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Microbial secrets of the deep

11/28/2005

A new study by Hailiang Dong, assistant professor of geology at Miami University, and doctoral student Gengxin Zhang under Dong's supervision, shows that the biosphere - the part of Earth capable of supporting life - extends far into the Earth's crust, and supports a diverse community of microbes.

Zhang and Dong are co-authors of “Microbial Diversity in Ultra-High-Pressure Rocks and Fluids From the Chinese Continental Scientific Drilling Project in China,” published in June in Applied and Environmental Microbiology. Other authors include Zhiqin Xu, Chinese Academy of Geological Sciences; Donggao Zhao, University of South Carolina; and Chuanlun Zhang, Savannah River Ecology Laboratory, University of Georgia.

This study is the first to systematically examine the abundance and diversity of microbial life in ultra-high-pressure (UHP) rocks at depths of more than 2,000 meters. Subsequent studies by Dong and Zhang (results in press) have examined the microbial community and their metabolic functions at depths of 4,000-4,500 meters.

Analysis of the novel microbes found in the deep subsurface may provide opportunities for discovering new pharmaceuticals, remediating contaminated environments or enhancing energy production, says Zhang.

Dong, a geomicrobiologist, was invited to collaborate with a multidisciplinary, international research team at the Chinese Continental Scientific Drilling (CCSD) project, the world’s deepest ongoing drilling project in China. The CCSD project - part of the International Continental Scientific Drilling program - reached its goal of drilling a 5,000-meter deep hole in the Dabie-Sulu ultra-high-pressure metamorphic (UHPM) belt last spring, nearly four years after the drilling started.

The CCSD site is geologically significant in that it is located at the subduction zone between two continental plates. In a collision nearly 240 million years ago, the Yangtze plate was subducted under (forced beneath) the Sino-Korean plate to a depth of at least 100 km. Rocks experienced UHP metamorphism before they were “rapidly” (in geological terms) brought closer to the surface. Thus, “by drilling to a depth of five kilometers it is possible to study rocks that used to be at a depth of 100 km,” says Dong.

Dong’s was the first study to look at microbes in hard, metamorphic rocks in a subduction zone under extreme conditions of high temperature, pressure and pH.

Results reveal that there is a unique microbial community in the rocks and geological fluids of the deep subsurface. Microbes related to known species of bacteria were found; Organisms were also found that may belong to novel bacterial lineages.

Samples were taken from five depths, between 529 and 2,026 meters. Significant changes in microbial diversity and community structure were associated with environmental gradients in the deep subsurface.

Bacteria found at lower depths tended to be anaerobic (live without oxygen), thermophilic (live at high temperatures) and alkaliphilic (live at high pH). They are metal-reducers: some are iron-reducing bacteria, others are related to selenate- or sulfate-reducing bacteria. Some were found to be related to bacteria that reduce nitrogen and that degrade aromatic (benzene ring-containing) compounds.

Further research may have implications for the understanding of the origin of life on Earth. Conditions at the beginning of life on Earth could have been similar to deep subsurface conditions of high temperature, pressure and pH, and without oxygen, says Dong. “Life may have originated on the surface of minerals...which can act as a catalyst for protein synthesis.”

Such research also may influence the search for life on other planets. Some scientists suggest that if water and energy - in the form of hydrogen and possibly also sulfur, methane and iron - are available, life may be hidden deep under Mars’ lifeless surface.

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