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

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 mumol mg protein(-1)). At higher salinities (1.03 M NaCl) ectoine became the main cytoplasmic solute (1.57 mumol 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 nonhalophiles. 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 muM at a salinity of 0.51 M NaCl, indicating that an uptake system of medium affinity is responsible for K+ accumulation in H. elongata.