Recovery of 16S ribosomal RNA gene fragments from ancient halite

During the last decade, sensitive techniques for detecting DNA have been successfully applied to archaeological and other samples that were a few hundred to a few thousand years old(1,2). Nevertheless, there is still controversy and doubt over claims of exceptionally ancient DNA(3). Additional accounts stretching back nearly a century suggest that microorganisms may survive over geological time in evaporite deposits(4,5). There is, however, often doubt over the age relationship between evaporite formation and the incorporation of microorganisms(6). Here, we have used petrographic and geochemical techniques (laser ablation microprobe-inductively coupled plasma-mass spectrometry) to verify the estimated geological age of halite (NaCl) evaporite samples. Fragments of 16S ribosomal RNA genes were detected by polymerase chain reaction amplification of DNA extracted from halite samples ranging in age from 11 to 425 Myr (millions of years). Haloarchaeal 16S rDNA amplicons were present in one sample (11-16 Myr), whereas other samples (65-425 Myr) yielded only bacterial 16S rDNA amplicons. Terminal restriction fragment length polymorphism analyses indicate complex and different populations of microorganisms or their free DNA in ancient halites of different ages.