By WILLIAM J. BROAD
reepy things from beneath the sea are clichés of modern exploration, but the abyss has now produced a surprise so bizarre as to have touched off hot international debate: tiny, mysterious, apparently living creatures far smaller than any known bacterium -- so small as to strain the limits on what is needed for independent life.
Dr. Philippa Uwins
| An electron micrograph shows filaments projecting from sandstone. The filaments are called nanobes because of their size, 20 to 150 nanometers, smaller than cells. |
Their discoverers call them nanobes (pronounced NAN-obes), because their size is in the realm of nanometers (or billionths of a meter). At 20 to 150 nanometers in length, they are smaller than cells, smaller than fungi, smaller than the smallest known bacterium and roughly the same size as viruses, which are considered nonliving parasites because they need hosts to reproduce. Thus, nanobes call into question the minimum size requirements for terrestrial life.
If alive, the find bears on the pervasiveness of terrestrial life, new forms of which are being discovered so deep in such abundance that some scientists suspect the planet has a hidden biosphere of microbes extending down miles whose total mass may exceed that of all surface life. The find might also influence hunts for microscopic aliens perhaps hidden in the solar system's deep netherworlds, the discovery of which would prove that life in the universe is not unique to Earth but an inherent property of matter.
Scientists in Australia at the University of Queensland discovered the tiny oddities four years ago in ancient sandstones retrieved from an oil drilling site some three miles below the Western Australian seabed. Described publicly for the first time in late 1998, the fuzzy tangles of filaments resembled fungi and appeared to reproduce quickly, forming dense colonies of tendrils. Laboratory analysis of them repeatedly found signs of DNA, or deoxyribonucleic acid, the master molecule of heredity and life.
"Our recent work provides further evidence" that the tangles are in fact alive, said Dr. Philippa J. R. Uwins, a scientist at the University of Queensland's center for Microscopy and Microanalysis, who leads the research. "They're extraordinary and incredibly pretty, the way they grow between the minerals. I'm fascinated by them."
Dr. Uwins's team has written two new papers, one being reviewed for publication and the other just submitted, that describe the most recent evidence and experiments. So far, the main findings are that the colonies grow spontaneously, contain DNA, are rich in biological elements like carbon, oxygen and nitrogen, and when cut in two show distinct outer and inner layers, including a possible nuclear area that holds DNA.
In a bid for skeptical rigor, the team has sought nonbiological explanations but has concluded that none can account for the observations. Its future research goals include quantifying the growth rate of the colonies and sequencing their DNA, which could help connect them to the known tree of terrestrial life.
It might seem that firm knowledge about the minimal size requirements for life would be old stuff. After all, it was several centuries ago that Antony van Leeuwenhoek first opened human eyes to the invisible world of microscopic life, describing a riot of "wee animalcules." But it turns out that the lower limits of life are still a biological mystery and their elucidation has only recently become a popular scientific objective.
The topic exploded in debate four years ago when scientists reported finding tiny fossil microbes in a 4.5-billion-year-old, potato-size Martian meteorite that crashed to earth in Antarctica long ago. At 20 to 200 nanometers, the putative Martian fossils were smaller than any known terrestrial life, and that discrepancy quickly sowed doubts about the veracity of the alien-life discovery.
Since then, many papers have tried to prove the existence of tiny terrestrial analogs and, by extension, the plausibility of the Martian fossils. The Australian team's paper is one of the genre's most recent entries.
To help sort through the rush of claims, the National Aeronautics and Space Administration asked the National Research Council of the National Academy of Sciences to convene an expert panel. It met in late 1998 and recently published a 148-page report, "Size Limits of Very Small Micro-organisms."
The 18 experts said that known terrestrial bacteria in the range of 200 nanometers probably marked the lower size limit for current life, but held out the possibility that primitive unknown microbes might have been as small as 50 nanometers, about the size of the Australian nanobes.
"A tremendous number of papers are coming out," said Dr. John A. Baross, a biologist at the University of Washington in Seattle who was on the panel's steering group.
"But there's no way a free-living cell is going to be less than 100 nanometers," he said, noting that such lilliputian sizes would seemingly leave too little room for the enzymatic and genetic machinery essential for life. For instance, a single ribosome, a kind of tiny factory that cells use in great numbers to make proteins, could fill a membrane sphere 50 to 60 nanometers wide.
Dr. Norman R. Pace, a microbiologist at the University of Colorado and another member of the research council's steering group, agreed that nanobes were highly unlikely to be alive. Their lower size limit of 20 nanometers, he noted, was about the width of 10 DNA molecules, making them too small to support all the other needed cellular machinery. "I don't think much of it," he said of the nanobe claim.
