Organisms from extreme environments have adapted to multiple stress factors induced by the local environment. The use of micro-:organisms from these environments can provide novel insights into cell and macromolecule function. Two such studies are presented in this thesis. . Firstly the tolerance of elevated magnesium chloride concentrations by the extreme halophile, Haloferax volcanii, which was isolated from the Dead Sea. The saturating salt conditions expose the Archaeon to 2.0 M MgCh and 2.1 M NaCI. The resultant osmotic pressure across the cell membrane is stabilised by saline equivalence in the cytoplasm. The role of the magnesium transport protein CorA is investigated for its role in magnesium homeostasis through the construction of a corA- knockout mutant and the development of a method to measure the intracellular Mg2+ concentrations ofH.fx. volcanii cells. Additionally the crystal structure of alanine dehydrogenase from the marine psychrophile Shewanella PA43 is compared against the model of the mesophilic enzyme from Vibrio proteolyticus, which shows high amino acid identity (77%). In this manner the comparison may highlight reasons for individual substitution events between the two enzymes that cannot be explained by current theories on protein cold adaptation. This type of analysis permits the assignment of functions for the presence/absence of specific amino acids which can be overlooked by gene alignments.