Microbe fixes nitrogen at a blistering 92 C

This may be another clue as to how life may have begun under the sea.


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Microbe fixes nitrogen at a blistering 92 C

May offer clues to evolution of nitrogen fixation

A heat-loving archaeon capable of fixing nitrogen at a surprisingly hot
92 degrees Celsius, or 198 Fahrenheit, may represent Earths earliest
lineages of organisms capable of nitrogen fixation, perhaps even
preceding the kinds of bacteria today's plants and animals rely on to
fix nitrogen.

The genetic analysis reported in the Dec. 15 issue of Science supports
the notion that the gene needed to produce nitrogenase an enzyme
capable of converting nitrogen gas, that's unusable by life, to a form
like ammonia that is useable arose before the three main branches of
life bacteria, archaea and eukaryotes diverged some 3.5 billion years
ago, according to oceanographer Mausmi Mehta, who recently received her
doctorate from the UW, and John Baross, UW professor of oceanography.
This is opposed to the theory that the nitrogenase system arose within
archaea and was later transferred laterally to bacteria.

"There's been lots of evidence that point to high-temperature archaea as
the first life on Earth but the question has been, 'So why cant we find
archaea that fix nitrogen at high temperatures"'" says Baross, whos been
on a 20-year quest to find just such a microbe. Archaea are
single-celled organisms that live under extreme environmental
conditions, such as the high temperatures and crushing pressures below
the seafloor. If heat-loving archaea were the first life on the planet,
they would have needed a usable source of nitrogen, Baross says.

Known as FS406-22 because of the fluid and culture samples it came from,
the archaeon discovered by the UW researchers is the first from a
deep-sea hydrothermal vent that can fix nitrogen, says Mehta, first
author on the Science paper.

It was collected at Axial Volcano on the Juan de Fuca Ridge off the
coast of Washington and Oregon. Fixing nitrogen at 92 C smashes the
previous record by 28 C, a record held by Methanothermococcus
thermolithotrophicus, an archaeon that was isolated from geothermally
heated sand near an Italian beach and fixes nitrogen at temperatures up
to 64 C.

Nitrogen is necessary for all life because it is an essential part of
amino acids and proteins. To be used by organisms, gaseous nitrogen must
be converted to other compounds, or "fixed," which can only be done by
certain bacteria and specific archaea. Nitrogen can be fixed into
ammonia, nitrate and other products that can be used by land and sea
plants, which in turn are eaten by higher animals.

Today's oceans contain nitrogen both as a dissolved gas and as nitrate.
Ocean water that percolates down into the seafloor can pick up enough
heat from volcanism deep in the earth to cause the fixed nitrogen to
revert to its gaseous form. Venting water hotter than 30 C contains very
little nitrate so organisms in areas where the subseafloor temperatures
are higher would lack nitrogen in a form they can use.

The discovery of FS406-22's nitrogen fixing capabilities at 92 C,
therefore, widens the realm of where life can grow in the subseafloor
biosphere and other nitrogen-limited ecosystems, perhaps even on other
planets, Mehta says.

Scientists have speculated since 1981 that nitrogen fixation was
occurring at hydrothermal vents because vent animals had completely
different nitrogen isotope ratios than non-vent deep sea animals.

The work that led to FS406-22 was supported by Washington Sea Grant,
based at the UW, and the NASA Astrobiology Institute. Mehta worked five
years doing some 600 enrichments, with FS406-22 being the only one she
was could wean off fixed nitrogen completely. An unimpressive looking
sphere of a microbe, FS406-22 is able to grow with gaseous nitrogen as
its sole source of nitrogen at temperatures ranging from 58 to 92 C,
with the fastest growth at 90 C.

The genetic analysis shows FS406-22 as having one of the deepest-rooted
genes and the most primordial characteristics in terms of gene sequence,
Baross says.

"We propose that among diazotrophic archaea, the nitrogenase from
FS406-22 might have retained the most ancient characteristics, possibly
derived from a nitrogenase present in the last common ancestor of modern
life," the co-authors conclude in their report.




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http://www.eurekalert.org/pub_releases/2006-12/uow-mfn121206.php
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