The divalent cation requirement of Dead Sea halobacteria

Pleomorphic Halobacterium strains isolated from the Dead Sea (H. volcanii, H. marismortui) require high concentrations of divalent cations (75 mM Mg²⁺) for growth. When suspended in medium containing less than 50 mM Mg²⁺ cells lose their native shape within minutes and become spherical. This occurs even at elevated sodium chloride concentrations. Concomitant with the morphological changes, a high mlecular weight component which is positive in Coomassie Brilliant Blue and in periodate Schiff stain is released into the surrounding medium. At divalent cation concentrations lower than 100 mM magnesium cells were shown to lose their viability and their ability to incorporate amino acids. The potency of different divalent cations or their combinations to enable growth and stabilize morphology and viability was studied. It is suggested that different mechanisms underlie the divalent cation requirement of the different functions.