At least three benefits accrue to the bacteria in the tubeworms.
The simplest benefit listed is that the tubeworms "provide the
bacteria with a home." This is a somewhat vague benefit, but one could
envision this as a benefit in that the bacteria are kept in a fixed
location near the vents supplying nutrients. Without being held within
the tubeworm, the bacteria would freely float in the water. Since the
hot water is vented and mixes with the cold surrounding water, mixing
would readily occur, carrying bacteria away from the nutrient-rich
vents. (I have not seen the "providing a home" benefit described this
specifically, so the details are really my elaboration.)
Another benefit is that the tubeworms use their hemoglobin-type
molecules to concentrate and bring nutrients to the bacteria contained
in the tubeworm's trophosome.
A third benefit is that a tubeworm reduces competition from competing
bacteria by forestalling invasion by other bacteria once some bacteria
have established themselves in the tubeworm's trophosome.
The "home" benefit:
"Some of the biggest and most dominant species, like the giant
tubeworms, mussels, and clams, house the bacteria inside their bodies
in a finely tuned symbioses. In other words, these animals give the
bacteria a place to live, and in turn, the bacteria provide the host
with a food supply."
"This worm, called Riftia pachyptila, is an unusual animal because it
has no mouth or digestive tract and no apparent way to eat! Instead of
eating food like other animals, Riftia allows bacteria to live inside
of it and provide its food. The worms have a special feeding sac,
called a trophosome, which provides the bacteria with shelter and
ingredients to make food. In turn, the bacteria use these ingredients
to make food for the worm. The trophosome and the bacteria inside it
are so important that they make up over half the weight of this
"A kind of symbiosis, or "living together" has developed, where vent
organisms provide a secure "house" for the bacteria, who in turn,
provide carbon compounds to the host animal."
Provision of nutrients to the bacteria:
"Tubeworms that live in the vicinity of hydrothermal vents and cold
seeps are called vestimentiferans, and their tentacles are bright red
because they contain hemoglobin (like our own red blood cells).
Vestimentiferans can grow to more than 10 feet long, sometimes in
clusters of millions of individuals, and are believed to live for more
than 100 years. They do not have a mouth, stomach, or gut. Instead,
they have a large organ called a trophosome, that contains
chemosynthetic bacteria. Hemoglobin in the tubeworm?s blood absorbs
hydrogen sulfide and oxygen from the water around the tentacles, and
then transports these raw materials to bacteria living in the
trophosome. The bacteria produce organic molecules that provide
nutrition to the tubeworm."
Protection against competition by other bacteria:
"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 explained. "Once the trophosome is established, infection
ceases, and no further infection appears to be possible at later
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."
"The tube worms, however, depend on these bacteria a bit more
directly. They have a symbiotic relationship with these bacteria,
which they shelter inside of a specialized internal organ. The
bacteria, in turn, provide a built in energy source for the worms,
which can do without a digestive system as a result.
Researchers have started studying the dynamics and development of this
symbiotic relationship. It turns out that the larval form of the worms
is mobile, bacteria-free and has a full digestive system. Only after
the larvae establish residency at a vent do the bacteria arrive. It
would be easy to imagine that the bacteria get inside the worms via
the structures that later ensure their access to the vent water. But
those imaginings would be wrong: the bacteria actually latch onto and
invade the outer surface of the worm in a process the most closely
resembles the pathology of a skin infection. Once inside, the bacteria
continue to migrate until they come in contact with a patch of
undifferentiated tissue inside the worm. Upon contact with the
bacteria, that tissue differentiates into the mature feeding organ.
This differentiation sends a body-wide signal that further bacteria
are not welcome; other cells near inappropriately located bacteria
commit cellular suicide, taking the bacteria down with them. This
apoptotic response is a normal facet of an immune response, indicating
that the whole process is like an infection where the immune response
is temporarily suspended. These results provide an excellent model of
how a complex symbiotic relationship can be built up from a series of
normal behaviors, such as bacterial infection, a regulated immune
response, and a developmental signal that triggers differentiation of
the feeding organ."
I'm pretty sure you're familiar with a diagram of a tubeworm, but
here's a diagram just in case: