They thrive where the notorious greenhouse gas rises from the sea floor: researchers report on tube worms, the basis of which is methane. A partnership with methane oxidizing bacteria enables them to do this. The two types of worms described for the first time thus capture methane, which could otherwise penetrate into the atmosphere. For this reason, the locations of these climate-relevant deep-sea inhabitants should be protected from disturbances caused by fishing nets and oil drilling, the researchers say.
Carbon dioxide is usually at the center of discussions about climate change – but another greenhouse gas also sets the world in a fever: methane does get into the atmosphere far less, but it has a 25-fold greater greenhouse effect than carbon dioxide. In addition to its anthropogenic release, natural sources of this gas are also known: it rises from deposits of organic material and also from the sea. There it flows from so-called “cold springs”, some of which are located at great depths on the sea floor. Fortunately, only a part of the methane escaping there reaches the surface because it is trapped deep – by living beings.
It is already known that the methane sources form veritable oases in the “deserts” of the deep sea. The basis of the communities is formed by bacteria that can use methane as food to build up biomass. They live freely in the water or form veritable mats on the bottom. They in turn feed on an illustrious community of other living things – from worms to crabs. However, there are also a few known animals that do not eat these microbes, but live in partnership with them: There are shells, sponges and worms that provide a home for the microbes in or on their bodies. In return, the bacteria provide them with food that comes from methane degradation. The scientists around Shana Goffredi from Occidental College in Los Angeles have now added two new representatives to these symbiotic creatures, who may be of great importance in methane mining in the area of cold sources.
Symbiosis or not?
In the course of their investigations of communities around methane sources in the deep sea, two representatives of the tube worms became the focus of the researchers. According to them, it became apparent that they are common around the world in cold sources. One of the two types belongs to the genus Laminatubus, the other to the Bispira. They are living things that sit in tubes and at the opening of which spring-like structures stretch into the water. So far, it was assumed that the tube worms with these structures fish for methane-oxidizing bacteria in the area of the cold springs and that they are incorporated as feed. As part of their study, Goffredi and her colleagues have now investigated the suspicion that these worms instead live in symbiosis with the bacteria – that is, they are colonized by them.
For this purpose, the researchers obtained samples of the animals by diving to methane sources, which are located at a depth of 1768 to 1887 meters off the coast of Costa Rica. The living worms were kept in special containers on board the research ship. In order to investigate whether they consume methane (CH4), the scientists provided them with a labeled version of the substance that contained the carbon isotope C13. As they report, after incubation with the labeled methane in the pool water, they found carbon dioxide that was enriched with carbon-13. That means: the worms or their bacterial partners had processed the methane. Subsequent investigations also confirmed that the carbon-13 from the methane had also passed into the biomass of the worms.
Climate-relevant deep-sea inhabitants
The scientists then also used microscopic investigations to track down the methane-oxidizing bacteria. Accordingly, they are representatives from the group of the Methylococcales, which exist in the feather-like structures of the tube worms. There they live protected and can collect methane from the water. How exactly the worms get the nutrients from their symbiotic partners is not yet completely clear. However, there are indications that the cells of the worms absorb and digest some bacteria. But it is also possible that there is a transfer of high-energy substances from the microbes to their partners.
“The two types of worm are now part of the small group of animals that are known to live closely with methane-oxidizing bacteria,” the researchers write. However, tube worms could play a particularly important role among these living beings, as can be seen from another result of the study. When exploring the area around the cold springs off the Pacific coast of Costa Rica, the scientists found that the worms cover large areas around the methane sources and are up to 300 meters away from them than other organisms in these deep-sea oases. Accordingly, they can apparently use even lower methane concentrations in the area.
Scientists assume that tube worms also play this role in other methane sources around the world. According to them, the question is how to define the boundaries of the areas of these ecosystems. If one wants to preserve the methane-degrading potential of these deep-sea dwellers, their potentially large spread should be taken into account. This could save the communities of the cold sources from damage caused by deep-sea fishing and oil production, the researchers say.
Source: Science Advances, doi: https://advances.sciencemag.org/content/6/14/eaay8562