Shedding new light on coral black band disease

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Image: A benthic cyanobacteria mat overgrowing live coral on an otherwise extremely healthy reef in Bonaire, The Netherlands. (Photo by Ethan Cissell)
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Credit: Photo by Ethan Cissell

UNC-Chapel Hill biologists are studying the links between microbial mats and a species of coral disease that has become an urgent conservation concern, and they are proposing mitigation strategies to reduce its spread. Coral reefs are valuable to marine ecosystems and the global economy. We spoke to sophie mccoy, Assistant Professor of Biology at the College of Arts and Sciences and Ph.D. Candidate Ethan Cissell on her research published in the journal Ecological Applications.

Q: What is the overall value of coral reefs to marine ecosystems and the economy?

A: It’s easy to take care of corals, they are such beautiful and charismatic animals. But corals also play a truly outsized role in providing tangible ecosystem services around the world. Corals form the skeleton, or underwater skyline, of reefs that support many other sedentary organisms, such as sponges and octocorals, which are valuable for water filtration and natural products, and which are also exploited by fish – including fisheries species. Physically, coral structures dampen waves and flooding from storms and protect shorelines from flooding and erosion. After all, healthy reefs are the backbone of lucrative tourism industries around the world, including in the United States.

Q: The increasing prevalence of coral disease is a pressing conservation concern. What have you found?

A: Coral diseases are becoming increasingly common on coral reefs around the world. Coral disease outbreaks can wipe out entire populations of endangered corals. This threatens the physical and economic services of the reefs, including coastal protection and food security. To work towards disease prevention, we also need to understand which aspects of coral reefs support the survival and spread of pathogens.

In this study, we identified benthic cyanobacteria mats as a possible refuge for black-band disease pathogens on coral reefs. Benthic cyanobacteria mats are microbial mats that grow on the sea floor in association with photosynthetic cyanobacteria, also called blue-green algae. Similar mats are also found in freshwater systems. Benthic cyanobacterial mats have spread worldwide on coral reefs and are related to water column cyanobacterial blooms that have become problematic on many coasts.

This paper identifies and describes priority research areas to specify links between cyanobacterial mats and coral blackband disease. We also discuss next steps to develop mitigation strategies to reduce the spread of black band disease.

Q: Why focus on Coral Black Bland Disease?

A: Black Band Disease, known as BBD, was one of the earliest coral diseases to be described and remains one of the easiest diseases to identify, as a black band separates healthy coral tissue from dead white skeleton. This black ring is actually caused by a microbial mat. This disease is described by the presence of a “microbial fingerprint” of multiple organisms, including cyanobacteria, sulfide-oxidizing, and sulfate-reducing bacteria. While BBD is still an active management challenge worldwide, many other coral diseases contribute to coral conservation challenges.

Q: Where are the coral reefs you analyzed in this study?

A: Our study focused on reefs in Bonaire, Netherlands. My research group chose to study Bonaire’s reefs because they are well-preserved due to long-standing conservation policies and are among the most pristine in the Caribbean. By studying processes in pristine ecosystems that are functioning properly, we can gain insights for the conservation and management of other coral reefs.

Q: What are the implications of these findings for decision makers?

A: We partnered with Caren Eckrich, a Natural Resources Manager at STINAPA, the National Park in Bonaire, Netherlands, for this study. Benthic cyanobacterial mats are normal components of all coral reefs, but are becoming more common with rising water temperatures and other runoff impacts on reefs. However, knowing that coral disease pathogens are associated with cyanobacteria mats may make management easier due to the visibility of the mats. For example, ongoing reef monitoring efforts should begin to include reef cover with cyanobacteria mats, which would add an inexpensive tool for management.

Other suggestions we make in this study help managers decide how best to allocate management and recovery resources, which are often limited. Management of fisheries, regulation of coastal development, control of runoff, and wastewater treatment are all local management strategies that are actively pursued to combat and control disease sources. A better understanding of coral disease transmission is critical to developing specific and effective strategies for coral monitoring, management and recovery, and creating evidence-based guidelines for sustainable coastal development.

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