SCIENCE OBSERVER
Rock of Ages
Christopher Brodie
What's the oldest living thing in the world? Well, 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.
And a creosote bush in the California desert clocks in at around
12,000 years. The King's lomatia plant in Tasmania is estimated to
be at least 44,000 years old and it's still growing (although it
doesn't seem to be able to reproduce sexually). Yet these venerable
organisms are infants, day-old newborns compared with a living
bacterium that may have been locked inside a salt crystal for the
past 250 million years.
In 2000, a trio of scientists led by Russell H. Vreeland at West
Chester University of Pennsylvania described in the journal
Nature the isolation of a dormant, yet living, bacterium.
The microbe came from pockets, or inclusions, of fluid trapped
inside 250-million-year-old salt crystals buried half a kilometer
deep in New Mexico. They gave it the unprepossessing name of
Bacillus (later, Virgibacillus) species 2-9-3.
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).
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.
Yet Vreeland and an expanding circle of collaborators have followed
up the original report with publications that seek to counter each
of these criticisms. In 2002, he and two West Chester University
colleagues reported in the International Journal of Radiation
Biology 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. Scratch objection number 1.
In April of 2005, the three authors from the Nature paper
teamed with Tim K. Lowenstein, a geologist at Binghamton University
in New York, and his student, Cindy L. Satterfield, in publishing a
detailed report in the journal Geology. To test the idea
put forth by critics that inclusions in the salt crystals were newer
than the surrounding rock, they measured the temperature of original
crystallization for samples from the same part of the Salado
Formation that yielded Virgibacillus sp. 2-9-3. The team
reasoned that if microbe-carrying fluid had recently reached the
deeply buried salt deposit and recrystallized, the temperatures of
those crystallizations would be similar. Instead, they found the
opposite: The results ranged from 17 to 37 degrees Celsius, or about
63 to 99 degrees Fahrenheit, a distribution that suggests seasonal
climatic variation. In other words, the crystals that formed around
pockets of fluid (and presumably bacteria) were created on or near
the surface instead of far underground.
A second, more definitive, line of investigation examined the
concentrations of various ions in the fluid inclusions. The balance
of ions in seawater changes over geological time, so measuring them
can provide an approximate date at which the saltwater crystallized.
The ion concentrations in the halite inclusions matched those of
oceans in the Permian period—a profile that is distinct from
the seawater of today and also from larger pockets of trapped brine
elsewhere in the Salado. As a final test, the team plans to use an
ultrasensitive mass spectrometer to date tiny, individual inclusions
by the rubidium-strontium method (87Rb decays into
87Sr with a half-life of 49 billion years). Scratch
objection number 2.
The third criticism, based on DNA similarities, has been harder to
dismiss. Despite a protocol of sterilization and controls that even
critics describe as "heroic," contamination remained a
potential source of the 2-9-3 bacterium based on its molecular
resemblance to current strains. Understandably, Vreeland defends the
work against charges of contamination. He even views the genetic
objections as the least valid, stating that of all the challenges
(geologic, chemical and genetic), "this is by far the weakest
of the critiques."
Although Vreeland admits that the DNA evidence classifies
Virgibacillus species 2-9-3 as one strain of another
bacterium isolated from the Dead Sea, he challenges the models that
predict that these strains diverged tens of thousands, not millions,
of years ago. In summarizing a 2002 letter to the journal
Molecular Biology and Evolution, Vreeland explained
that such models calculate a phylogenetic history based on specific
generation and mutation rates, but "you can pick whatever rate
you want." Instead of an intergenerational interval of days or
months, he cites evidence that some spore-forming organisms can lie
dormant for hundreds or thousands of years between growth episodes.
Thus, he suggests that as the 2-9-3 strain lay locked in the salt,
its relatives might have grown—and evolved—far less than
critics believe.
Skepticism lingers—even among some of Vreeland's
collaborators—but that hasn't prevented them from conducting
further studies in this highly visible area of science. And of
course, they may yet be persuaded by new data. In a paper published
online August 30, 2005 in the journal Extremophiles,
Vreeland presented evidence that four strains of Permian microbes
(2-9-3 and three others that were found later) are different enough
from modern relatives in a number of categories that they could not
arise from contamination.
Many questions remain, as Vreeland readily admits. Wouldn't organic
molecules crucial to life, including DNA, spontaneously degrade in
250 million years even in the absence of ionizing radiation?
Vreeland responds, "That is something we have to look at."
Do the older, nonviable inclusion pockets contain the remains of
expired microbes? Says Vreeland, "I can't answer that."
And, perhaps most interestingly of all, what is the mechanism? How
do these ancient organisms manage to survive so long? His answer:
"I have no idea how that happens."
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