Coral bleaching and viruses

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The Role of Viruses in Coral Bleaching

Introduction to Coral Bleaching and Viral Infections

Coral bleaching is a phenomenon where corals lose their symbiotic algae, known as zooxanthellae, leading to a pale or white appearance. This process is often triggered by environmental stressors such as temperature anomalies, high irradiance, and pollution. Recent research has highlighted the significant role that viruses play in coral bleaching events, adding a new layer of complexity to our understanding of coral health and disease.

Viral Outbreaks During Bleaching Events

Studies have shown that viral outbreaks are commonly associated with coral bleaching events. For instance, a study on the Pacific reef-building coral genus Acropora revealed high titers of viral particles during a natural outbreak of viral infection coinciding with mass bleaching. The identified viruses included retroviruses, herpesviruses, and nucleocytoplasmic large DNA viruses (NCLDVs), as well as megavirus-like particles associated with the algal endosymbiont Symbiodinium. This suggests that viral infections may be a common but previously undocumented component of natural bleaching events.

Nucleocytoplasmic Large DNA Viruses (NCLDVs) and Coral Bleaching

NCLDVs have been frequently implicated in coral bleaching. Research on corals from Mo’orea, French Polynesia, found that viral metagenomes from bleached corals contained significantly more eukaryotic virus sequences, particularly NCLDVs, compared to non-bleached corals. This indicates that NCLDVs may play a role in the onset and severity of coral bleaching. Similarly, another study identified NCLDV sequences in bleached corals, further supporting their involvement in bleaching processes.

Herpes-Like Viruses in Coral Health

Herpes-like viruses are also prevalent in coral tissues. Metagenomic analyses of the viral communities associated with healthy and partially bleached corals revealed that herpes-like sequences accounted for a significant portion of the total sequences. These viruses were found in both healthy and bleaching corals, suggesting they could infect various components of the coral holobiont, including the coral animal, algae, and microbial communities. This widespread presence indicates that herpes-like viruses may play a crucial role in both the preservation and breakdown of coral health.

Sunscreens and Viral Infections

Interestingly, human activities such as the use of sunscreens have been shown to exacerbate viral infections in corals. Laboratory and in situ experiments demonstrated that sunscreens containing organic ultraviolet filters could induce the lytic viral cycle in symbiotic zooxanthellae, leading to rapid and complete bleaching of hard corals. This highlights the potential impact of recreational activities on coral health through the promotion of viral infections.

The Coral Microbiome and Viral Interactions

The coral microbiome, which includes bacteria, archaea, fungi, protists, and viruses, plays a significant role in coral health and disease. Viruses within this microbiome can dynamically interact with their hosts, influencing microbial community dynamics, coral bleaching, and disease. For example, bacteriophages and archaeal phages are abundant in healthy corals and are likely involved in complex ecological networks and genetic material transfer. These interactions underscore the importance of considering the entire coral microbiome when studying coral health and disease.

Conclusion

The role of viruses in coral bleaching is multifaceted and significant. Viral infections, particularly those involving NCLDVs and herpes-like viruses, are closely associated with bleaching events. Human activities, such as the use of sunscreens, can exacerbate these infections, further stressing coral populations. Understanding the complex interactions between viruses and the coral microbiome is crucial for developing strategies to protect and preserve coral reef ecosystems in the face of environmental change.