Novel insights into the role of potassium for osmoregulation in Halomonas elongata

Abstract.

The role of K⁺ in osmoregulation of the halophilic bacterium Halomonas elongata was investigated. At lower salinities (0.51 M NaCl), K⁺ was the predominant cytoplasmic solute (1.25 µmol mg protein^–1). At higher salinities (1.03 M NaCl) ectoine became the main cytoplasmic solute (1.57 µmol mg protein^–1), while the K⁺ content remained unchanged. In response to osmotic upshock, cells of H. elongata simultaneously accumulated ectoine and K⁺ glutamate. The ectoine and K⁺ glutamate levels in osmotically stressed cells exceeded the level of cells adapted to high salinities. The increase in K⁺ glutamate was long lasting (>120 min) and not transient, as described for non-halophiles. Regulation of the synthesis of ectoine and glutamate was proven to occur mainly at the level of enzyme activity. Limitation of K⁺ inhibited the growth of salt-adapted H. elongata cells, especially at high salinities, and caused a decrease of the intracellular organic solute content, inhibition of respiration, and an abolition of the cell's ability to respond to osmotic stress. The saturation constant K _S for K⁺ was estimated to be 105 µM at a salinity of 0.51 M NaCl, indicating that an uptake system of medium affinity is responsible for K⁺ accumulation in H. elongata.