This is a microscopic view of the tubular structures in a 25-micrometer-thick, polished slice of brown volcanic glass. These are thought to be the borings made by autolithotrophic microbes inside the oceanic crust. Credit: Scripps Institution of Oceanography

A team of researchers recently announced that they have found the deepest-living microbes on the planet. These bacteria eat into rock at the bottom of the sea floor, some burrowing down as far as 500 meters (1,640 feet), although most of the microbial activity seems to be in the upper 300 meters (984 feet) of the ocean crust.

"We've documented how extensive these microscopic organisms are eating into volcanic rock, leaving worm-like tracks that look like someone has drilled their way in," says one of the paper's co-authors, Hubert Staudigel of Scripps Institution of Oceanography at the University of California, San Diego. "We've basically determined the depth of the biosphere."

This study is featured in the current edition of Geochemistry, Geophysics, Geosystems, as well as the September 28, 2001, edition of the journal Science. Other authors include Harald Furnes, Ingunn H. Thorseth, and Ole Tumyr of the Geological Institute, University of Bergen, Bergen, Norway; Terje Torsvik of the Department of Microbiology, University of Bergen, Bergen, Norway; and Karlis Muehlenbachs of the Department of Geology, University of Alberta, Edmonton, Canada.

The team studied samples of glassy silica rock that was formed by high heat and pressure. The rock, often referred to as "glass" because of its similarity to the black volcanic glass obsidian, is made from super-cooled lava that's ejected from deep-water volcanic vents.

A scanning electron microscope view of a "biofilm" covering a glass surface altered by microbial activity. Filaments of biological matter (F) are attached to fresh glass (FG). Credit: Scripps Institution of Oceanography

Looking at drill samples from the north to central Atlantic Ocean, the Lau Basin, and the Costa Rica Rift, as well as a wide range of other marine settings, the team found little tunnels in the glass.

The scientists claim that as bacteria absorb the silica nutrients in the glass, they release acid generated by their metabolic processes. This acid corrodes the glass, creating pitting, granular textures, and the tiny tunnels. Chemical interactions between oceanic crust and seawater can also alter volcanic glass, but the researchers say that the sort of structures they found are quite different from those caused by non-biological processes.

Other indications of biological activity found in the volcanic glass are filament-like features reminiscent of organic remains, and a similarity in size between microorganisms and the tubes and microtubes in the granular structures (between 0.2 to 2 micrometers).

The authors admit that the presence of organic carbon and the identification of DNA within the granular and tubular textures could be the result of drilling contamination or sample preparation. However, they feel that the textures and structures produced by the rock corrosion could only have been produced by living organisms.

According to the study, this microbial action occurs wherever ocean water circulates through the ocean crust. Thus, water seems to be a necessary condition for the microbial action.

Steven D'Hondt, an oceanographer with the University of Rhode Island and NAI member, says if bacteria do create the tunnels, this need for water may point to alternate energy sources other than the silica in the glass.

"The circulation of water could conceivably be important if the microbes relied on nutrients, electron donors like organic carbon, or electron acceptors such as oxidants dissolved in the circulating seawater," says D'Hondt. "These explanations underscore the possibility that microbes living in and altering oceanic crust may actually rely for energy primarily on material photosynthesized in the ocean and then circulated through the crust, rather than on energy derived from glass alteration."

This map shows the ages of the crustal rocks that make up the floor of the Atlantic Ocean. Red represents the youngest rocks; the deepest red marks the Mid-Oceanic Ridge, where continental plates are pulling apart and new crust is being formed. Older rocks are yellow, green, and blue. Credit: UCMP Berkeley

"This alternative interpretation -- of dependence on dissolved organic carbon in the circulating seawater -- might also help to explain why alteration is generally limited to the upper few hundred meters of the crust, even though fracture density is high at greater depths," D'Hondt adds.

The researchers say that the bioalteration of the glass likely occurred during the first 6 million years of crustal history. This does not mean that these structures were created during the first 6 million years of the Earth's history, because oceanic crust is constantly being formed. Their conclusion instead suggests that most alteration within any piece of oceanic crust occurs within 6 million years of the formation of that crust.

D'Hondt says that if crust-dwelling microbes are active only in such relatively young crust, then the most deeply buried active microbes may occur in layers of sediment that collect over older oceanic crust. These sediments tend to collect more heavily in some regions than others -- along continental margins, for instance, the sediment overlying older crust may be 5 to 10 kilometers thick. Microbial cells and active sulfate reduction previously have been identified in such sediments, buried more than 800 meters below the seafloor.

In any event, says D'Hondt, "their results do not demonstrate that microbes are active today -- or at any single point in time -- in all crust shallower than several hundred meters. Even if one accepts the paper's assumptions, its data do not really allow good estimates of how long microbes may remain active in any volume of oceanic crust."

"I suspect that the maximum subsurface depth of Earth's biosphere will ultimately be considered less important than discoveries of extreme habitats and novel metabolic pathways utilized by buried microbes," concludes D'Hondt.

Related Web Pages

Scripps Institution of Oceanography press release (Scripps)
The mechanism behind Plate Tectonics (UCMP Berkeley)
How Small Can Life Be? (NAI)