Washington,
DC (4/16/99)- Thiomargarita namibiensis, a giant bacterium discovered
off the coast of Namibia, has a repertoire of survival techniques that would
be the envy of any extremophile.
left
Light-photomicrograph of three cells of Thiomargarita.
The prokaryote, measuring up to 0.75 mm wide, is 100 times larger than its
nearest competitor in the bacterial size contest. An international team of
biologists were stunned to discover the organism while studying the sediments
in the coastal waters of Namibia. The "Sulfur Pearl of Namibia" has adapted
to an environment low in oxygen and high in hydrogen sulfide that would be
toxic to most life forms.
"When I told them, my colleagues at first didn't believe me because the bacteria
were so big. But I've been working with exotic bacteria for a while now and
I knew immediately that these were sulfur bacteria," said Heide Schulz, of
the Max Planck Institute for Marine Microbiology.
Microscopic analysis revealed that much of the volume of the cell is taken
up by a vacuole. The bacteria uses the vacuole to store the nitrates that
it uses to oxidize sulfide. The researchers noted that nitrate concentrations
within the cell could be up to 10.000 times higher than in the surrounding
sea water This combination of the oxidation of sulfide with the reduction
of nitrate provides the bacteria with an energy source which is not accessible
for most bacteria in the absence of oxygen. The massive vacuole allows Thiomargarita
to "hold its breath" until the appropriate nutrients become available.
Dr. Schulz was part of a team of scientists who were looking for two other
kinds of sulfur bacteria, Beggiatoa and Thioploca, which they
had found off the Pacific coast of South America. Both areas feature the hydrographic
similar features, particularly an upsurgence of deep ocean water rich with
the nutrients on which phytoplankton and other marine organisms depend. But
the scientists found only minor levels of Beggiatoa and Thioploca,
but quite a lot of Thiomargarita.
The genetically similar Thioploca and Thiomargarita, have
evolved separate adaptations to the same ecological challenge of surviving
in the high sulfide environment. While nitrate is found in sea water, it does
not penetrate the oxygen-poor, sulfide-rich sediment where these bacteria
are found. Thioploca cells form filaments that cling to each other
and secrete a sheath of mucous film. This sheath provides a vertical tunnel
through the sediment up to the overlying water, allowing the Thioploca
filaments to 'commute' between their food source and the nitrate they need
to metabolize it. Thiomargarita, in contrast, do not commute. Rather,
they form strands of single, unattached cells evenly separated by a mucous
sheath, and wait for nutrients to pass by.
The discovery of these organisms should stimulate research into the origins
of life on planet Earth. The biosphere depends on the constant recycling of
key elements including carbon, nitrogen, and sulfur. Microorganisms are major
contributors to this recycling as they facilitate reduction and oxidation.
This turn facilitates the transfer of elements to the oceans, sediments, and
atmosphere, and to other organisms.
The appetite of these bacteria for sulfide and nitrate also suggest another
role for them. It might be possible to utilize the bacteria to remediate coastal
waters polluted by excess nitrates from agricultural runoff.
The research appears in the April 16, 1999 issue of Science.
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