A Single Cell Living 250 Million Years?
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CELL BIOLOGY: A SINGLE CELL LIVING 250 MILLION YEARS?
The following points are made by Christopher R. Brodie (American Scientist 2005 Nov-Dec):
1) What is the oldest living thing on Earth? A Madagascar radiated tortoise given to the Tongan royal family by Captain James Cook in the 1770s was at least 188 years old when it died in 1965. A creosote bush in the California desert has been dated at approximately 12,000 years. The King's lomatia plant in Tasmania is estimated to be at least 44,000 years old and is still growing (although it does not seem to be able to reproduce sexually). But the age of these organisms may be dwarfed by the apparent age of a recently discovered living bacterium: 250 million years.
2) In 2000, a research team described the isolation of a dormant but living bacterium from pockets (inclusions) of fluid trapped inside 250-million-year-old salt crystals buried half a kilometer deep in New Mexico. The bacterium was named Bacillus (later, Virgibacillus) species 2-9-3, and the report elicited strong skepticism from many quarters. Biological chemists doubted that nucleic acids could remain pristine over such time periods. Even had the bacterium hibernated as a hardy spore, its DNA surely would have broken down over 250,000 millennia, if not from the barrage of ultraviolet light during its long-ago residence on the surface, then from naturally occurring terrestrial radiation over the Earth's evolution. Geologists questioned the age of the fluid inclusions, arguing that certain features of the Salado Formation (the source of the halite crystal) suggested that flaws in the rock had permitted the intrusion of more recent fluid (which, by inference, had carried more recent bacteria into the ancient rock).
3) Geneticists pointed out that one of the bellwether genes that the group had sequenced -- one that encodes the so-called 16S ribosomal subunit -- was far too similar to its counterpart in another strain of bacteria. According to this critique, either the "ancient" bacterium was actually a contaminant, or its descendants had inexplicably failed to change in the past 250 million years.
4) But the research group that discovered the bacterium has followed up the original report with publications that seek to counter each of these criticisms. In 2002, the group reported their calculation that the degree of genetic damage caused by normal traces of radioactive potassium-40 in the surrounding rock was not great enough to rule out a quarter-billion years of bacterial survival. In April of 2005, an enlarged group of researchers reported a test of the idea that inclusions in the salt crystals were newer than the surrounding rock. The results suggest the crystals that formed around pockets of fluid (and presumably bacteria) were created on or near the surface instead of far underground.
5) Many questions remain. Would not organic molecules crucial to life, including DNA, spontaneously degrade in 250 million years even in the absence of ionizing radiation? Do the older and nonviable inclusion pockets contain the remains of expired microbes? And, perhaps most interestingly of all, what is the mechanism? How do these ancient organisms manage to survive so long?[1-3]
- Vreeland RH, Rosenzweig WD, Lowenstein T, Satterfield C, Ventosa A. Fatty acid and DNA analyses of Permian bacteria isolated from ancient salt crystals reveal differences with their modern relatives. Extremophiles. 2005 Aug 30
- Vreeland RH, Straight S, Krammes J, Dougherty K, Rosenzweig WD, Kamekura M. Halosimplex carlsbadense gen. nov., sp. nov., a unique halophilic archaeon, with three 16S rRNA genes, that grows only in defined medium with glycerol and acetate or pyruvate. Extremophiles. 2002 Dec;6(6):445-52.
- Maughan H, Birky CW Jr, Nicholson WL, Rosenzweig WD, Vreeland RH. The paradox of the "ancient" bacterium which contains "modern" protein-coding genes. Mol Biol Evol. 2002 19(9):1637-9
Christopher R. Brodie and American Scientist http://www.americanscientist.org