Earth's Asteroid-Proof Life Zone: The Planet's Largest Microbial Habitat Found beneath Seafloor

Written By My Infomation on 5/4/11 | 5/04/2011


Tiny ancient microbes beneath the sea floor influence the Earth's long-term carbon cycle. Distinct from life on the Earth's surface, these microbes may account for one-tenth of the Earth's living biomass but many of these minute creatures are living on a geologic timescale. The team examined how the microbial world differs in the sub-sea floor from that in the surface waters, with profound implications for our understanding of possible lifeforms elsewhere in the Solar System.
A large reservoir of seawater exists in Earth's crust, which is thought to be the largest llife habitat on Earth. This seawater aquifer supports a dynamic microbial ecosystem that is known to eat hydrocarbons and natural gas, and may have the genetic potential to store carbon. Scientists are interested in better understanding the natural processes taking place below the seafloor, which also give rise to economically important ores along the seafloor and may play a role in earthquakes.
An international team of scientists report on the first observatory experiment to study the dynamic microbial life of an ever-changing environment inside Earth's crust. During the four-year subsurface experiment, the research team deployed the first in situ experimental microbial observatory systems below the flank of the Juan de Fuca Ridge, which is located off the coast of Washington (U.S.) and British Columbia (Canada).


Miami (UM) Rosenstiel School of Marine and Atmospheric Science professor Keir Becker  and UM Rosenstiel alumnus Andrew Fisher installed the sub-surface observatory technology known as CORK (Circulation Obviation Retrofit Kit), which seals the sub-surface borehole for undisturbed observations of the natural hydrogeological state and microbial ecosystem inside Earth's crust.



"Similar to a cork in a wine bottle, our technology stops fluids from moving in and out of the drilling hole," said Becker, a UM Rosenstiel School professor of marine geology and geophysics. "Ocean water is blocked from entering the hole and flushing out the natural system."



These natural laboratories allow scientists to investigate the hydrogeology, geochemistry, and microbiology of ocean crust.



"The paper is important since it is the first in-situ experiment to study subsurface microbiology," said Becker, a co-author of the paper.



The new research paper, "Colonization of subsurface microbial observatories deployed in young ocean crust", was published in last month's issue of Multidisciplinary Journal of Microbial Ecology, a publication of the journal Nature.



A study of the seafloor beneath the Peruvian Margin back in 2006, made some estimates that micribial cells could double every 100 to 2,000 years," according to  Jennifer F. Biddle, PhD. in biochemistry and former postdoctoral fellow in geosciences, Penn State. "Now we have the first comprehensive look at the genetic makeup of these microbes."



"The Peruvian Margin is one of the most active surface waters in the world and lots of organic matter is continuously being deposited there," said Christopher H. House, associate professor of geoscience.



The researchers used a metagenomic approach to determine the types of microbes residing in the sediment 3 feet, 53 feet, 105 feet and 164 feet beneath the ocean floor. The use of the metagenomics, where bulk samples of sediment are sequences without separation, allows recognition of unknown organism and determination of the composition of the ecosystem.



"The results show that this subsurface environment is the most unique environment yet studied metagenomic approach known today," says House. "The world does look very different below the sediment surface."



The researchers found that a large percentage of the microbes were Archaea, single-celled organisms that look like bacteria but are different on the metabolic and genetic levels. The percentage of Archaea increases with depth so that at 164 feet below the sea floor, perhaps 90 percent of the microbes are Archaea. The total number of organisms decreases with depth, but there are lots of cells, perhaps as many as 1,600 million cells in each cubic inch.



If the rest of the world is like the Peruvian Margin, then at least one tenth and as much as a third of the Earth's biomass could be these tiny microbes living in the dark mud. However, this population lives at an unusual rate. Single-celled organisms usually consume food for energy and then rather than grow larger, simply divide and reproduce themselves. While the Bacteria Escherichia Coli, as an example, doubles its numbers every 20 minutes, these Archaea double on the order of hundreds or thousands of years and consume very little energy.



"In essence, these microbes are almost, practically dead by our normal standards," says House. "They metabolize a little, but not much."



According to House, organisms metabolizing at such slow rates is what we could expect to find in other areas of our solar system because such environments have much less energy available than on Earth. Perhaps, similar organisms may be in hydrothermal vents beneath the ice of Europa -- the second moon of Jupiter -- or in subsurface aquifers of Mars.



"We do not expect the microbes in other places to be these microbes exactly," says House. "But, they could be living at a similar slow rate."



Biddle notes that these microbes could survive major Earth impacts by asteroids, so the subsea floor could be a refuge for life during extinction events. Now this study shows they may be a reservoir of novel genetic material as well. Her future research will focus on understanding the lifestyle of the microbes.

"For example, how do they die?" asks Biddle. "It is a simple question that we cannot answer."

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