How Small Can Life Be?
By Leslie Mullen, Science Communications
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 or weeks.
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 1992.
"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 Folk.
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 nanometers.
"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 interesting."
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 detection."
"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 for them.
"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 biological roles.
"[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."