[meteorite-list] How Small Can Life Be?
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:32 2004
How Small Can Life Be?
By Leslie Mullen, Science Communications
NASA Astrobiology Institute
As advanced microscopes enable us to peer deeper into the realms of inner
space, biologists have been faced with a vexing question: Is there a size
limit on life? If so, then just how small can something be before it can no
longer be defined as "life"?
Some scientists believe that life can be very small indeed. Called nanobes,
nanobacteria, or nano-organisms, these miniscule structures borrow their
name from their unit of measurement, the nanometer. A nanometer is one
billionth of a meter. That's about the length of 10 hydrogen atoms laid out
side by side. The period at the end of this sentence is approximately one
million nanometers in diameter.
While the tiniest bacteria measure 200 nanometers across, nanobes are even
smaller. They can range anywhere from 20 to 150 nanometers long.
What first caught the attention of some scientists was the way nanobes are
shaped. They look remarkably like bacteria, forming spheres, chains of
beads, filaments, or bean- or sausage-like shapes.
Nanobes seem to share other important qualities with bacteria. For one
thing, nanobes are often found grouped together in clusters. Also, some
scientists claim they can grow colonies of nanobes by culturing them in the
lab. The nanobes seem to spontaneously grow on metal, glass, plastic or
organic surfaces which are left in water or exposed to oxygen for a few days
One scientist who firmly believes that nanobes are alive is Robert Folk of
the University of Texas at Austin. In 1990, Folk discovered bacteria-like
structures about 100 nanometers in size in Italian hot-spring deposits. He
went public about his findings at a Geological Society of America meeting in
"NASA scientist Chris Romanek heard my talk, and decided to look for nanobes
in the Martian meteorite," says Folk. "He found them, and the rest is
History - or Scandal, if you prefer."
The discovery of nanometer-sized shapes in the Martian meteorite ALH84001
was reported in 1996 in headlines all around the world. The researchers
studying the 4.5-billion-year-old meteorite said the shapes, which measured
20 to 200 nanometers across, were the fossils of Martian microbes. Was this
definitive proof of ancient life on Mars - or were the structures too small
to be considered "life"?
To answer this question, NASA asked the National Research Council of the
National Academy of Sciences to convene an expert panel. It met in late 1998
and published the report, "Size Limits of Very Small Micro-organisms."
An organism able to live and reproduce on its own needs certain equipment to
accomplish these tasks - and that equipment takes up space. For instance, a
single ribosome, a tiny factory that cells use to make proteins, is usually
25 to 30 nanometers wide. A typical modern cell can house several hundred
thousand ribosomes. Based on such life requirements, the 18 experts on the
panel concluded that 200 nanometers probably marked the lowest size limit.
In other words, anything smaller than 200 nanometers could not be considered
"life" as we know it.
"Several lines of evidence suggest that the volume of a sphere about 200
nanometers across is needed to house the chemistry of a cell that has a
biology familiar to us," says Andrew Knoll, paleobiologist from Harvard
University, member of the NASA Astrobiology Institute, and one of the
editors of the report. "As long as molecules have volume there will be a
lower limit to organism size."
However, the panel also said it was possible that primitive microbes could
once have been as small as 50 nanometers in diameter.
"Simpler forms of life are conceivable and probably existed early in the
history of life," says Knoll. "One might envision a simple cell with only
one class of informational macromolecule that would fit into a 50 nanometer
sphere. The key, of course, is making the distinction between cells of
familiar biochemistry and cells that may exist but for which we have no
direct observational knowledge."
Folk disagrees with the determination of the panel, however. He believes the
nanobe structures, which he says commonly range between 50 and 100
nanometers across, are viable life forms.
"The limit adopted by biologists is 200 to 250 nanometers on the basis that
[the structures] must be large enough to contain a DNA or RNA strand, and
have the ribosomes, etc., necessary to carry on metabolism," says Folk. "My
opinion is that scientists do not know enough to set arbitrary limits on
life. After all, pre-Pasteur, nobody even thought there were things such as
germs, and pre-1890 nobody knew there were viruses."
