Extremely halophilic Archaea from ancient evaporite deposits.

Extremely halophilic Archaea (halobacteria) were isolated from salt deposits of diverse geographical locations and ranging in geological age from Permian (250-260 million years (Ma)) to Recent. The diversity of these halobacteria was established by analyses of polar lipid patterns, protein electrophoresis patterns and ribotype bands; isolates from any one salt deposit were shown to be unique. Those halobacterial groups which predominate in present-day salt lakes and solar salterns, namely Halobacterium, Halorubrum and Haloarcula spp., were also shown to comprise the majority of isolates from ancient salt deposits. The inaccessibility of the deep subsurface mines, the nature of the sampling, and the 'extreme' requirements of these organisms for survival make contamination by contemporary halobacteria from surface sites highly unlikely. It is proposed that (some of) these salt mine isolates are autochthonous - presumably remnant populations derived from ancient hypersaline seas. This thesis aimed to determine whether these organisms are, in fact, 'living fossils' which have been revived. The approach used to test this hypothesis was to use the 16S rRNA gene as a molecular chronometer for the phylogenetic analysis of selected isolates representing most sites, and seek a correlation between phylogenetic position and geological age. This method proved not to be appropriate, possibly because of the natural phylogenetic diversity of the halobacteria. However, phylogenetic analysis has revealed new lineages in salt mines, distinct from known genera. That some of these halobacteria are 'living fossils', which remain locked within the salt in a state of dormancy until revived, is still an open question.