Great barrier reef

https://www.downtoearth.org.in/climate-change/heat-stress-rare-disease-wipe-out-goniopora-coral-colonies-on-great-barrier-reef

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Picture Courtesy: Down to earth

Context:

A combination of extreme marine heatwaves and a rare infectious disease, Black Band Disease (BBD) has caused 75% mortality among Goniopora (flowerpot) corals at One Tree Reef, Great Barrier Reef.

Current Status:

• At One Tree Reef, 75% of Goniopora colonies died, and surveys of more than 700 colonies showed a uniform pattern.
• The ongoing global bleaching event has affected 84% of coral reefs worldwide, making it the most extensive on record.
• Heat stress remained unusually prolonged, with 74 consecutive days above 28°C, 21 days exceeding 29°C, and temperature peaks reaching 34–35°C.

What are Corals?

Corals are marine invertebrates composed of numerous tiny, genetically identical organisms known as polyps, which live together in colonies and secrete calcium carbonate to form the solid structures that make up coral reefs. A defining characteristic of corals is their close association with microscopic algae called zooxanthellae that reside within their tissues and contribute significantly to their nutrition and coloration.

Symbiotic relationship with Zooxanthellae

Corals and zooxanthellae maintain a mutualistic partnership in which the coral provides the algae with shelter and the compounds required for photosynthesis, while the zooxanthellae, in return, supply the coral with energy-rich organic molecules produced through photosynthesis. These nutrients enable corals to grow and build their calcium carbonate skeletons, and the pigments of the algae impart the vivid colours characteristic of healthy coral reefs.

Different types of Corals:

Hard Corals (Scleractinian Corals): Hard corals are the primary reef builders, producing aragonite-based calcium carbonate skeletons that create the rigid framework of coral reefs. They typically inhabit shallow, warm, clear, and well-lit waters where conditions favour photosynthesis. Common examples include Acropora, Porites, and Montipora.

Soft Corals (Alcyonacean Corals): Soft corals lack the solid skeletons of hard corals and therefore do not contribute to reef formation. Their bodies are flexible and often tree-like, supported internally by small structures called spicules. They are more tolerant of deeper or turbid waters compared to hard corals. Examples include sea fans, sea whips, and sea pens. 

Importance of Corals:

• Support exceptional biodiversity by providing habitat, breeding grounds, and feeding areas for a wide range of marine organisms, including fish, crustaceans, molluscs, and numerous invertebrates.
• Protect coastlines by absorbing wave energy and reducing the impact of storms, coastal erosion, and flooding on beaches and human settlements.
• Sustain fisheries and livelihoods, as millions of people, especially in tropical regions depend on reef-associated fish species for food security and income.
• Boost tourism and local economies by attracting divers, snorkelers, and other visitors, thereby generating significant revenue for coastal communities.
• Regulate marine biogeochemistry through their calcium carbonate structures, which help maintain nutrient cycles, enhance water clarity, and support overall productivity in tropical marine ecosystems.

What Is Coral Bleaching?

Coral bleaching occurs when corals lose their characteristic colour after expelling or losing their symbiotic zooxanthellae, the algae that normally provide them with food through photosynthesis and confer vibrant pigmentation. Elevated sea temperatures are the primary trigger for bleaching, although factors such as pollution, sedimentation, and changes in salinity can also induce stress. Once the algae are lost, corals turn white or pale, their energy supply declines sharply, and they become increasingly vulnerable to disease, reduced growth, and mortality.

Picture Courtesy: usgs

Why this event at One Tree Reef is a red flag?

• Thermally tolerant corals are now dying: Goniopora corals are widely recognised as thermally tolerant and long-lived, yet their sudden collapse shows that even resilient species are losing the capacity to endure accelerating ocean warming.

• Heat–disease feedback loop: Exceptionally high sea temperatures weaken coral immunity, accelerate the growth and virulence of microbial pathogens, and prolong bleaching events, thereby creating conditions that favour explosive disease outbreaks.

