A
third of Earth's organisms live in rocks and sediments, but their lives have
been a mystery
By some estimates, a third of Earth's
organisms live in our planet's rocks and sediments, yet their lives are almost
a complete mystery.
This week, the work of microbiologist
James Holden of the University of Massachusetts-Amherst and colleagues shines a
light into this dark world.
In the journal Proceedings of the
National Academy of Sciences (PNAS), they report the first detailed data on
methane-exhaling microbes that live deep in the cracks of hot undersea
volcanoes.
"Evidence has built that there's an
incredible amount of biomass in the Earth's subsurface, in the crust and marine
sediments, perhaps as much as all the plants and animals on the surface,"
says Holden.
"We're interested in the microbes
in the deep rock, and the best place to study them is at hydrothermal vents at
undersea volcanoes. Warm water there brings the nutrient and energy sources
these microbes need."
Just as biologists studied the habitats
and life requirements of giraffes and penguins when they were new to science,
Holden says, "for the first time we're studying these subsurface
microorganisms, defining their habitat requirements and determining how they
differ among species."
The result will advance scientists'
comprehension of biogeochemical cycles in the deep ocean, he and co-authors
believe.
"Studies such as this add greatly
to our understanding of microbial processes in the still poorly-known deep
biosphere," says David Garrison, program director in the National Science
Foundation's Division of Ocean Sciences, which funded the research.
The project also addresses such
questions as what metabolic processes may have looked like on Earth three
billion years ago, and what alien microbial life might look like on other
planets.
Because the study involves
methanogens--microbes that inhale hydrogen and carbon dioxide to produce
methane as waste--it may also shed light on natural gas formation on Earth.
One major goal was to test results of
predictive computer models and to establish the first environmental hydrogen
threshold for hyperthermophilic (super-heat-loving), methanogenic
(methane-producing) microbes in hydrothermal vent fluids.
"Models have predicted the
'habitability' of the rocky environments we're most interested in, but we
wanted to ground-truth these models and refine them," Holden says.
In a two-liter bioreactor at UMass
Amherst where the scientists could control hydrogen levels, they grew pure
cultures of hyperthermophilic methanogens from their study site alongside a
commercially available hyperthermophilic methanogen species.
The researchers found that growth
measurements for the organisms were about the same. All grew at the same rate
when given equal amounts of hydrogen and had the same minimum growth
requirements.
Holden and Helene Ver Eecke at UMass
Amherst used culturing techniques to look for organisms in nature and then
study their growth in the lab.
Co-investigators Julie Huber at the
Marine Biological Laboratory on Cape Cod provided molecular analyses of the
microbes, while David Butterfield and Marvin Lilley at the University of
Washington contributed geochemical fluid analyses.
Using the research submarine Alvin, they
collected samples of hydrothermal fluids flowing from black smokers up to 350
degrees C (662 degrees F), and from ocean floor cracks with lower temperatures.
Samples were taken from Axial Volcano
and the Endeavour Segment, both long-term observatory sites along an undersea
mountain range about 200 miles off the coast of Washington and Oregon and more
than a mile below the ocean's surface.
"We used specialized sampling
instruments to measure both the chemical and microbial composition of
hydrothermal fluids," says Butterfield.
"This was an effort to understand
the biological and chemical factors that determine microbial community
structure and growth rates."
A happy twist awaited the researchers as
they pieced together a picture of how the methanogens live and work.
At the low-hydrogen Endeavour site, they
found that a few hyperthermophilic methanogens eke out a living by feeding on
the hydrogen waste produced by other hyperthermophiles.
"This was extremely exciting,"
says Holden. "We've described a methanogen ecosystem that includes a
symbiotic relationship between microbes."
The research was also supported by the
NASA Astrobiology Institute and the National Oceanic and Atmospheric
Administration.
-NSF-
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