Black band disease (BBD) is a coral affliction recognized by its distinct appearance on reefs worldwide. It manifests as a dark, mat-like band, ranging from deep red to black, that separates living coral tissue from the white skeleton left in its wake. This band moves across a coral colony, consuming living tissue at a rate that can reach up to one centimeter per day, leaving behind a trail of dead coral that alters the reef’s structure.
The Microbial Cause of the Disease
Black band disease is caused by a polymicrobial consortium, a community of different microorganisms working together. The visible structure of the dark band is formed by a dominant, filamentous cyanobacterium, most commonly Roseofilum reptotaenium. These cyanobacteria are rich in a pigment called phycoerythrin, which gives the band its color and creates the physical mat that rests on the coral’s surface.
Living within this cyanobacterial framework is a diverse array of other microbes, including sulfate-reducing bacteria (SRB). These bacteria are anaerobic, meaning they thrive in environments without oxygen. Their role is complemented by various heterotrophic bacteria that break down decaying coral tissue.
This assembly of organisms forms a self-sustaining microenvironment. The cyanobacteria provide organic matter through photosynthesis, which fuels the other members of the consortium. The interactions between these microbial groups create the toxic conditions at the band’s leading edge, enabling its steady advance across a healthy coral colony.
How Black Band Disease Kills Coral
The death of coral tissue occurs through an attack generated at the base of the microbial mat. The process begins as the dense mat physically smothers the coral polyps, blocking sunlight needed by the coral’s symbiotic algae. The mat also consumes available oxygen through respiration, especially at night, creating a localized, oxygen-deprived zone (anoxia) against the coral’s surface.
Within this anoxic environment, the sulfate-reducing bacteria in the consortium begin their work. These bacteria respire sulfate, which is plentiful in seawater, and release hydrogen sulfide as a byproduct. This chemical is highly toxic to coral tissue, causing cellular damage and death.
This creates a positive feedback loop at the band’s leading edge. As the coral tissue dies from anoxia and sulfide poisoning, it releases organic matter. This material serves as fuel for the microbial community, powering its growth and allowing the band to continue its migration across the colony.
Environmental Triggers and Spread
The development and spread of black band disease are strongly influenced by environmental conditions. Elevated seawater temperature is a primary trigger, as outbreaks are more common and progress faster during warmer months. This link suggests that rising ocean temperatures from climate change may increase the disease’s frequency and severity on a global scale.
Nutrient pollution (eutrophication) is another significant factor. Runoff from agriculture or sewage introduces excess nitrogen and phosphorus into reef waters. These nutrients act as a fertilizer for the cyanobacteria that form the disease mat, promoting their growth and making the disease more aggressive.
This disease is not confined to one region; it has been documented in coral reef ecosystems around the world, including the Caribbean, the Red Sea, and throughout the Indo-Pacific. The disease can be transmitted when fragments of the microbial mat break off and settle on a new, healthy coral.
Impacts on Coral Reef Ecosystems
The impact of black band disease extends beyond a single colony to the entire reef ecosystem. The disease is damaging because it often infects and kills large, slow-growing stony corals, such as brain and star corals. These massive colonies are the reef-building species that create the complex three-dimensional structure defining the habitat. The rapid tissue loss can remove these foundational species in a single season.
When these large corals are lost, the structural complexity of the reef diminishes, which has cascading effects on other organisms. The nooks and crannies of a healthy reef provide shelter for fish, invertebrates, and other marine life, protecting them from predators. Without this habitat, these populations may decline due to increased vulnerability.
This degradation can lead to a “phase shift” on the reef. As the coral framework erodes, the substrate becomes dominated by turf or macroalgae, which colonize the bare skeleton. This transforms a diverse coral reef into a less complex, algae-dominated ecosystem that supports fewer species, altering the local food web and reducing the reef’s overall health.