Icebound microbes breathe iron

Members of a microbial community from a pool of water deep under the Arctic ice power their metabolism by "breathing" iron, a study in this week's Science reports. The previously unknown mechanism may explain how microbes survived during a period 600 million years ago, when the earth's oceans were covered in ice, the authors say. Blood Falls at the Taylor Glacier Image: Benjamin Urmston The identification of the bacterial ecosystem's oddball respiration is a "remarkable discovery," said Alan

Written byTia Ghose

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Members of a microbial community from a pool of water deep under the Arctic ice power their metabolism by "breathing" iron, a study in this week's Science reports. The previously unknown mechanism may explain how microbes survived during a period 600 million years ago, when the earth's oceans were covered in ice, the authors say.

Blood Falls at the Taylor Glacier
Image: Benjamin Urmston

The identification of the bacterial ecosystem's oddball respiration is a "remarkable discovery," said Alan J. Kaufman, a biogeochemist at the University of Maryland in College Park, who was not involved in the study. It's impressive, he said, that the group was able to "look with such detail at the microbiology of the consortium of organisms that is basically eking out a living in an environment where there's no new food." Jill Mikucki, a geomicrobiologist at Dartmouth College in Hanover, NH, who led the research, first discovered the microbial community several years ago in a pool of marine brine seeping out from Blood Falls, a frozen, rust-laced waterfall at the mouth of the Taylor Glacier in East Antarctica. In this study, Mikucki's team investigated the microbes' metabolism by studying their use of different minerals. Normally, microbes living in deep-sea environments bypass the need for oxygen or sunlight by getting their energy from the breakdown of sulfate to hydrogen sulfide. This feature of their metabolism shifts the fraction of heavy and light isotopes of sulfur in the water they live in, Mikucki told The Scientist. So her group compared the fraction of light and heavy isotopes of sulfur, as well as oxygen and other elements, in brine samples that seeped out of the glacier from the subglacial lake to seawater from nearby. The group found no evidence of a shift from heavy to light sulfur isotopes in the brine samples from Blood Falls. Instead, the surrounding water showed a change in the ratio of oxygen isotopes, suggesting that they were gleaning energy by converting sulfate to an intermediate compound. That process frees an oxygen atom, which would then convert iron to a soluble form. The soluble iron creates rust when exposed to air and is responsible for the reddish hue of the falls, Kaufman said. Other analysis showed the microbes lacked the genes to make sulfate reductase, a key enzyme in the sulphur reaction, further suggesting that the microbes take a different tack. Timothy Lyons, a biogeochemist at the University of California Riverside said the most convincing evidence for that chemical pathway was the absence of sulfate reductase genes, so follow-up studies should confirm the finding. According to Mikucki, the study may provide clues to how early organisms survived the "snowball earth" -- two periods around 710 million and 635 million years ago when the earth was entombed in ice. It's possible that the ancient microbes deep in the ice had no oxygen and little organic matter, and therefore used a similar form of respiration to the one used by microbes at Blood Falls, she said. Kaufman, however, disagreed. The comparison between the Blood Falls microbial community and early life is the paper's "weakest link," he said, because not all scientists believe the seas were completely iced over during these periods. In fact, some isotopic evidence of sulfate reactions suggests the opposite, meaning ancient organisms might have had access to air and sunlight for respiration, unlike the Blood Falls microbes. Editor's Note: The title for this story has been updated from a previous version.
**__Related stories:__***linkurl:Ancient microbes repair DNA;http://www.the-scientist.com/news/display/53546/
[27th August 2007]*linkurl:Hot bacteria near Antarctica;http://www.the-scientist.com/article/display/23292/
[1st April 2006]*linkurl:Life six feet under...and high above?;http://www.the-scientist.com/article/display/18137/
[20th July 1998]