Ocean acidification describes the ongoing decrease in the pH of the Earth’s oceans, a direct result of increased absorption of atmospheric carbon dioxide. This chemical change in seawater poses a significant challenge to marine life, with particularly profound implications for corals.
Understanding Ocean Acidification
The ocean acts as a vast sink, absorbing a significant portion of the carbon dioxide (CO2) released into the atmosphere from human activities. When CO2 dissolves in seawater, it reacts with water to form carbonic acid (H2CO3), a weak acid.
This carbonic acid then quickly dissociates, releasing hydrogen ions (H+) into the water. An increase in hydrogen ions leads to a decrease in the water’s pH, making it more acidic. This process of the ocean becoming more acidic is distinct from global warming, though both are driven by the same increase in atmospheric CO2.
Impacts on Coral Skeleton Formation
Corals construct their rigid external skeletons primarily from a form of calcium carbonate called aragonite. This process, known as calcification, depends on the availability of carbonate ions (CO3^2-) in the surrounding seawater. As ocean pH decreases due to acidification, the increased concentration of hydrogen ions binds with carbonate ions, forming bicarbonate (HCO3-).
This chemical reaction effectively reduces the concentration of free carbonate ions available for corals to build their skeletons. Scientists monitor the “aragonite saturation state,” which indicates how readily aragonite can form or dissolve. A lower aragonite saturation state makes it energetically more difficult for corals to extract the necessary building blocks, hindering their ability to grow and repair. In severely undersaturated conditions, existing coral skeletons can even begin to dissolve. This impacts the growth and structural integrity of various coral types.
Broader Physiological Effects on Corals
Beyond the direct impact on skeleton formation, ocean acidification imposes several other physiological stresses on corals. Corals may experience reduced growth rates as they expend more energy on calcification in an increasingly acidic environment. This leaves less energy for other vital biological processes. For instance, a coral might divert energy from growth to maintain its existing skeleton.
Acidification can also impair coral reproduction, affecting various stages from fertilization to larval development. Lower pH levels can reduce fertilization success and hinder the normal development of coral larvae. Compromised physiological health can also increase a coral’s susceptibility to diseases and other environmental stressors. While distinct from coral bleaching, acidification can exacerbate a coral’s vulnerability to bleaching events by weakening its overall resilience.
Consequences for Coral Reef Ecosystems
Coral reefs are intricate and highly biodiverse ecosystems, often referred to as the “rainforests of the sea.” They provide shelter, nursery grounds, and foraging areas for many marine species, including fish and invertebrates. The decline in healthy coral structures due to impaired calcification and increased dissolution directly translates to a loss of physical habitat.
This degradation of reef architecture leads to cascading effects throughout the marine food web. Species that rely on the complex reef structure for protection or food sources may decline in number, impacting biodiversity and ecosystem stability. The overall resilience of the entire marine environment is diminished as these foundational ecosystems weaken. Human communities also face consequences, as healthy reefs provide coastal protection from storms, support substantial fisheries, and underpin tourism industries globally.