• Collapse of reef structural complexity: The death of large, structure-forming Goniopora colonies reduces the three-dimensional architecture of reefs, which in turn diminishes habitat availability for fish, crustaceans, and juvenile marine organisms.

• Disruption of Coral–microbiome relationships: Heat stress destabilises the delicate relationship between corals and their microbial symbionts, including zooxanthellae and beneficial bacteria.

• Early indication of evolutionary limits: While corals possess some adaptive capacity through symbiont switching, genetic variation, and acclimatisation, the rapid pace of warming suggests these evolutionary mechanisms are being outstripped.

• Breakdown of recovery cycles: Coral reefs historically relied on cooler intervals between heatwaves to recover, regenerate tissue, and rebuild energy reserves. The increasing frequency and duration of marine heatwaves eliminate these recovery windows.

What are the key stress factors driving coral mortality?

• Marine heatwaves: Marine heatwaves have emerged as the primary driver of widespread coral bleaching, and the temperatures rising to 34–35°C at One Tree Reef, far above the normal thermal tolerance of most coral species. Such extreme warming weakens coral immunity and disrupts the symbiosis between corals and zooxanthellae, leading to mass bleaching and physiological collapse. A similar pattern was observed during the 2016 Great Barrier Reef bleaching event, when heatwaves caused the loss of 30% of shallow-water corals within a single season

• Prolonged thermal Stress: Extended periods of elevated temperature are often more damaging than brief heat spikes, as they prevent corals from recovering once bleaching begins. When thermal stress persists beyond two to three weeks, corals exhaust their stored energy reserves, suffer metabolic dysfunction, and become increasingly susceptible to disease. A well-documented case occurred in Kiribati’s Phoenix Islands, where 18 consecutive months of elevated temperatures between 2014–2016 led to widespread bleaching and mortality exceeding 75% in some reef sections.
• Disease outbreaks: Coral diseases frequently follow bleaching events, as stressed corals have reduced immunity and are more vulnerable to pathogenic microbes. Outbreaks such as Black Band Disease, White Syndrome, and Stony Coral Tissue Loss Disease can rapidly destroy coral tissue and accelerate mortality. In the Caribbean, the spread of white-band disease during the 1980s caused a over 90% decline in Acropora palmata and Acropora cervicornis, fundamentally altering reef structure across the region.

• Species-specific vulnerability: Different coral species exhibit varying levels of tolerance to heat stress, disease, and environmental changes, which means that climate events can alter reef community composition by selectively eliminating more sensitive taxa. For instance, Acropora species, though fast-growing and major reef builders, are highly susceptible to bleaching and disease and have declined sharply in many parts of the Indo-Pacific. In contrast, massive corals like Porites often survive bleaching but may suffer reduced growth and reproduction.

What are the ecological and global implication of coral mortality?

Structural collapse of reef ecosystems: The loss of long-lived corals reduces reef complexity, leading to declines in fish diversity, spawning habitats, and reef-building capacity. After the 2016 Great Barrier Reef bleaching, reefs that lost over 50% of corals recorded sharp drops in habitat-dependent fish species

Human impact: Coral degradation threatens food security for over one billion people who rely on reef fisheries and undermines coastal protection, as reefs absorb up to 97% of wave energy, reducing storm damage.

Climate change: Climate change is outpacing coral adaptation, with the IPCC projecting 70–90% reef loss at 1.5°C warming and over 99% at 2°C. Severe bleaching in remote, low-pollution regions such as the Phoenix Islands Protected Area shows that even pristine reefs are no longer protected from rising ocean temperatures.

Conclusion:

The mortality of Goniopora corals on the Great Barrier Reef showcases a dangerous new climate reality: even resilient corals are failing under extreme heat–disease interactions. This event is a global warning, emphasizing that without rapid emission reductions, coral reefs are critical ecosystems supporting a billion people, may face widespread collapse.

Source: Down to Earth