Article: Giant Deep-Sea Tubeworm's Meal Ticket Comes in as a Skin Infection

Giant Deep-Sea Tubeworm's Meal Ticket Comes in as a Skin Infection

19 May 2006- Giant tubeworms found near hydrothermal vents more than a mile
below the ocean surface do not bother to eat: lacking mouth and stomach,
they stand rooted to one spot. For nourishment, they rely completely on
symbiotic bacteria that live within their bodies to metabolize the
sulphurous volcanic soup in which they both thrive.

But the microscopic larvae of these giants are born bacteria-free, with a
complete digestive system. Juveniles swim, hunt, and eat before permanently
settling down and taking up with their microbial partners. Now the idea that
the larvae acquire their symbionts by eating them has been overturned. By
collecting the giant worms' tiny spawn from traps laid on the ocean floor,
oceanographers have shown that the sulfur-eating bacteria infect the larvae
through their skin.

Andrea Nussbaumer and Monika Bright of the University of Vienna, and Charles
Fisher, professor of biology at Penn State, report their findings this week
in the British journal Nature.

Previous groups had shown that, after a larva quits swimming and attaches
itself to the bottom of the ocean near a volcanic vent, its mouth disappears
and its stomach shrinks away, even as it grows a specialized organ called
the trophosome that houses the symbiotic bacteria it collects. "It is an
absolutely obligate symbiosis for the worm," Fisher explains. "If the larvae
do not get the right symbiont, they die."

The prevailing hypothesis was that the appropriate bacteria were gathered
into the stomach during feeding, somehow escaped digestion, and by remaining
in the stomach caused it to undergo metamorphosis into the trophosome.

But those conclusions were based on a very small set of observations, due to
the extreme difficulty of obtaining the tubeworm's larval and juvenile
stages. The only way to collect these delicate organisms is directly from
the ocean floor, at 2500 meters depth, in the deep sea vehicle Alvin. Bright
invented "tubeworm artificial settlement cubes," or "baby traps" as the team
calls them, to collect young, just-settled larvae and juveniles. They left
the traps at the bottom near an active hydrothermal vent and returned the
next season to collect them, bring them back to land-based laboratories, and
analyze them carefully using molecular techniques and fluorescence- and
electron microscopy.

By a painstaking reconstruction of electron micrographs of thin slices of
larvae and juvenile worms, the team showed that the symbionts do not enter
through the mouth, but through the skin, in a process akin to infection by
pathogenic bacteria. These bacterial partners then crawl inward, through
various larval tissues, not to the stomach but to an adjacent, "mesodermal"
tissue. Upon their arrival, the bacteria appear to induce the immature
mesodermal tissue to differentiate and form the trophosome, where they
proliferate and provide sustenance to the growing worm indefinitely. In
return the bacteria get a safe habitat and a reliable source of food.

"The symbiont, and only the symbiont, is capable of invading the skin of the
tubeworm larvae. It migrates through several layers of tissue towards the
interior of the host, and into the future trophosome," Bright explains.
"Once the trophosome is established, infection ceases, and no further
infection appears to be possible at later stages."

The researchers found that after the trophosome is established, further
infection appears to be prevented, in part by a wave of programmed cell
death in tissues where straggling bacteria remain.

"Biologists are realizing that symbiosis is not an oddity in nature, but
rather the norm," Fisher says. "Most - if not all - animals and plants exist
in symbiosis with some forms of microbes. We currently understand the early
stages of symbiont acquisition for only a very few of the multitudes of
symbioses. Since symbiosis is so widespread, understanding the mechanisms of
symbiont acquisition is a first order question for modern biologists."

"In this tubeworm," Bright adds, "the symbiont acquisition process resembles
the infection processes of pathogenic bacteria." "It may be," Fisher says,
"that understanding the early stages of symbiotic interactions will help us
to understand the early stages of host-pathogen interactions, and
vice-versa."

Full Text at Penn State University (includes hi res images of tubeworms)
http://www.science.psu.edu/alert/Fisher5-2006.htm

Posted by
Robert Karl Stonjek


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