|
Deep-sea Tubeworms Get Versatile 'Inside' HelpScientists find first known organism that makes organic carbon by two different means |
|
|
When scientists found lush thickets of 6-foot-tall, red-tipped
tubeworms on the seafloor in 1977, they realized that life could thrive
without sunlight in extreme environments.
When they discovered that the
tubeworms had no mouth, digestive tract, or anus, they learned that
bacteria live inside the tubeworms’ bodies in a remarkable organ called a
trophosome. In exchange for a fertile place to live, the bacteria
convert carbon dioxide into organic carbon by using chemical
energy—much the way chloroplasts provide nutrition for plants via
photosynthesis using the sun’s energy.
Now, a team of 12 scientists has found that the symbiotic bacteria on
which the gutless, buttless tubeworms depend are surprisingly
versatile: They can use two different ways to metabolize carbon dioxide
and can switch back and forth to accommodate fast-changing
environmental conditions. The findings were reported in the Jan. 12
issue of the journal Science.
Scientists have known microorganisms that use the so-called Calvin
cycle to convert, or “fix,” carbon dioxide into a form that livings
things can use, and other microorganisms that use another metabolic
pathway known as the reductive tricarboxyclic (rTCA) cycle. “But this is the first
occurrence of an organism that can switch between the two carbon
fixation pathways, or even use both at the same time,” said Stefan
Sievert, a microbiologist at Woods Hole Oceanographic Institution (WHOI) and
co-author of the study led by Thomas Schweder and Stephanie Markert of
the Institute of Marine Biotechnology in Germany. Co-authors included
scientists from the Max-Planck-Institute for Infection Biology and
Ernst-Moritz-Arndt University in Germany and Scripps Institution of
Oceanography, University of California Santa Cruz, and SymBio Corp. in
California.
Solving a longstanding conundrum
Bacteria like these probably played fundamental roles in the evolution
of life on Earth. But their environment—both inside living
tubeworms and around deep-sea hydrothermal vents that spew hot,
metal-rich fluids—is difficult to imitate in laboratories, so scientists
have never been able to culture them.
The team used a novel
approach: They analyzed the bacteria’s genome along
with their proteome, the proteins transcribed and produced by the
genes. (The genome is like the bacteria's blueprint, while the proteome
is, in a way, the bacteria’s tool box, Sievert said.) The proteome
revealed telltale enzymes, which the bacteria use to harness chemical energy and to fix inorganic
carbon.
The combined genomic and proteomic approaches offer a valuable way to
investigate the metabolic capabilities and history of these
microorganisms, said Charles Fisher of Pennsylvania State University
and Peter Girguis of Harvard University, who wrote a perspective article on the
research in Science.
The new study solves one mystery that had been puzzling scientists for
decades. They had found that tubeworm tissues contain more of a heavier
stable carbon isotope than expected if the Calvin cycle were the only
one at work. Use of the rTCA cycle explains this conundrum, because it
results in the incorporation of more of the heavy stable carbon
isotope, compared to the Calvin cycle, Sievert said.
“We were always suspicious that something else was going on and that
maybe the Calvin cycle wasn’t the whole story,” said Sievert, who,
together with then-WHOI postdoctoral scholar Michael Hügler (now at
University of Kiel, Germany), provided essential data to prove that the
rTCA cycle operated in the symbionts.
The Calvin cycle works with plenty of oxygen around, Sievert explained,
but requires substantially more energy than the rTCA cycle, which, on
the other hand, is inhibited by higher oxygen concentrations. Such
metabolic flexibility is an asset in habitats dependent on the
fluctuating flows of fluids emanating from hydrothermal vents, he said.
A relationship with give and take
For a long time, the means by which the tubeworms (Riftia pachyptila) acquired
the symbionts had remained a mystery as well, with many investigators
thinking that the worms may pick up the bacteria in their larval stage,
when the worms still have a mouth. However, just last year, Fisher and
Andrea Nussbaumer and Monika Bright of the University of Vienna
discovered that the bacteria enter through the tubeworms’ skin in a
process that is akin to an infection.
Once the bacteria get inside the tubeworms, both benefit from
this unusual arrangement, which is called endosymbiosis (from “endo,”
meaning “inside”). The tubeworms’ feather-like red plumes act as gills,
absorbing oxygen from seawater and hydrogen sulfide from vent fluids.
This feat is accomplished by a special type of hemoglobin in their
blood that can transport oxygen and sulfide at the same time (human
hemoglobin transports only oxygen). The bacteria inside the tubeworms
oxidize hydrogen sulfide to create energy. The tubeworms get a steady
supply of organic carbon and can grow prolifically, tacking on roughly
31 inches (80 centimeters) of white tube to their bodies every year.
The mysteries of tubeworms and their endosymbiotic microbes only
continue to grow, Sievert said. In certain situations, the
endosymbionts may even
burn internally stored carbon, Sievert said, giving the bacteria and
the tubeworms even more metabolic flexibility to adapt to fluctuating
conditions.
“How all this is regulated is currently not well understood,” Sievert
said. "Clearly, this study has opened a new door, paving the way
for exciting discoveries to come.”
— Kristen M. Kusek
The research was funded by Deutsche Forschungsgemeinschaft, the U.S.
National Science Foundation, the U.S. National Aeronautics and Space
Administration's Astrobiology Institute, the Woods Hole Oceanographic
Institution, and the University of California.
Posted: January 12, 2007 [top] |
|
