Saturday, December 16, 2006
A heat-loving archaeon capable of fixing nitrogen at a surprisingly hot 92 degrees Celsius, or 198 Fahrenheit, may represent Earth's 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 December 15 issue of the journal 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 (info), 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 can't we find archaea that fix nitrogen at high temperatures?' says Baross, who's 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.
Continued at "Microbe fixes nitrogen at a blistering 92 C" [Evolution, Extremophiles, Ocean Deeps]
Based on the Science paper:
"Nitrogen Fixation at 92 degrees C by a Hydrothermal Vent Archaeon"
A methanogenic archaeon isolated from deep-sea hydrothermal vent fluid was found to reduce N2 to NH3 at up to 92 C, which is 28 C higher than the current upper temperature limit of biological nitrogen fixation. The 16S ribosomal RNA gene of the hyperthermophilic nitrogen fixer, designated FS406-22, was 99% similar to that of non-nitrogen fixing Methanocaldococcus jannaschii DSM 2661. At its optimal growth temperature of 90 C, FS406-22 incorporated 15N2 and expressed nifH messenger RNA. This increase in the temperature limit of nitrogen fixation could reveal a broader range of conditions for life in the subseafloor biosphere and other nitrogen-limited ecosystems than previously estimated.
*Info on Archaea:
The Domain Archaea wasn't recognized as a major domain of life until quite recently. Until the 20th century, most biologists considered all living things to be classifiable as either a plant or an animal. But in the 1950s and 1960s, most biologists came to the realization that this system failed to accomodate the fungi, protists, and bacteria. By the 1970s, a system of Five Kingdoms had come to be accepted as the model by which all living things could be classified. At a more fundamental level, a distinction was made between the prokaryotic bacteria and the four eukaryotic kingdoms (plants, animals, fungi, and protists). The distinction recognizes the common traits that eukaryotic organisms share, such as nuclei, cytoskeletons, and internal membranes. (More)
**Info on the Axial Volcano:
Axial Volcano rises 700 meters above the mean level of the ridge crest and is the most magmatically robust and seismically active site on the Juan de Fuca Ridge between the Blanco Fracture Zone and the Cobb offset. It represents the product of intense volcanic activity from the Cobb-Eikelberg hotspot juxtaposed on the extensional field of the spreading center. Axial Volcano was first studied in the late 1970s and then mapped in greater detail by NOAA/VENTS with SeaBeam in the early 1980s. Following the initial discovery of venting in the northern portion of the caldera in 1983, a concentrated mapping and sampling effort was made in the mid-late 1980s.The summit of Axial Volcano is marked by an unusual rectangular shaped caldera (3 x 8 km) that lies between the two rift zones. (More)