The rough 100-nanometer size limit for living things, Dr. Baross of the University of Washington noted, "doesn't rule out biological entities that are 20 or 30 or 50 nanometers wide. They're just not going to be free-living and self-replicating. We don't know if these things might exist and have some function. They might be signaling or doing all sorts of novel things."
The Australian nanobes, he added, might well represent such components of life, perhaps working together in a primordial kind of communalism only now coming to light and soon to rewrite the textbooks.
"They can't be anything like the traditional micro-organism we know about," he said of the nanobes. "We have to think about them in a different way, and one is that they are components" that function as a living organism only in totality, the whole being greater than the sum of the parts.
"We're very interested in whether these small things are biological in nature," Dr. Baross said, "and, if so, what their function is."
In the research council's report, Dr. Jeffrey G. Lawrence, a biologist at the University of Pittsburgh, laid out a detailed analysis of such hypothetical communal life made up of extraordinarily tiny components, calling the aggregate a meta-cell.
"Such organisms need not maintain a full complement of genes," Dr. Lawrence wrote, referring to the basic unit of heredity. His own computer simulations, he added, predicted the existence of stable meta-cell components that held as few as a single gene.
"One may consider the meta-cell to be a single-celled organism whose genome is distributed through a network," Dr. Lawrence wrote.
In contrast to a hypothetical meta-cell component bearing a single gene, the smallest known living bacterium has 470 genes, making the theoretical entity seem quite puny.
Dr. Jack W. Szostak, a geneticist at Harvard, said in the research council's report that certain terrestrial environments like compressed sediments might be conducive to the evolution of very small whole organisms of about 50 nanometers.
"Given present uncertainties," he wrote, "it seems wise to be prepared to detect life-forms of a wide range of sizes."
In Australia, Dr. Uwins stumbled on her nanobes while examining just the kind of compressed sediments that Dr. Szostak proposed as a possible realm of diminutive life. The sandstone samples came from deep petroleum exploration wells off western Australia.
Today, Dr. Uwins and her Australian colleagues Dr. Richard I. Webb, Dr. Anthony P. Taylor and Dr. Thomas Loy are working hard to cut through the theorizing by proving the existence of extremely small life. The team's main tool is a Jeol 890 scanning electron microscope, an instrument that is able to magnify objects nearly one million times. By contrast, most electron microscopes have magnifications less than half as strong.
"That's an extreme speciality machine, very expensive and very powerful," said Steve Hamilton of Jeol U.S.A., in Peabody, Mass. "There are only two or three in the United States."
Stunning electron micrographs of the nanobes, printed in the November-December 1998 issue of American Mineralogist, a journal of the Mineralogical Society of America, show riots of filaments and tendrils, their ends often swollen and suggestive of reproductive budding. Constrictions along some of the filaments "most likely represent septa," or cavities within the nanobes, the team wrote.
Dissective cuts through the axes of some of the filaments "demonstrate that nanobes have an amorphous membrane structure," the team wrote. Such an outer covering, the scientists added, "is consistent with biological material and excludes the presence of crystalline mineral compounds."
The colonies of nanobes grew so rapidly and so large, the team reported in American Mineralogist, that within weeks of becoming established on growth substrates they became visible to the naked eye, appearing as dense colonies of opaque, white, brown or gray filaments.
In trying to nail down the life issue, the team treated the nanobes with three kinds of DNA stains, in each case getting positive results.
In its new research, the team is doing molecular and structural analyses to see if the organisms are related to bacteria or fungi, or belong to a different evolutionary tree altogether. New photomicrographs have revealed details of nanobe interiors, have pinpointed areas rich in DNA and have found "a whole range of interesting morphologies that look like life-cycle stages in fungi," Dr. Uwins said in an interview. "They're striking."
The researchers say they are increasingly confident that their investigations are going to bolster the Martian meteorite find.
Today, a top goal of the Mars exploration program is to see if life started on Mars early in its history, with experts eager to look for Martian microbes and microbial fossils.
Dr. Baross of the University of Washington said the kind of ferment now churning in biology would aid the hunt.
"It's developing the tools and attracting really smart people into this field," he said. "Most of us in microbiology are prepared for any kind of surprise," including organisms smaller than expected theoretically and tiny alien life.
"Dogma in microbiology is out the window in the past few years," Dr. Baross added. "The field has rediscovered itself. It's essentially a new science."