Hydrothermal vent field at Axial Volcano seen through the porthole of the submersible Alvin. Credit: Mark Spear/WHOI

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.

Alvin extends its mechanical arm to a high-temperature black smoker at Endeavor Segment. Credit: Bruce Strickrott/WHOI

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.

Source: National Science Foundation