Ocean acidification (OA) and coral bleaching (CB) are two distinct phenomena driven by the ocean’s absorption of anthropogenic carbon dioxide (CO₂). Coral bleaching is the visible, acute response of corals to environmental stress, while ocean acidification is a slower, chronic alteration of ocean chemistry. The core answer to whether OA causes CB is no, as they affect coral physiology through different mechanisms. However, they are not independent, as both stressors arise from increasing atmospheric CO₂. Their interaction severely compromises the long-term health and survival of reef ecosystems, creating a far greater threat than either one alone.
The Primary Trigger of Coral Bleaching
Coral bleaching is triggered by sustained periods of elevated sea surface temperature, a form of heat stress. Corals maintain a symbiotic relationship with tiny single-celled algae called zooxanthellae, which live within the coral polyp’s tissues. These algae use photosynthesis to convert sunlight into energy, supplying the coral host with up to 90% of its nutritional requirements. The coral’s color is derived directly from the pigments within these algal cells.
When water temperature exceeds the coral’s upper thermal limit, the photosynthetic machinery within the zooxanthellae malfunctions. This breakdown produces harmful reactive oxygen species, which damage the coral host’s tissues. The coral polyps respond by expelling the symbiotic algae, leaving the translucent tissue covering the white calcium carbonate skeleton. This white appearance is bleaching. A bleached coral is not immediately dead, but it is starving and can only survive for a short period, hoping temperatures drop quickly enough to re-acquire its algal partners.
How Ocean Acidification Impedes Skeleton Building
Ocean acidification is a direct chemical consequence of the ocean absorbing roughly one-quarter of the excess CO₂ emitted into the atmosphere. This absorbed CO₂ reacts with seawater to form carbonic acid, which increases the concentration of hydrogen ions and lowers the ocean’s pH. This shift in ocean chemistry directly impacts the carbonate system by reducing the concentration of carbonate ions (CO₃²⁻) available in the water.
Corals and other calcifying marine organisms build their skeletons from calcium carbonate in the mineral form of aragonite. The process, called calcification, requires corals to extract calcium ions and carbonate ions from seawater to precipitate the solid skeletal structure. As ocean pH drops, the reduced availability of carbonate ions makes it energetically taxing for corals to secrete their skeletons. This chemical challenge slows the overall calcification rate, meaning corals grow slower and produce less dense structures.
Ocean acidification impedes skeletal thickening, leaving corals with skeletons that are more porous and fragile. A 20 to 60 percent decline in coral calcification is projected by mid-century under continued CO₂ emissions. This weakened structure makes the corals more susceptible to breakage from storms and physical damage from bioeroding organisms. Ocean acidification is a chronic, structural weakening that undermines the physical foundation of the entire reef.
The Synergistic Threat: Impaired Recovery and Resilience
While ocean acidification does not cause acute coral bleaching, the two stressors combine to create a synergistic threat that is far more damaging than either one alone. Acidification acts as a background stressor that lowers the coral’s overall physiological health, making it less resilient to temperature spikes. Corals experiencing the effects of ocean acidification may bleach at a lower thermal threshold than those in a healthy chemical environment. The dual exposure accelerates the onset of bleaching events.
The most profound interaction occurs during the period immediately following a bleaching event. When a coral expels its zooxanthellae, it loses its primary energy source and must rely on stored energy reserves to begin the recovery process. Ocean acidification impairs this recovery by compromising the coral’s ability to repair and rebuild its damaged skeleton. The reduced availability of aragonite slows the metabolic processes required for the coral to heal tissue damage and re-establish the symbiotic relationship with new algae.
A coral attempting to recover requires high rates of calcification for skeletal maintenance and growth, but the acidifying conditions inhibit this process. The coral is already weakened from starvation and thermal stress, and chronic chemical stress prevents it from strengthening its structure and returning to a healthy state. This combined effect significantly prolongs recovery time, increasing the likelihood that the coral will succumb to disease, predation, or a subsequent bleaching event before it can fully heal.
The Fate of Reef Ecosystems Under Dual Stress
The dual pressure of recurrent bleaching and chronic acidification is rapidly altering the ecological structure of reef systems worldwide. The loss of calcification and structural integrity at the individual coral level translates directly into the degradation of the larger reef framework. As the weakened skeletons are eroded by biological and physical forces, the three-dimensional complexity of the reef habitat begins to collapse.
This loss of structural complexity reduces the habitat and shelter available for the countless fish, invertebrates, and other species that rely on the reef for survival. The transition from a structurally complex coral reef to a flatter, rubble-dominated environment results in a corresponding decline in biodiversity. The robust calcium carbonate structures that protect coastlines from storm surges also become more vulnerable to dissolution.
If current trends continue, the combined threats of thermal stress and acidification are projected to reduce many reefs to a state of net dissolution, where the rate of skeletal breakdown exceeds the rate of growth. This outcome signifies the functional collapse of the reef ecosystem. This impacts the ecological services, such as fisheries and coastal protection, that these complex habitats provide to human communities. The dual stressors fundamentally challenge the capacity of reef-building corals to maintain their role as the primary engineers of one of the planet’s most diverse ecosystems.