An archaic bacteria that may be the oldest nitrogen fixing organism

Dec 15, 2006 09:33 GMT  ·  By

If hell does exist, only some archaic bacteria could enjoy it!

A heat-loving archaeon able to fix nitrogen at a surprisingly 92 degrees Celsius (198 Fahrenheit), may be the oldest organism capable of nitrogen fixation, preceding the bacteria types today's plants and animals rely on to fix nitrogen.

"The genetic analysis 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 usable - arose before the three main branches of life - bacteria, archaea and eukaryotes - diverged some 3.5 billion years ago" said oceanographer Mausmi Mehta and John Baross, professor of oceanography at the University of Washington.

Previous theories say the nitrogenase system emerged 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 can't we find archaea that fix nitrogen at high temperatures?'" said Baross.

Archaea are archaic bacteria 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, this archaeon is first from a deep-sea hydrothermal vent that can fix nitrogen," says Mehta.

The archaeon was sampled at Axial Volcano (photo) on the Juan de Fuca Ridge off the coast of Washington and Oregon. Previous record was held by Methanothermococcus thermolithotrophicus, an archaeon isolated from geothermally heated sand near an Italian beach, fixing nitrogen at temperatures only up to 64 C. Nitrogen is needed by all living things as it enters in the molecules of amino acids that form proteins, including enzymes, and DNA/RNA.

But gaseous nitrogen must be "fixed" (turned in compounds usable by plants), task which can only be done by certain bacteria and specific archaea. Nitrogen fixed into ammonia, nitrate and other products is used by land and sea plants, which are eaten by 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 fixed nitrogen so organisms in these areas would lack nitrogen. "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.

Researchers have suspected since 1981 that nitrogen fixation was occurring at hydrothermal vents because nitrogen isotope ratios in vent animals were completely different than non-vent deep sea animals.

FS406-22 is able to use gaseous nitrogen as its sole source of nitrogen at temperatures ranging from 58 to 92 C, with an optimum 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".

Photo credit: NOAA