Links to Other ASM Pages:



• Table of Contents
• ASM News Issues

Ancient Bacteria May Be Oldest Life Form

Two views of the crystal from which ancient bacteria were isolated. (Reproduced with permission from Nature.)

Alarm bells ring as microbiologists—some convinced, others skeptical—contemplate the purported longevity of persistently dormant, recently revived, and perhaps extraordinarily ancient microorganisms. Russell Vreeland of West Chester University in Pennsylvania and his colleagues claim to have reawakened very old bacteria from spores inside a 250-million-year-old salt crystal. If the results hold, "then it is the oldest living organism," says Vreeland's collaborator William Rosenzweig. Indeed, the cells from which those spores presumably formed were alive and active before the time of the dinosaurs. "Our plans are to study the bacterium and learn everything it wants to tell us about its physiology, morphology and genetics," adds Vreeland, who is convinced these studies will provide insights into very early life.

However, this study and others like it trigger a great deal of skepticism, some of it still raging following an announcement some five years ago by Raul Cano and colleagues at California Polytechnic State University. They claimed to have revived bacteria from spores within the intact carcass of a bee that was trapped in amber 25 to 30 million years ago. Vreeland's microorganisms derive from spores that he says are as much as 10 times older.

Cano's experiments became associated with a then-recent novel and movie, Jurassic Park, in which ingenious but unscrupulous scientists revived dinosaurs through cloning techniques. "Jurassic Park was neat, but this beats it hands down," says Paul Renne, a geologist at the University of California, Berkeley, referring to Vreeland's revitalized spores. "The idea of having a living glimpse of what life looked like 250 million years ago is pretty spectacular."

Vreeland and his collaborators obtained the controversial spores from intact salt crystals (see figure) found among hundreds of pounds of rock salt retrieved from an underground nuclear waste site near Carlsbad, N.M. The deep-set formation is at least 250 million years old, and is "one of the best if not the best-characterized salt formation in the world," says Vreeland, whose findings are reported in the 19 October issue of Nature (407:897-900). The West Chester University researchers carefully removed spores from a fluid-filled region of one crystal, from which they cultured a Bacillus isolate, designated 2-9-3, in a medium containing 20% NaCl. A second spore isolated from the same fluid and another from fluid in a second crystal have yet to be studied.

Steps taken while the material was extracted from the crystal reduced the chance of contamination with outside microbes to less than 1 in 100 million, Vreeland says. But proof that those techniques were aseptic cannot be absolute, he admits. Indeed, much of the intense debate over these findings revolves around the integrity of the salt crystal and whether its contents were contaminated before Vreeland's extractions began. "The fact that the salt crystal had no clearly visible imperfections does not exclude the trivial explanation of fine cracks permeable to present-day bacteria," says Tomas Lindahl of the Imperial Cancer Research Fund, Hertfordshire, United Kingdom, who is a critic of this and other research claiming to have recovered live, preserved bacteria from ancient substances. "Unlike amber or rocks or permafrost, salt is not an impermeable material," points out Chris McKay, a biologist at the National Aeronautics and Space Agency's Ames Research Center.

However, Vreeland remains confident that the microorganisms he is studying are ancient and that the spores from which they derived were hermetically trapped within the crystal during its formation. Thus, he maintains that what grew out from the spores is not a modern-day bug that entered that crystal recently through some microscopic crack, or during dissolution and reformation of the crystal. "The crystal shows no evidence of having been penetrated or recrystallized since the time of formation," he says. "The three fluid inclusions that we sampled were completely filled with fluid. If they had been penetrated, there would have been air bubbles. [The inclusions] were rectangular indicating there had been no movement of fluid through the matrix, and they did not contain any extraneous material that might have been carried in from the outside."

Lindahl says that gene sequence analysis of the Bacillus provides additional reasons for doubting whether the microorganism derived from the extracted spores is genuinely ancient. Thus, he points out that the DNA sequence encoding the 16S ribosomal RNA (rRNA) of the Bacillus 2-9-3 isolate that Vreeland and his colleagues are studying closely resembles that of Bacillus marismortui, which was isolated in modern times from the Dead Sea. Lindahl says this similarity also suggests that isolate 2-9-3 is a mere contaminant, either the same as, or closely related to, the Dead Sea microbe.

Vreeland says that this genetic similarity between the two microbes is no surprise to him. The close overlap in rRNA sequences between the two reflects the vital function of that gene and the fact that its structure was maintained is consistent with the two bacteria adapting to similarly salty environments. "Much of our collective desire to see differences (in sequences) is based on the idea that the nucleic acid is a `clock' that changes at a `known' rate of 1% every 50 million years," he says. "But the clock is based on modern organisms looking backward, so maybe the calibration is off. Alternately, the Dead Sea formed 6-8 million years ago. If B. marismortui was in salt itself until then and released at that time, then the `clock' is right on the money."

Resolution of the debate could hinge on what researchers find when they probe other salt deposits for microbes. For now, the possibility that a 250-million-year-old life form retained its vitality is causing intense excitement in some circles. "Their results are the best evidence yet for the extremely long-term survival of microorganisms," says John Parkes, Department of Earth Sciences at the University of Bristol, United Kingdom. "If bacteria can survive for this length of time, why should they die at all?"

Brian Hoyle