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Settling on the SeafloorDeep in the ocean, larvae search for 'home, sweet home' |
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| Enlarge ImageBiologist Stace Beaulieu from the Woods Hole Oceanographic Institution packed winter clothes for an expedition to the equatorial Pacific to work in a shipboard lab with the temperature set to 2°C (35°F). The cold temperature prevented decomposition of microbes and larvae that settled on experimental panels retrieved from the seafloor. This would foul the research. (Photo by Lauren Mullineaux, WHOI) |
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| Enlarge ImageAt the experiment site at the Tica Vent on the East Pacific Rise in 2004, basalt panels were placed in the center of a tubeworm colony. The blue T-handle in the lower center of the photo shows the location of one of the sensors deployed to measure chemicals, acidity, and temperature at the site. (Photo by Stace Beaulieu, Woods Hole Oceanographic Institution) |
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| Enlarge ImageScientists Stace Beaulieu of WHOI (left), Naomi Ward of The Institute for Genomic Research, and Breea Govenar of Pennsylvania State University wear gloves to keep panels used to collect colonizing seafloor larvae free of human bacteria that could contaminate research experiments. Their work was done in May and June 2005 during an expedition to the Galápagos Rift. (Photo by Amy Nevala, WHOI) |
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People may search for a long time, but they know it when they see
it—the right job in the right town, or the right house in the right
neighborhood. Then they settle down, set up shop, and put down roots.
At the bottom of the ocean, tiny larvae of deep-sea animals, floating
through the black depths, make a similar quest. The offspring of
tubeworms, giant clams, and other organisms that form thriving
communities of life around seafloor hydrothermal vents must find the
right conditions for them to settle down, live, grow, and reproduce.
How they do that has remained a mystery. In the 28 years since
tubeworms and other organisms were discovered living in volcanic areas
along mid-ocean ridges, scientists have been puzzling over how they
survive there. Woods Hole Oceanographic Institution biologist Tim Shank
calls the vent sites “Earth’s largest chemosynthetic community,” where
inhabitants live without sunlight and instead convert chemicals from
vent fluids into energy and nutrient sources.
“There’s no other place on the planet like this,” Shank said, “yet we
know little about how young organisms move around, assemble, and form
new communities.”
Understanding the factors that determine why larvae settle is a key
first step to understanding the bigger picture of how seafloor life has
developed across the wide expanse of the ocean throughout time.
In a similar way, scientists have studied how and why human populations
developed and moved out of Africa, crossed the Bering Strait into the
New World, or later journeyed across oceans via ships during the
colonial era. Over the short term, social scientists also seek to
understand the factors that cause demographic shifts—from urban to
suburban communities, for example, or from the U.S. East Coast to the
Southwest.
Equivalent population movements occur on the seafloor, and they all
begin with organisms finding places to accommodate their basic needs.
For larvae of hydrothermal vent organisms, those factors include basalt
(volcanic seafloor rock) on which to settle, and the right acidity and
temperature of seawater.
Setting the bait
To begin learning exactly why larvae prefer one swath of seafloor to
another, Shank and fellow WHOI biologists Stefan Sievert and Stace
Beaulieu teamed up last year using a grant from the WHOI Deep Ocean
Exploration Institute. They began in February 2004 by deploying small
basalt panels as an experiment at a hydrothermal vent site called Tica
Vent on the East Pacific Rise. This mountainous undersea area is
located about 3,700 kilometers (2,000 nautical miles) west of Central
America.
During three cruises in 2004 to the East Pacific Rise, the science team
began a series of studies that was repeated this spring at the
Galápagos Rift, located off the Galápagos Islands near the equator, to
see how microbes and larvae settle over time.
Traveling to Tica Vent on the research vessel Atlantis, the biologists
worked at a lush and thriving community of giant tubeworms and mussels.
Using the submersible Alvin, they placed 17 pre-cut, 4-inch squares of
rock basalt previously collected from the site. The squares acted as
seafloor petri dishes, growing microbes and providing a settlement
surface for larvae in their natural surroundings.
“Microbes might coat the surface of the basalt and serve to indicate to
the larvae that a particular spot is good for settling,” said Beaulieu.
She suspects they may act as a conditioner for larval settlement, in
the same way that a paint primer brushed on walls allows subsequent
paint layers to adhere better.
Nearby sensors monitored the temperature and chemistry of the vent
fluid, including acidity and hydrogen sulfide levels, for a clearer
picture of how the vents influence the larvae’s settlement.
The scientists returned to collect and replace the squares over the
following four, nine, and 13 days, as well as on later expeditions
after two and nine months. This allowed time for the microbes and
larvae to settle over different time spans.
“To the naked eye, the squares recovered after four, nine, and 13 days
looked relatively bare,” said Beaulieu. To peer closer, she headed to
the ship’s lab to use a special microscope to photograph where larvae
settled on the squares. The lab’s temperature was set at 2°C (35°F) to
prevent seafloor microbes and the larvae that settled and
metamorphosized—known to scientists as “recruits”—from rapidly
decomposing, and thus fouling the research.
That’s why Beaulieu packed winter clothes for the trip to the
equatorial Pacific.Using forceps, Beaulieu plucked out tiny recruits,
one by one, and placed them in tubes.
“For up to nine hours after Alvin returned from the dive, we labored at
the scope,” she noted. “While many scientists and crew were deep
asleep, we were sequestered in the bio lab’s walk-in freezer, forceps
at hand.
“It can be difficult to take photos through a microscope on a rocking ship,” she added.
Getting IDs via DNA
Unlike the recruits, microbes are too small to pluck off. So after
Beaulieu was finished, the squares were frozen for subsequent studies
to identify organisms by their DNA. To do that, the scientists first
needed to extract DNA from the samples.
“This is especially exciting because it’s the first time it has been
done,” she said. Back at WHOI in February 2005, Beaulieu developed a
technique using a jewelry- and tool-cleaning machine. It sends
ultrasonic waves to the surface of the squares, which break up cells
that may have accumulated there. DNA is then extracted into a liquid
solution that is purified to remove other organic compounds, such as
proteins.
With relatively pure DNA in hand, Sievert has begun analyses to
identify microbes that may have colonized the larval panels, while
Beaulieu and Shank search for DNA to identify larval recruits plucked
from the basalt surfaces.
To the naked eye and even under a microscope, “the surfaces sometimes
appear to be clean, with no settled larvae,” Beaulieu said. “But now
we’re developing techniques that tell us ‘Not so fast—it appears that
somebody is home, after all.’ ”
— Amy E. Nevala
Posted: July 8, 2005 [top] |
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