Osmotic Effects on Membrane Permeability in a Marine Bacterium

Abstract

When cells of Alteromonas haloplanktis 214 (ATCC 19855) were preloaded with .alpha.-[14C]aminoisobutyric acid or the K+ in the cells was labeled with 42K by incubation in a buffered salt solution containing 0.05 M MgSO4, 0.01 M KCl and 0.3 M NaCl, the cells retained their radioactivity when resuspended in the same salt solution. When NaCl was omitted from the solution, 80-90% of the radioactivity was lost from the cells. Cells suspended at intermediate concentrations of NaCl also lost radioactivity. New steady-state levels of the intracellular solutes were established within 15 s of suspending the cells; the percentage of radioactivity retained at each level decreased proportionately as the osmolality of the NaCl in the suspending solution decreased. With minor variations in effectiveness, MgCl2, LiCl and sucrose could substitute for NaCl on an equiosmolal basis for the retention of radioactivity by the cells. KCl, RbCl and CsCl were appreciably less effective as replacements for NaCl, particularly when their osmolalities in the suspending solutions were low. The amount of .alpha.[14C]aminoisobutyric acid taken up by the cells at the steady-state level increased to a maximum as the NaCl concentration in the suspending medium increased to 0.3 M. At suboptimal levels of NaCl, LiCl or sucrose could substitute for NaCl in increasing the steady-state levels. The porosity of the cytoplasmic membrane of this organism is apparently determined by the difference between the osmotic pressure of the cytoplasm and the suspending medium. The lesser effectiveness of K+, Rb+ and Cs+ than Na+, Li or Mg2+ in permitting the retention of solutes by the cells is attributed to the greater penetrability of the hydrated ions of the former group through the dilated pores of a stretched cytoplasmic membrane.