Effect of growth at low Na+ concentration on the capacity of a marine bacterium to establish ion gradients and transport α-aminoisobutyric acid

Abstract

Cells of a histidine-auxotrophic, streptomycin-resistant mutant of marine bacterium Alteromonas haloplanktis 214 were grown at or near the lowest concentration of Na+ permitting growth (30–33 mM Na+). When suspended in solutions containing 10 mM KCl and either 30, 100, or 300 mM NaCl, the intracellular to extracellular K+ ratios after equilibration were found to be 11.6, 20.8, and 43.2, respectively. The corresponding Na+ ratios were 1.0, 0.46, and 0.62. The K+ ratios were similar to those obtained with cells of the parent organism grown at more nearly optimum Na+ concentrations, whereas the Na+ ratios were somewhat larger. Cells of the parent organism grown at 32 mM Na+ transported α-aminoisobutyric acid (AIB) at only one-third the rate and to less than one-quarter of the extent of cells grown at 130 mM Na+ even when the NaCl concentration during transport was raised to optimum levels. The Km for uptake of AIB by cells grown at 32 and 130 mM Na+ was the same but the Vmax was higher for cells grown at 130 mM. The Vmax for cells grown at both concentrations of Na+ increased as the Na+ concentration in the uptake medium increased. It was concluded that none of the observations made could account for the fact that both parent and mutant of A. haloplanktis grow at 30–32 mM Na+ only after a very long lag period, and then grow at near normal rates once logarithmic growth begins despite the fact that the osmotic pressure of the medium is very low.