A research team from Germany and the United States, led by Professor Iliana Baums from the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg and Dr. Samuel Vohsen from Lehigh University, has made an intriguing discovery of two unusual bacterial species within the tissues of deep-sea corals in the Gulf of Mexico. These newly identified coral symbionts possess an extremely simplified genome and are unable to derive energy from carbohydrates, as detailed in their article published in the journal Nature Communications. “These species are impressive examples of how few genes are needed for a functional organism,” says Baums, who co-authored the paper.
The team examined several colonies of two soft coral species, Callogorgia delta and Callogorgia americana, which thrive in the dark depths of the Gulf of Mexico, ranging from 300 to 900 metres. They identified two previously unknown, closely related species belonging to the mollicutes class of bacteria. Typically, mollicutes are known to exist as parasites in the cells of various organisms, including plants, animals, and humans, and can sometimes lead to diseases. Based on their genetic findings, the researchers propose a new family named Oceanoplasmataceae for these two bacterial species.
Further analysis showed that these bacteria are the primary symbionts of the corals, residing in a gelatinous tissue layer that plays a role in the corals’ immune defense and nutrient transport. One of the species, Oceanoplasma callogorgiae, has a mere 359 genes that code for proteins involved in various metabolic processes, while the other, Thalassoplasma callogorgiae, contains 385 protein-coding genes. In contrast, the well-known intestinal bacterium Escherichia coli has over 4,000 such genes, and humans possess around 21,000.
The researchers are puzzled by how these two newly discovered microbes can sustain their metabolism with such a limited genome: “These bacteria don’t even carry genes for normal carbohydrate metabolism, in other words, for obtaining energy from carbohydrates — something that basically every living organism has,” Baums explains. According to the research to date, their only source of energy is the amino acid arginine, which is provided by the host coral. “But the breakdown of this amino acid provides only tiny amounts of energy. It is astonishing that the bacteria can survive on so little,” says Vohsen. The bacteria also obtain other essential nutrients from their host.
It is still uncertain whether the microbes act solely as parasites or if the corals gain some advantages from their symbiotic relationships. Genetic analyses conducted by scientists reveal that the two bacterial species employ various defense strategies known as CRISPR/Cas systems to eliminate foreign DNA. These systems are also utilised in biotechnology for gene editing. The researchers propose that these mechanisms might benefit the host corals by helping them combat pathogens. Another theory suggests that the bacteria could supply nitrogen to their host by breaking down arginine.
For Baums, who studies both the ecology and evolution of corals, these symbionts present a chance to deepen our understanding of this diverse group of organisms. “I always find it amazing that corals can colonise so many different habitats despite being very simple animals in terms of their genetic blueprint,” she remarks. She emphasizes the importance of symbionts in enabling corals to adapt to varying environmental conditions: “They provide essential metabolic functions that corals themselves do not possess.” For instance, tropical corals, which inhabit shallow, sunlit waters, depend on photosynthetic algae for nourishment and energy. In contrast, cold-water corals, many of which exist in dark, nutrient-scarce deep-sea environments, are believed to rely on bacteria to transform nutrients or derive energy from chemical compounds.
[image: A field of the soft coral Callogorgia sp. with its ophiuroid symbiont. Gulf of Mexico – NOAA]
