Corals often enter into symbioses with single-celled algae or bacteria that live inside the cnidarians. But most of the knowledge about these unusual communities comes from shallow water areas. Now marine biologists have also examined deep-sea corals for “roommates” and discovered two previously unknown bacterial species. They have some unusual characteristics, including an extremely small genome. This suggests that the symbiotic bacteria are no longer viable without their host.
Because of their solid, reef-building skeletons, corals are often mistaken for plants or lifeless rock. But they are cnidarians that often enter into symbioses with single-celled organisms such as algae or bacteria. For example, tropical shallow-water corals are usually home to so-called zooxanthellae – photosynthesizing algae that supply their host with carbohydrates in return for a safe roof over their heads. However, what the symbioses of deep-sea corals look like far from sunlight is still only partially understood.
Looking for bacteria in the deep sea
In the Gulf of Mexico, researchers led by Samuel Vohsen from Pennsylvania State University have now partially closed this knowledge gap. Over four years, the team took samples from coral colonies of the species Callogorgia delta and examined the cnidarians for single-celled inhabitants. Corals of this species grow along the continental slope between 400 and 900 meters deep, making them deep-sea corals adapted to cooler, darker water. The reefs of these corals create habitat for other animals such as brittle stars and serve, among other things, as an egg-laying site for the chain catshark (Scyliorhinus retifer).
But who do the deep-sea corals harbor within their skeleton? Vohsen and his colleagues found a diverse microbial community inside Callogorgia delta – including two previously completely unknown bacterial species from the Mollicutes group. The team named their new discoveries Oceanoplasma callogorgiae and Thalassoplasma callogorgiae and assigned them to their own, newly created family of bacteria: the Oceanoplasmataceae. Further analysis revealed the most unusual feature of this family: a tiny genome. While Thalassoplasma has just 385 genes that encode proteins for different metabolic functions, Oceanoplasma only has 359. For comparison: the intestinal bacterium Escherichia coli has more than 4,000 such genes, while we humans have around 21,000.
Parasite or beneficial insect?
“These bacteria don’t even have genes for normal carbohydrate metabolism, i.e. for obtaining energy from carbohydrates – something that every living thing actually has,” explains senior author Iliana Baums from the University of Oldenburg. Additional research has shown that the two newly discovered microbes can only use the amino acid arginine as an energy source. You get them from the host coral. “However, very little energy can be obtained from the breakdown of the amino acid. The fact that this is enough for the bacteria to live is truly astonishing,” emphasizes Baums. The bacteria also apparently receive other essential nutrients from their host.
But what does the host coral receive in return for its generous donations of nutrients? Vohsen and his colleagues are still unsure whether the two new bacterial species are equal “roommates” or parasites that benefit from the coral but give it nothing in return. If the microbes return the favor, they may do so by providing the coral with nitrogen by breaking down arginine. This is in short supply in the ocean depths. Perhaps the bacteria also actively protect the coral from pathogens. This is at least suggested by various defense mechanisms in the bacteria’s genome, with which they can remove foreign genetic material. This would also be consistent with the fact that the bacteria live in a gelatinous layer of tissue in the coral, through which nutrients are transported and which also serves as immune defense.
Source: Carl von Ossietzky University of Oldenburg; Specialist article: Nature Communications, doi: 10.1038/s41467-024-53855-5