A Chemistry Problem With Biological Consequences
While coral bleaching captures headlines, ocean acidification represents an equally serious — and less visible — threat to reef ecosystems. As the ocean absorbs carbon dioxide from the atmosphere, seawater chemistry changes in ways that directly undermine corals' ability to build and maintain their skeletons. Understanding this process is essential for grasping the full scale of the challenge facing coral reefs.
How Ocean Acidification Works
The ocean naturally absorbs a significant portion of the CO₂ emitted into the atmosphere. When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which then dissociates to release hydrogen ions. More hydrogen ions mean lower pH — in other words, more acidic water.
Since pre-industrial times, ocean surface pH has dropped measurably. While this may sound small, the pH scale is logarithmic, meaning even modest numerical changes represent substantial shifts in chemical conditions. The concentration of carbonate ions — the building blocks corals use for their calcium carbonate skeletons — decreases as pH falls.
What Acidification Does to Corals
Corals build their hard skeletons through a process called calcification, depositing calcium carbonate (CaCO₃) in the form of aragonite. Ocean acidification directly impairs this process in several ways:
- Reduced calcification rates: Corals grow more slowly and produce weaker, more porous skeletons under more acidic conditions.
- Increased dissolution: At sufficiently low carbonate saturation levels, existing coral structures begin to dissolve faster than they can be rebuilt.
- Impaired larval development: Coral larvae show reduced settlement success and survival in more acidic water.
- Altered behavior: Some fish species that reef ecosystems depend on show impaired sensory and behavioral responses in acidified water.
Interaction With Other Stressors
Ocean acidification does not act in isolation. It interacts with and amplifies other threats:
| Stressor | How Acidification Makes It Worse |
|---|---|
| Thermal bleaching | Weakened skeletons make post-bleaching recovery harder |
| Storm damage | Structurally weaker reefs break apart more easily |
| Bioerosion | Boring sponges and urchins erode weakened coral faster |
| Disease | Stressed corals are more susceptible to bacterial infection |
Are Any Corals More Resilient?
Research suggests that some coral species are more tolerant of acidification than others. Massive brain and star corals tend to fare better than branching species. Some corals in naturally acidic environments — such as volcanic CO₂ seeps — have shown adaptations to lower pH, giving scientists clues about what future reef communities might look like.
The Only Real Solution
Unlike some local threats, ocean acidification cannot be managed at the reef level — it is a global problem with a global cause. The only effective solution is reducing CO₂ emissions at scale. Local conservation measures can reduce other stressors and improve reef resilience, but they cannot counteract the chemistry of an acidifying ocean. This makes climate action the single most important factor in the long-term survival of coral reefs.