Halobacteria from Ancient Salt : Evidence for Living Fossils
William GRANT*a, Renia GEMMELLb, and Terry MCGENITYc
a Department of Microbiology and Immunology, University of Leicester, PO Box 138, Leicester LE1 9HN, UK
b Institute of Virology and Environmental Microbiology, Mansfield Road, Oxford OX 1 3SR, UK
c Postgraduate Research Institute for Sedimentology, The University of Reading, Whiteknights, Reading RG6 6AH, UK
There are a series of accounts stretching back nearly a century, of the supposed long-term survival of microorganisms, usually bacteria, in a range of environments that are believed to be of ancient origins, uncontaminated by present-day activities. Ancient salt deposits (evaporites) are claimed to harbour microorganisms derived from the original populations that inhabited these sites when they were salt lakes many millions of years ago. In recent years we have shown that ancient evaporite deposits do indeed harbour populations of halophilic archaea of types similar to those populating present day salt lakes (1). We have previously established that halobacteria become entrapped within the fluid inclusions found within halite crystals grown in the laboratory (2) providing a conceptual basis with which to understand how halophilic archaea are to be found within crystals of ancient primary salt. We have embarked on sequence analyses of 16S rRNA genes from halophilic archaea isolated from evaporites to establish whether the phylogenetic position of these isolates is consistent with ancient origins. Members of one group of halophilic archaea (the haloarculas) also, unusually, possess two or more dissimilar 16S rRNA genes (3) offering an alternative strategy that may shed light on the longevity of ancient evaporite isolates. If the gene multiplicity is due to gene duplication in the past, then fewer differences would be expected between a haloarcula revived from dormancy after millions of years as compared to an isolate that has had time to evolve and mutate the genes. If the duplication is due to gene transfer, then crossover mechanisms would reduce gene differences between gene sets with time. We have compared dissimilar 16S rRNA gene sequences in haloarculas from ancient evaporites with contemporary surface isolates. Key questions about the rate of evolutionary change could be answered by revived microorganisms or the recovery of ancient DNA. In addition, planets like Mars are likely to have evaporite deposits where traces of life might exist, protected from the currently inimicable environment. Our experience with terrestrial deposits will inform the search for life on other planets.
1. Norton, C.F., McGenity, T.J., and Grant, W.D. (1993) J. Gen. Microbiol., 139, 1077-1081.
2. Norton, C.E and Grant, W.D. (1988) J. Gen. Microbiol., 134, 1365-1373.
3. Mylvaganam, S. and Dennis, P.P. (1992) Genetics, 130, 399-410.
