MINUSCULE filamentous structures found in sandstone deep beneath the seabed
off Western Australia are the smallest organisms ever discovered, claim
geologists at the University of Queensland in Brisbane. But other experts are
withholding judgment until the team provides more evidence that the structures
really are alive.
Philippa Uwins and her colleagues call the filaments 鈥渘anobes鈥�. They are
between 20 to 150 nanometres in diameter. If they are alive, they would prove
that the cellular machinery of life can squeeze into tighter spaces than was
thought possible. The smallest known bacteria are Mycoplasma, which
lack cell walls and range in size from 150 to 200 nanometres.
The existence of living nanobes, if proven, would also add weight to the
controversial claim in 1996 by NASA researchers that the Martian meteorite
ALH84001 contains fossilised nanobacteria. Some of those 鈥渇ossils鈥� were just 20
nanometres across.
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Uwins and her colleagues describe the nanobes in American
Mineralogist (vol 83, p 1541). They come from a sandstone core taken from
more than 3 kilometres below the seabed, where the temperature exceeds 150
掳C. Until now, life hasn鈥檛 been found above 113 掳C.
When the filaments were brought into the laboratory and exposed to air at 22
掳C, they appeared to grow. 鈥淭hey spread onto the microscope mounts,鈥� says
Uwins.
She claims the nanobes fulfil other criteria for life: they move away from
the electron beam during scanning electron microscopy and have structures much
like cell walls. The nanobes also gave positive signals with three different DNA
probes.
But experts warn that chemical processes can show similar patterns of growth.
They add that the DNA could be from bacteria from rocks nearer to the surface.
鈥淚t doesn鈥檛 take much to contaminate deep sediments,鈥� warns John Parkes, a
microbiologist at the University of Bristol.
Parkes also questions why the nanobes appear to grow under aerobic
conditions: oxygen is toxic to most organisms that live deep in the Earth. But
he says Uwins might convince the sceptics if she sequenced the nanobes鈥� DNA, and
finds that it is distinct from that of other microorganisms.