Super-heated, mineral-rich water pours out of a hydrothermal vent on the Axial volcano in the Pacific Ocean. A species of archaea discovered here recently could provide insights into the origins of life on Earth.
Credit: NOAA
SYDNEY: A nitrogen-eating microorganism, discovered living in hot water around deep-sea vents, could provide new insights into the origins of life on Earth.
The new species of archaea - an ancient form of life similar to bacteria - is the first microorganism found around so-called 'black smokers' to convert nitrogen into a useable form, according to a study published today in the U.S. journal Science.
The primitive organism uses the enzyme nitrogenase to take nitrogen gas dissolved in water and convert - or 'fix' it - into ammonia, which living organisms require for growth.
"This does have implications for how life began on earth: the enzyme responsible for nitrogen fixation is already believed to be very ancient, and early life is believed to have lived at high temperatures," said lead author Mausmi Mehta of the University of Washington in Seattle. "Our research suggests that early life on Earth may have had an enzyme similar to the one found in our organism."
The species, which has been given the unremarkable moniker of FS406-22, fixes nitrogen at temperatures up to 92°C. This makes the new species a 'hyperthermophile' - a heat-loving organism which thrives at extreme temperatures, from 80 to 120°C.
"It's exciting to find nitrogen fixers coming out of vents because for years scientists have speculated that nitrogen fixation may be going on there and this is the first description of such an organism that we know of," said Mehta.
Archaeans are common in the hot water around hydrothermal vents where fissures in the Earth's surface release mineral-rich water heated by underground magma. The single-celled archaeans form the basis of the deep-sea vent food chain by consuming methane - as well as nitrogen - and turning it into organic matter.
The amount of nitrogen available in a useable form is thought to act as a limit to life around vents, which include species of giant tubeworms, clams and shrimps. The ability of a vent organism to fix nitrogen therefore means that more life may be able to live on the seafloor independent of nitrogen-fixing ecosystems on the surface.
Researchers have long thought that nitrogen-fixing organisms could be living around the vents, "because vent animals have completely different nitrogen isotope ratios than non-vent animals in the deep sea," said Mehta.
Researchers isolated the organisms from a sample of 23°C water taken from a vent near the 1,400 metre-deep Axial volcano in the northeast Pacific Ocean. In the laboratory, they confirmed that it grew best at 90°C.
"The fact that [the organism] flourishes in such high temperatures means that there must be a habitat below the seafloor, underneath the vents - called the subseafloor biosphere - that reaches those temperatures," said Mehta. The seafloor around the vent is thought to be porous up to 100m down, so the new organism could live at higher temperatures below the seafloor.
The authors proposed that the heat-loving species might have retained ancient characteristics of a common ancestor which lived at high temperatures, before the evolutionary separation of bacteria, archaea and eukaryotes.
Archaea, Greek for 'ancient ones', are single-celled organisms similar to bacteria. However they also possess some characteristics in common with eukaryotes, a group that includes all plants and animals.
The researchers point to genetic studies suggesting that the enzyme nitrogenase arose before the evolutionary separation of archaea and bacteria, and that mutations in the nitrogenase gene later gave rise to chlorophyll, a key ingredient in photosynthesis.
They also suggest an ancient origin for nitrogenase is likely, because the enzyme is inactivated by oxygen, and oxygen levels in the atmosphere on Earth only rose after the beginning of photosynthesis.