Folk, for one, believes the nanobes in the Martian meteorite are definitely
fossil structures indicative of past life. He also believes similar life
forms are present in the Martian meteorite Dhofar 019, as well as the
non-Martian Allende and Murchison carbonaceous meteorites.
But Knoll says he knows of no reason to insist that the structures found in
Martian meteorite ALH 84001 are fossils of ancient Martian life. He says the
problem with these structures is not that they are too small, but rather
that there is no way to tell if the structures themselves are specifically
"life." Instead, they could be any number of other structures that naturally
form through non-biological processes.
"The size argument is a red herring if we don't know the relevant
biochemistry," says Knoll. "The problem is that the structures in question
are not diagnostically biological."
Proof of Life?
Four years ago, scientists at the University of Queensland discovered
nanobes in ancient Australian sandstones. Although some of the structures
were as small as 20 nanometers across, the fuzzy tangles of filaments looked
a lot like fungi. They also appeared to reproduce quickly, spontaneously
forming dense colonies of tendrils, on Petri dishes that were exposed to
oxygen and kept at 22 degrees Celsius (72 F). Laboratory analysis of the
filaments repeatedly found signs of DNA (deoxyribonucleic acid).
According to Philippa J.R. Uwins, one of the lead scientists of the
Queensland team, all the nanobes they discovered seem to have the enzymatic
and genetic material considered essential for life.
Folk believes this research should have made other scientists accept the
idea that life could be smaller than previously thought.
"Uwins, of course, should have broken the life-barrier for biologists," says
But Knoll doesn't find this example of possible nanometer-sized life to be
especially compelling. Although the Queensland structures stain positively
for DNA, Knoll says there are other substances that can give a "false
positive" for DNA.
If nanobes are ever proven to be alive, they would challenge our
understanding of life on Earth. Based on everything we know about biology,
it does not seem possible for modern living organisms to be smaller than 200
"If current nanobes can be shown to be living entities, then Earth harbors
life forms whose chemistry we do not understand," says Knoll. "That would be
Although such a revelation would change our comprehension of life, Knoll
doesn't think it would dramatically affect astrobiology.
"We already acknowledge that unfamiliar life is possible," says Knoll. "I
don't think that it would change the philosophy or search strategy for life
"Until more advanced forms are discovered, nanobacteria are astrobiology,"
says Folk. "Nanobacteria are the primordial life form on Earth, as well."
Since his discovery of nanobes in Italian hot spring deposits, Folk says he
has found nanobes in such things as bird bath scum, decayed leaves in
streams, brownish water from old flower bouquets, air filters, tap and well
water, hair, feces, blood, gallstones, chicken egg shells, clam shells, and
teeth. He says that nanobes are virtually everywhere - one only need look
"I would say, cattily, that those who say NO [to the existence of nanobes]
simply have not looked for small life forms," says Folk. "All those who have
looked, have found them. Over half a dozen labs have succeeded in culturing
colonies of organisms of this minute size, and some of these labs have
succeeded in obtaining DNA, detecting the organic chemistry of living
tissue, and even revealing structure of cell walls or membranes."
Despite such assertions, Knoll maintains that those who insist nanobes are
alive have yet to prove their claims.
"No one has as yet convinced a skeptical microbiological community that the
very small structures under discussion are living entities fully capable of
self-replication," says Knoll. "Or that if they are, what novel biochemistry
makes this possible."
While the issue of nanobes continues to be debated, the Queensland group is
trying to determine the exact nature of nanobe genetic material. They also
plan to analyze the growth rates of their nanobe cultures.
Folk, meanwhile, is working hard to prove that the structures are widespread
in nature. He is currently studying both modern and ancient rocks and
minerals, as well as samples of Martian meteors, for evidence of nanobes.
Folk is also conducting studies to see how nanobes may play important
"[I'm studying] the possible role of nanobacteria in symbiotically
precipitating hard parts of organisms, from clam shells to dinosaur teeth,"
says Folk. "Also, in a joint project with the Mayo clinic, [I'm conducting]
an intense study with Dr. Brenda Kirkland on [the role of nanobes in] human
arterial plaque and diseased heart tissue."
Received on Mon 16 Jul 2001 01:46:25 PM PDT