The discovery of the hydrothermal vents in the deep sea was one of the scientific sensations of the 1970s: A completely foreign animal world revealed itself to the marine researchers - such as the tube worms, whose existence is based solely on endosymbiotic bacteria. But how do these beneficial germs get inside the worms?
Hot and poisonous. The environment of the deep-sea hydrothermal vents appears uninviting at first glance. In thick, black clouds, hot water shoots out of crevices in the earth at over 300 degrees, in which, in addition to many minerals, above all a strong cell toxin is dissolved: sulfide - better known in its gaseous version, hydrogen sulfide, which smells like rotten eggs.
But the secret of the unusual living world lies precisely in this substance. Because bacteria know how to use the toxin as a rich source of energy by oxidizing it with the oxygen that is sufficiently available in the deep sea. And the fauna that lives here feeds on the bacteria – directly or indirectly. Based on the chemosynthetic activity of the sulfur bacteria, hydrothermal vents represent habitats that exist independently of solar energy and the photosynthesis based on it.
The tube worms feel particularly at home here, such as the species Riftia pachyptila. The sedentary animals, which can be up to one meter tall, have no mouth or anus and – apart from gill tufts – do not show any other special organs for nutrition. They basically consist of a closed sack. This structure - the trophosome - is in turn packed with symbiotic bacteria that diligently oxidize sulfide, multiply vigorously and thus feed their host. The worm's only job is to provide its symbionts with enough oxygen and sulfide.
So far, so good. But how does Riftia get its nutritious guests in the first place? After all, every worm life begins without bacteria, so the animals have to become infected with their future suppliers during their youth. The question is not easy to answer, as the tube worms are used to the pressures prevailing at a water depth of 2500 meters - which does not exactly simplify their laboratory breeding.
In contrast to the adult animals, the larvae still have everything a worm needs to live freely - i.e. a mouth, anus and an intestine. For this reason, marine researchers previously assumed that the Riftia larvae simply eat their symbionts, but then do not digest them. Rather, the bacteria remain in the intestine, which then turns into that closed sac. The trophosome would therefore be nothing more than remodeled intestines.
Andrea Nussbaumer and Monika Bright from the University of Vienna, together with Charles Fisher from the University of Pennsylvania, have now tackled the mystery of tube worms. The biologists didn't even try to breed the animals in the laboratory, but moved their breeding to the deep sea: in the hydrothermal vents of the East Pacific, they sank "artificial tubeworm settlement cubes" - known as TASC (tubeworm artificial settlement cubes) for short - simple structures made of the building material hard PVC.
A year later, the marine biologists returned to the site to see if anything had changed on their TASCs. In fact, all three tubeworm species known here – Riftia pachyptila, Oasisia alvinae and Tevnia jerichonana – had made their home on the cubes. In this way, different age stages of the worms ended up in the researchers' nets and finally under the microscope.
Bright and her colleagues first confirmed something well known: the worm babies are free of symbionts. But even in the intestines of older stages, no symbionts could be detected. Conclusion: The larvae eat the bacteria completely, a symbiont colonization via the intestine fails.
The researchers struck gold elsewhere: on the skin of the worm larvae. Apparently, the bacteria do not use the mouth opening, but the body surface of their hosts as an entry gate. From here, the germs migrate into deeper layers and finally get into the mesodermal tissue – i.e. the cells from which muscles, for example, later develop.
Finally, the researchers are convinced that the infected mesoderm - and not the gut-forming endoderm - converts to the trophosome. Once the symbiote sac is complete, all other bacteria-containing cells die off, leaving the beneficial germs only where the worm wants them.
From now on, nothing stands in the way of a frugal existence in the midst of a hot, toxic environment.