Abstract
Live bacteria and archaea have been isolated from several rock salt deposits of up to hundreds of millions of years of age from all around the world. A key factor affecting their longevity is the ability to keep their genomic DNA intact, for which efficient repair mechanisms are needed. Polyploid microbes are known to have an increased resistance towards mutations and DNA damage, and it has been suggested that microbes from deeply buried rock salt would carry several copies of their genomes. Here, cultivable halophilic microbes were isolated from a surface sterilized middle-late Eocene (38–41 million years ago) rock salt sample, drilled from the depth of 800 m at Yunying salt mine, China. Eight unique isolates were obtained, which represented two haloarchaeal genera, Halobacterium and Halolamina. We used real-time PCR to show that our isolates are polyploid, with genome copy numbers of 11–14 genomes per cell in exponential growth phase. The ploidy level was slightly downregulated in stationary growth phase, but the cells still had an average genome copy number of 6–8. The polyploidy of halophilic archaea living in ancient rock salt might be a factor explaining how these organisms are able to overcome the challenge of prolonged survival during their entombment.
Highlights
Cultivable bacteria and archaea have been isolated from rock salt up to hundreds of millions of years of age (MYA), representing species of Haloarcula, Halobacterium, Halococcus, Haloterrigena, Natronobacterium, Natronomonas, and Virgibacillus [1,2,3,4,5,6,7,8,9]
Our results indicate that polyploidy is common in buried halophilic archaea, and possibly helps them preserve their genomes in a functional state for a prolonged time
The age and origin of these isolates have been debated [20,57,58]. If these microbes have been buried inside halite deposits during sedimentation, the oldest viable cells known would be from Permian era, around 225–280 MYA [1,4,5]
Summary
Cultivable bacteria and archaea have been isolated from rock salt up to hundreds of millions of years of age (MYA), representing species of Haloarcula, Halobacterium, Halococcus, Haloterrigena, Natronobacterium, Natronomonas, and Virgibacillus [1,2,3,4,5,6,7,8,9]. These findings have inspired discussion about the theoretical maximum age an organism can reach, and their possible tactics for survival [10,11,12]. Dating of liquid inclusion brine has provided substantial evidence of the age of the isolates [18,19]
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