Scientists say a third of Earth's
organisms live in our planet's rocks and sediments - and the amount
could even be greater than what we find on the surface.
This
week, microbiologist James Holden of the University of
Massachusetts-Amherst along with colleagues shone a light on this dark
work, reporting on the first detailed data on methane-exhaling microbes
that live deep in the cracks of hot undersea volcanoes.
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.'
A hydrothermal vent field at Axial Volcano seen through the porthole of the submersible Alvin
The submersible Alvin extends its mechanical arm to a high-temperature black smoker at Endeavor Segment
'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.'
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,' said 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.
Life at the bottom of the ocean: A smoking
hydrothermal sulfide spire at Endeavor Segment, Juan de Fuca Ridge in
the Pacific Ocean
'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
specialised 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.'
