Summary: Two freshwater bacteria, a sp. and a sp., were grown in continuous culture under steady-state conditions in L-lactate-, succinate-, ammonium- or phosphate-limited media. In sp., NAD-independent and NAD-dependent L-lactate dehydrogenases, aconitase, isocitrate dehydrogenase and glucose 6-phosphate dehydrogenase activities increased up to 10-fold as the dilution rate was decreased from 0.5 to 0.02 h, regardless of whether the growth-limiting nutrient was carbon, ammonium or phosphate. In contrast, 2-oxoglutarate dehydrogenase and succinate dehydrogenase activities were not influenced by , and NADH oxidase activity increased with sp. gave different results in some respects, but it also exhibited an increase in the activity of several enzymes at low values. Such increases may emanate from release of catabolite repression, and catabolite repressors for the five enzymes in sp. showing such increases are probably compounds of carbon, nitrogen and phosphorus. It is likely that increased enzyme syntheses in low cultures represent the normal physiological state for bacteria in aquatic environments where growth occurs slowly under nutrient limitations. Such increases probably permit a more effective utilization of nutrients present at sub-saturating concentrations.


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