[meteorite-list] Chomping on Nano-Nuggets (ALH84001)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:56 2004
Message-ID: <200309171729.KAA17265_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=596&mode=thread&order=0&thold=0

Chomping on Nano-Nuggets
By Leslie Mullen
Astrobiology Magzine
September 17, 2003

Nanobacteria are not alive, but instead are the result of enzymes that break
down organic material, according to a new study published in the journal
Geology.

Eight years ago, features resembling bacteria and measuring 20 to 100
nanometers across were discovered in the Martian meteorite ALH84001. NASA
scientists interpreted these features to be the fossilized remnants of
ancient life, but many scientists rejected that conclusion.

A nanometer is one millionth of a millimeter. The period at the end of this
sentence is about one million nanometers long. The tiniest bacteria measure
about 200 nanometers in size, and many believe that life can't get much
smaller than that.

A committee formed under the auspices of the US National Academy of Sciences
determined that, due to the size requirements of such vital elements as
enzymes and genetic material, organisms smaller than 200 to 300 nanometers
in diameter could not be self-sustaining and therefore could not be
considered to be "life."

Others contend that life can be that small, and as proof they claim to have
grown nanobacteria in the laboratory. In addition to the nanobacteria in the
Martian meteorite, spheroidal features measuring 50 to 200 nanometers have
been found in sedimentary rocks on Earth. Some claim that these spheroids
are the fossilized remains of once living nanobacteria.

The new study, conducted by Jürgen Schieber of Indiana University in
Bloomington and Howard Arnott of the University of Texas at Arlington,
suggests an alternative explanation for nanometer-sized features. The
scientists report that protein balls measuring 40 to 120 nanometers across
are produced when bacterial enzymes cause organic material to decay.

Schieber and Arnott dipped tiny pieces of bean, squid and beef into the muck
from a pond, to ensure that the samples became coated with the full spectrum
of naturally occurring decay bacteria. The samples were then buried in clay
to simulate the burial of organic matter in sedimentary rock.

Over the next two weeks, the researchers found the tissue samples
experienced "explosive" bacterial growth, and balls measuring 40 to 120
nanometers in size were widespread. The scientists say that these
"nannoballs" compare well with published examples of nanobacteria.

"Because gradual decay of tissues always led to formation of nannoballs, we
surmised that the latter resulted when microbial enzymes interacted with the
buried samples," the scientists write. The scientists also exposed tissues
to various purified protein-degrading enzymes in separate experiments, and
this confirmed that such enzymes were responsible for the nannoballs.

The enzymes snip the larger tissue elements like cell walls and muscle
fibers into nanometer-sized units. Once snipped, the tissues contract into
balls due to elastic forces. This enzymatic breakdown of organic matter may
act as an aid to decomposition, the scientists suggest, reducing material to
bite-sized nuggets for bacteria to ingest.

"Bacteria are osmotrophs and can only take in dissolved molecules liberated
by exoenzymes utilized outside of the cell," write the scientists. "Seeing
no subunits smaller than our nannoballs, we assume that in the subsequent
degradation step, the nannoballs are broken down by further enzyme action
into soluble molecules that can be ingested by bacteria."

Nannoballs are not always consumed by bacteria, say the scientists, because
under certain conditions the tissues can become mineralized. This
mineralization preserves the nannoballs, turning them into fossils in just a
few weeks.

Although the nannoballs are not fossilized life forms, they can act as
"biomarker" evidence for bacterial life.

"Most if not all alleged nannobacterial structures in sedimentary rocks are
probably by-products of bacterial degradation of organic matter and not
evidence for minute life forms called nannobacteria," the scientists
conclude. "Nonetheless, mineralized nannoballs may indicate bacterial enzyme
action on organic tissues and serve as a visual proxy for microbial
activity."

Kathie Thomas-Keprta, an astrobiologist with Lockheed Martin at NASA's
Johnson Space Center, has studied the magnetite and carbonate mineralogy of
the martian meteorite ALH84001. She says that if microbes on Earth produce
nannoballs as they degrade certain minerals, as they do with the tissues in
this new study, then the nannoball-like texture observed on the surface of
carbonate globules in ALH84001 may be a product of such microbial etching.

However, she says it's still possible that the features in ALH84001 are the
fossilized remains of microbial life. Part of the problem with the debate
over the size constraints of life, says Thomas-Keprta, is that microbes can
shrink substantially after death.

"The size of a viable organism may be vastly different from the size of that
organism when fossilized or mineralized," she states. "We do not understand
how the size of organisms changes with fossilization or mineralization, nor
do we know if particular categories of organisms can be better preserved
than others."

While the physical shape and size, or morphology, of a structure is not
enough to determine whether it was once a living microorganism, certain
surface textures might be evidence of past biological activity. A granular
surface texture composed of nannoballs, in conjunction with other
biomarkers, may provide further evidence that certain morphological features
were once microbial life.

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Schieber, J. & Arnott, H. J. Nanobacteria as a by-product of enzyme-driven
tissue decay. Geology, 31, 717 - 720, (2003).
Received on Wed 17 Sep 2003 01:29:37 PM PDT