Illustration of a vibrant coral reef with glowing bacteria interacting with the coral, symbolizing the hologenome concept.

Coral's Secret Weapon: How Bacteria Help Corals Survive and Evolve

Jordan Keane in Science & Nature August 2025 • 4 min read.

"Unlocking the hologenome concept and its implications for understanding coral resistance and adaptation."

Imagine a world where every living thing, including ourselves, isn't just a single entity but a complex partnership. This is the realm of the 'hologenome,' a concept that's changing how we understand evolution and life itself. At the heart of this idea is the profound relationship between a host organism and all the microbes it carries – its microbiome.

Corals, those vibrant and essential architects of marine ecosystems, offer a fascinating example of this partnership in action. For years, scientists have been puzzled by how corals can resist diseases and adapt to changing environments. The answer, it turns out, lies in their close relationship with bacteria.

This article explores the groundbreaking research that led to the hologenome concept, focusing on the specific case of coral bleaching. We'll delve into how corals acquire beneficial bacteria to resist pathogens, and how this symbiotic relationship shapes their very evolution, offering new insights into the intricate web of life.

The Coral Probiotic Hypothesis: A Turning Point in Understanding Disease Resistance

Illustration of a vibrant coral reef with glowing bacteria interacting with the coral, symbolizing the hologenome concept.

For years, scientists knew that rising ocean temperatures led to coral bleaching, where corals expel the colorful algae living in their tissues and turn ghostly white. In 1996, it was discovered that Vibrio shiloi bacteria were responsible for this bleaching disease in Oculina patagonica corals. But something unexpected happened between 2004 and 2006: the corals became resistant to V. shiloi.

This led to the coral probiotic hypothesis, suggesting that corals acquire beneficial bacteria to protect themselves from pathogens. This was supported by the fact that treating O. patagonica with antibiotics to kill the beneficial bacteria made them sensitive again to V. shiloi infection and bleaching.

Beneficial Microbes as Protectors: Symbiotic bacteria protect the host from pathogens, proves to be a general phenomenon.
Beneficial Effects of Intestinal Microbiota: Studies shows animal’s treatment with antibiotics are much more sensitive to infection with pathogens.

This finding inspired a broader consideration of beneficial microbes in multicellular life. The research led to the understanding that the first evidence for the beneficial effects of intestinal microbiota in terrestrial mammals came from studies in the 1950s, showing that animals treated with antibiotics were much more sensitive to infection with pathogens.

The Hologenome Concept: Redefining Life and Evolution

The findings about corals and their bacterial partners contributed to the development of the hologenome concept of evolution, postulating that the holobiont with its hologenome is a level of selection in evolution. This means, each animal and plant is composed of abundant and diverse microbial symbionts. In some cases, the numbers of symbiotic microorganisms and their combined genetic information far exceed that of their host.

This theory recognizes new dimensions of genetic variation: the changes in numbers of associated microorganisms, the acquisition of new strains, and horizontal gene transfer

As we continue to unravel the intricate relationships within the hologenome, we move closer to understanding the true complexity of life and open new avenues for conservation, disease treatment, and a deeper appreciation for the interconnectedness of all living things.

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Everything You Need To Know

What is the 'hologenome' concept, and why is it important in understanding coral?

The 'hologenome' concept suggests that a living organism, like a coral, should be viewed as a collective entity (a holobiont) composed of the host (coral) and all its associated microbes (its microbiome). The hologenome is the sum of the genetic information of the host and its microbiome. It is important because it reframes our understanding of evolution, considering the host and its microbes as a single unit of selection. This is particularly relevant to understanding how corals adapt, resist diseases, and evolve in response to environmental changes.

How did the discovery of Vibrio shiloi's role in coral bleaching lead to the 'coral probiotic hypothesis'?

The discovery that Vibrio shiloi causes bleaching in Oculina patagonica corals was initially a key finding. However, the observation that these corals later developed resistance to V. shiloi led to the coral probiotic hypothesis. This hypothesis proposes that corals can acquire beneficial bacteria to protect themselves from pathogens, offering a natural defense mechanism. The sensitivity of O. patagonica to V. shiloi after antibiotic treatment further supported the role of beneficial bacteria in coral health.

What evidence supports the idea that beneficial microbes protect organisms from pathogens, and how does this relate to the 'hologenome' concept?

Evidence supporting the role of beneficial microbes in pathogen defense comes from multiple sources. Studies in terrestrial mammals, dating back to the 1950s, demonstrated that animals treated with antibiotics are more susceptible to infections. This phenomenon, along with similar observations in corals, supports the idea that symbiotic bacteria protect the host from pathogens. This concept ties directly into the 'hologenome' concept, which posits that the holobiont (host plus microbes) functions as a unit of selection in evolution, emphasizing the crucial role of the microbiome in the host's survival and adaptation.

In what ways does the 'hologenome' concept challenge traditional views of evolution?

The 'hologenome' concept challenges the traditional view of evolution, which primarily focuses on the individual organism as the unit of selection. By considering the holobiont (host plus its associated microbes) and its combined genetic information (hologenome), this concept suggests that the interactions and co-evolution of the host and its microbiome play a significant role in evolutionary processes. The fact that symbiotic microorganisms can sometimes outnumber host cells and contribute more genetic information further emphasizes the importance of considering the holobiont as a whole in understanding evolutionary adaptation.

Can the 'hologenome' concept offer insights into coral conservation efforts?

Yes, the 'hologenome' concept can significantly enhance coral conservation efforts. Understanding that corals rely on their symbiotic relationship with bacteria for disease resistance, stress tolerance, and overall health opens up possibilities for targeted interventions. For example, researchers could potentially identify and introduce beneficial bacteria to corals, bolstering their resilience to stressors like rising ocean temperatures or pollution. This approach, informed by the 'hologenome' concept, could lead to more effective and sustainable strategies for preserving coral reefs.