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Discovery of Unique Bacterial Symbionts in Deep-Sea Corals

A research team from Germany and the United States, led by Professor Iliana Baums of the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg and Dr. Samuel Vohsen from Lehigh University, has encountered two remarkable bacterial species within the tissues of deep-sea corals in the Gulf of Mexico. This groundbreaking finding, detailed in a paper published in the scientific journal Nature Communications, reveals these newly identified coral symbionts possess a significantly reduced genome and are unable to derive energy from carbohydrates. Baums, who co-authored the article, remarked, “These species exemplify how minimal genetic resources can still support a functional organism.”

The investigation focused on two soft coral species, Callogorgia delta and Callogorgia americana, thriving at ocean depths between 300 to 900 meters, a habitat devoid of light. Researchers identified two new bacterial species belonging to the mollicutes class, which are commonly known as parasites that inhabit various organisms, sometimes leading to disease. Their genetic analyses led to the proposal of a new family, Oceanoplasmataceae, for these bacteria.

In-depth exams indicated these bacteria are the primary symbionts of the studied corals, residing in a jelly-like tissue layer integral to the corals’ immune defense and nutrient transport systems. One species, Oceanoplasma callogorgiae, comprises a mere 359 genes related to metabolic functions, while its counterpart, Thalassoplasma callogorgiae, has 385 protein-coding genes. To put this into perspective, the intestinal bacterium Escherichia coli contains over 4,000 of these genes, and the human genome comprises nearly 21,000.

Energy Source and Metabolic Mysteries

Amino acid is their only source of energy

The researchers are puzzled by how these two novel bacterial species can thrive with such minimal genetic material. “They do not possess genes necessary for typical carbohydrate metabolism—an essential energy acquisition method for nearly all known living organisms,” Baums noted. Current understanding suggests that their sole energy source is the amino acid arginine, supplied by the host corals. Vohsen added, “However, metabolizing this amino acid yields only a small amount of energy. It’s remarkable that these bacteria can persist on such limited resources.” Additionally, they rely on the corals for other vital nutrients.

The nature of the relationship between the bacteria and the corals is still uncertain—whether they are entirely parasitic or if the corals derive some benefits from them. Genetic studies indicate that these bacterial species employ various CRISPR/Cas systems, known for their role in genetic editing in biotechnology, to eliminate foreign DNA. The scientists speculate these mechanisms might aid corals in warding off pathogens, while another theory posits that the bacteria could provide nitrogen to their coral hosts through arginine breakdown.

Insights into Coral Adaptation

For Baums, the symbionts present an invaluable opportunity to deepen the understanding of coral evolution and ecology. She expressed her fascination with corals’ ability to inhabit diverse environments despite their relatively simple genetic architecture. Baums explains that symbiotic relationships are vital for corals to adapt to various environmental conditions, offering metabolic capabilities the corals themselves do not possess. For instance, tropical corals rely on photosynthetic algae for energy in sunlit waters, whereas many deep-sea corals depend on bacteria to convert nutrients or extract energy from chemical compounds in dark and nutrient-scarce ecosystems.

Baums is a noted evolutionary ecologist and coral researcher affiliated with the Helmholtz Institute for Functional Marine Biodiversity and holding positions at both the University of Oldenburg and the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven. The collaborative study involved contributions from scientists at the Max Planck Institute for Marine Microbiology in Bremen, Kiel University, and Pennsylvania State University in the US.

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