Na+ responses of two marine bacteria that did not appear to require Na+ for growth

Abstract

Two Gram-negative heterotrophic marine bacterial strains had been reported not to require Na+ when grown on a chemically defined medium solidifed with purified agar and prepared without added Na+. When these strains were tested in a chemically defined liquid medium they required at least 3 mM Na + for growth. The agar used in the plating medium was found to contribute 3.3 mM Na+. Increasing the concentrations of Na+ in the liquid medium above 3 mM increased the rate and extent of growth of both organisms and decreased the lag periods. Optimal Na+ concentrations for growth varied from 100 to 500 mM depending on the organism and the carbon source in the medium. Na+ was also required for the transport of the carbon source into the cells. For the maximal rate of transport of L-glutamate, one organism required only 10 mM Na +, the other, 50 mM. For acetate and succinate transport the optimal Na+ concentrations varied from 30 to 200 mM depending on the substrate and the organism. When the initial rate of transport of glutamate into one of the organisms was plotted against Na+ concentration the reponse curve was sigmoid and a Hill plot of the data indicated that the transport protein may possess three binding sites for Na+. Evidence was obtained indicating that both organisms possess a Na+-stimulated NADH oxidase. The results indicate that there are marine bacteria that grow to a limited extent at appreciably lower concentrations of Na+ than have been realized previously and for these a much more definitive examination of the requirement for Na+ is necessary.Key words: marine bacteria, Na+ requirement, growth, membrane transport, NADH oxidase.