Mechanism of Chloride Transport in Halophilic Archaebacteria

Abstract

The halophilic archaebacteria, i.e. the various Halohacterium and Natronobacterium species (and possibly others), accumulate K CI rather than organic compounds to balance the high osmotic pressure of the nearly saturated NaCI solution which constitutes their environment. Because of this, chloride ions present a special bioenergetic problem to them, which is not normally of great importance in other organisms. The problem is that while the uptake and retention of the positively charged K ions are consistent with the inside negative membrane potential across the cytoplasmic membrane, the negatively charged chloride ions will be ejected rather than accumulated by any passive permeability pathway. Thus, to reach the several molar intracellular concentrations of K CI required, chloride has to be transported into the cells by an active mechanism, i.e. one which is capable of “uphill” transport, against the prevailing direction of the electrochemical potential of chloride.