A research group has uncovered two extraordinary types of bacteria residing in the tissue of deep-sea corals from the Gulf of Mexico. These previously unidentified coral symbionts possess an incredibly small genome and are unable to extract energy from carbohydrates, according to the team’s findings published in the scientific journal Nature Communications. The bacteria fall under a newly identified family.
Led by Professor Iliana Baums from the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg (HIFMB) and Dr. Samuel Vohsen from Lehigh University in the US, a German-American research team has found two highly unusual bacterial species in the tissue of two deep-sea corals from the Gulf of Mexico. These little-known coral symbionts exhibit a significantly reduced genome and do not have the capability to generate energy from carbohydrates, the team reports in an article published in the scientific journal Nature Communications. “These species are remarkable examples of how minimal genetic information can sustain a functioning organism,” Baums, a co-author of the paper, states.
The research team examined various colonies of two soft coral species, Callogorgia delta and Callogorgia Americana, which thrive in the depths of the Gulf of Mexico anywhere from 300 to 900 meters, a region devoid of light. Upon investigation, the scientists discovered two closely related yet previously unknown species belonging to the mollicutes class of bacteria. Typically, mollicutes exist as parasites either on or within the cells of plants, animals, and humans, occasionally causing diseases. Based on their genetic research, the scientists suggest the establishment of a new family named Oceanoplasmataceae, to which these bacteria belong.
Further studies indicated that these bacteria act as the primary symbionts for the corals, residing in a gelatinous tissue layer that contributes to the corals’ immune defenses and nutrient transportation. One of the species, identified as Oceanoplasma callogorgiae, includes a mere 359 genes that encode proteins for various metabolic processes. The other species, Thalassoplasma callogorgiae, has 385 protein-coding genes. In contrast, the intestinal bacterium Escherichia coli contains over 4,000 such genes, while the human genome has about 21,000.
Amino acids as their sole energy source
Researchers are puzzled over how the metabolism of these newly found microbes might function with such limited genetic material: “These bacteria lack genes typically associated with normal carbohydrate metabolism, which is a requirement for energy extraction from carbohydrates—something almost all life forms possess,” Baums clarifies. Current research suggests that their only energy source comes from the amino acid arginine supplied by the host coral. “However, the energy obtained from breaking down this amino acid is minimal. It’s astonishing that these bacteria can survive with so little,” notes Vohsen. Additionally, the bacteria acquire other essential nutrients from their coral host.
It’s still uncertain whether the bacteria function solely as parasites, or if the corals derive some benefits from their presence. According to the genetic analysis conducted by the scientists, the two bacterial species utilize diverse defense mechanisms known as CRISPR/Cas systems to eliminate foreign DNA. These systems are also applied in biotechnological gene editing. The researchers propose that these mechanisms could aid the host corals in warding off pathogens. Another possibility is that the bacteria generate nitrogen for their host during arginine breakdown.
For Baums, whose research delves into coral ecology and evolution, the presence of these symbionts provides a chance to deepen our understanding of the evolutionary history of corals. “I’m continually amazed that corals can adapt to such a variety of environments, despite being quite simple organisms in terms of genetics,” she shares. Symbionts play a critical role in allowing corals to adjust to various environmental conditions, as they furnish metabolic capabilities that corals themselves may lack. For instance, tropical corals in shallow waters depend on photosynthetic algae for nourishment and energy, while cold-water corals, often found in dark, nutrient-sparse areas, are believed to rely on bacteria for nutrient conversion or for drawing energy from chemical compounds.
Baums, an evolutionary ecologist specializing in corals, conducts her research at the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg (HIFMB), where she also holds a joint professorship with the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven. Alongside Professor Baums and Dr. Vohsen, researchers from the Max Planck Institute for Marine Microbiology in Bremen, Kiel University, and Pennsylvania State University in the US contributed to this study.
