolii.

Rhizobium trifolii was grown in a defined medium in chemostat cultures. Extracellular polysaccharide production was found in carbon-sufficient as well as in carbon-limited cultures. Extracellular polysaccharide production in carbon-limited cultures was strongly dependent on the growth rate. In mannitol-limited cultures, asparagine was always totally depleted from the culture medium. Only when the asparagine supply was not sufficient to meet the nitrogen need of the culture, ammonia assimilation took place. Excess organic nitrogen was excreted as ammonia. Whether ammonia assimilation or ammonia excretion took place was also dependent on the growth rate. Respiration-coupled proton translocation measurements showed the presence of three energy conserving sites in an electron transport chain which is branched. Assuming a H+/P ratio of 4, a P/O ratio of 2.33 was found. Growth yield calculations indicated a P/O ratio of approximately 2. Sulphate limitation in the chemostat culture resulted in a decrease in the efficiency of oxidative phosphorylation and in a less stringent coupling between growth and energy yielding processes.