Ocean Acidification (OA) and Coral Bleaching (CB) are two distinct threats to coral reefs, often confused by the public. OA refers to the ongoing decrease in the ocean’s \(\text{pH}\) level, fundamentally changing its chemistry. CB is a biological process where corals expel the symbiotic algae living in their tissues. While separate, OA does not directly cause coral bleaching, but both are linked to the same global process, creating a compounding crisis for reef survival.
The Chemical Process of Ocean Acidification
Ocean acidification begins with the ocean absorbing a significant portion of the excess carbon dioxide (\(\text{CO}_2\)) released into the atmosphere by human activities. This absorption is a natural process, but the current rate of atmospheric \(\text{CO}_2\) increase is overwhelming the ocean’s natural buffering systems. Once dissolved in seawater, the \(\text{CO}_2\) reacts with water (\(\text{H}_2\text{O}\)) to produce carbonic acid (\(\text{H}_2\text{CO}_3\)).
This newly formed carbonic acid immediately dissociates into a bicarbonate ion (\(\text{HCO}_3^-\)) and a free hydrogen ion (\(\text{H}^+\)). The release of these hydrogen ions lowers the ocean’s \(\text{pH}\) and increases its acidity. Since the Industrial Revolution, the average \(\text{pH}\) of the ocean surface has dropped by approximately 0.1 units, representing about a 30 percent increase in acidity due to the logarithmic nature of the \(\text{pH}\) scale.
This increase in hydrogen ions has a secondary, highly detrimental effect on coral organisms. Hydrogen ions readily bond with the carbonate ions (\(\text{CO}_3^{2-}\)) already present in the seawater. This chemical reaction essentially locks up the available carbonate, converting it into additional bicarbonate.
The reduction in free carbonate ions poses a direct challenge to corals and other calcifying organisms. Corals require carbonate ions to combine with calcium ions (\(\text{Ca}^{2+}\)) to create calcium carbonate (\(\text{CaCO}_3\)), the building block for their hard skeletons. Less available carbonate means corals must expend significantly more energy to build and maintain these structures.
The Biological Mechanism of Coral Bleaching
Coral bleaching is a biological response rooted in the partnership between the coral animal and microscopic algae called zooxanthellae. These algae live within the coral’s tissue, perform photosynthesis, and transfer up to 90 percent of the food energy to the coral host. The zooxanthellae also provide the coral with its vibrant colors; when expelled, the coral appears white.
The primary trigger for mass coral bleaching events is sustained, elevated sea surface temperatures, known as thermal stress. When water temperatures rise even a small amount, typically just 1 to 2 degrees Celsius above the average maximum for a prolonged period, the zooxanthellae become stressed. This stress causes the algae’s photosynthetic machinery to malfunction.
The dysfunctional photosynthesis leads to the overproduction of harmful reactive oxygen species (ROS), which are essentially toxic compounds. Because these toxins can damage the coral host’s tissues, the coral polyps react by actively expelling the zooxanthellae from their cells.
The expulsion of the symbionts leaves the coral tissue translucent, revealing the white calcium carbonate skeleton underneath, hence the term “bleaching.” A bleached coral is not dead, but it is starving, having lost its primary source of nutrition. If water temperature returns to normal quickly, the coral may reacquire zooxanthellae and recover. However, prolonged bleaching often leads to disease and death.
How These Two Threats Affect Coral Structure and Survival
Ocean acidification and coral bleaching represent two different modes of attack on the coral organism: one is chronic and structural, and the other is acute and metabolic. Ocean acidification creates a long-term, slow-motion decline in the structural integrity of the entire reef. The reduced availability of carbonate ions forces corals to slow their calcification rate, meaning they build their skeletons more slowly.
Research shows that this \(\text{pH}\)-driven stress impedes the thickening and reinforcement of the coral skeleton, even if the coral continues to grow upward. This results in weaker, less dense structures that are more susceptible to physical damage from storms and biological erosion. The entire reef framework becomes fragile and unstable over time.
Coral bleaching, conversely, is an immediate, life-threatening event that determines survival over a matter of weeks or months. Its impact is focused on the coral’s energy supply, leading to mass starvation and potential mortality across vast reef systems. The acute nature of bleaching makes it the visible, headline-grabbing crisis of reef loss.
These two distinct threats act synergistically, where the presence of one amplifies the damage caused by the other. A coral chronically stressed by low \(\text{pH}\) conditions and possessing a weaker skeleton is less capable of surviving a subsequent thermal bleaching event. The energetic demands of coping with acidification can also lower the thermal threshold required to trigger the expulsion.
Even if a bleached coral survives thermal stress, its recovery is significantly hampered by an acidified environment. Rebuilding tissue and repopulating zooxanthellae requires high energy. The process of repairing or strengthening the skeleton is already slowed by the lack of carbonate building blocks. This weakened state prolongs recovery time, increasing the overall risk of death.
The Compounding Crisis of Shared Origins
The connection between ocean acidification and coral bleaching is their shared fundamental cause: the massive increase in anthropogenic carbon dioxide emissions. Excess \(\text{CO}_2\) drives global warming, leading to the elevated sea surface temperatures responsible for thermal bleaching. The ocean’s absorption of that same atmospheric \(\text{CO}_2\) triggers the chemical reactions leading to acidification.
Corals face a dual environmental assault that makes adaptation difficult. The planet’s \(\text{CO}_2\) output simultaneously attacks the coral’s internal energy source through heat stress and compromises its physical structure through \(\text{pH}\) changes. This shared origin means that mitigating climate change and reducing \(\text{CO}_2\) emissions addresses both threats at their source.