250 million year-old bacteria shows INTRASpecies molecular “clock” has stopped

Paradox of Ancient Bacterium

The isolation of microorganisms from ancient materials and the verification that they are as old as the materials from which they were isolated continue to be areas of controversy. Almost without exception, bacteria isolated from ancient material have proven to closely resemble modern bacteria at both morphological and molecular levels. This fact has historically been used by critics to argue that these isolates are not ancient but are modern contaminants introduced either naturally after formation of the surrounding material (for further details, see Hazen and Roeder 2001<$REFLINK> and the reply by Powers, Vreeland, and Rosenzweig 2001<$REFLINK> ) or because of flaws in the methodology of sample isolation (reviewed recently in Vreeland and Rosenzweig 2002<$REFLINK> ). Such criticism has been addressed experimentally by the development of highly rigorous protocols for sample selection, surface sterilization, and contamination detection and control procedures. Using the most scrupulous and well-documented sampling procedures and contamination-protection techniques reported to date, Vreeland, Rosenzweig, and Powers (2000)<$REFLINK> reported the isolation of a sporeforming bacterium, Bacillus strain 2-9-3, from a brine inclusion within a halite crystal recovered from the 250-Myr-old Permian Salado Formation in Carlsbad, NM.

The evidence presented here clearly indicates that isolate 2-9-3 should be considered a strain of S. marismortui under the established standards of 16S rRNA systematics, which state that isolates sharing >97% identity should be considered as the same species (Stackebrandt and Goebel 1994<$REFLINK> ). But does such a close relationship to modern bacteria mean that isolate 2-9-3 is itself modern? The answer to this question must be sought by resolving what appears to be an increasingly common paradox. We have a large set of rigorous geological and microbiological data which can be interpreted in favor of the antiquity of these organisms, and an equally large set of rigorously obtained molecular data which can be interpreted in favor of their modernity. As it stands, our present molecular work can neither confirm nor disprove the age of isolate 2-9-3.