Ocean Acidification Quiz: CO2, Chemistry, and Coral Crisis

Ocean acidification refers to the ongoing decrease in the pH of Earth's oceans resulting from the absorption of excess atmospheric carbon dioxide produced primarily by burning fossil fuels and deforestation. As CO2 dissolves in seawater, it forms carbonic acid, which dissociates to release hydrogen ions, lowering the pH of the ocean.

The chemistry of ocean acidification follows a clear reaction pathway. CO2 plus water forms carbonic acid (H2CO3), which dissociates into bicarbonate and a free hydrogen ion. The increase in hydrogen ion concentration lowers pH. Since industrialization, ocean surface pH has decreased from approximately 8.2 to 8.1, representing a 26 percent increase in hydrogen ion concentration.

Since pre-industrial times, ocean surface pH has dropped from approximately 8.2 to about 8.1. Although this may seem small, the pH scale is logarithmic, meaning this 0.1 unit decrease represents approximately a 26 percent increase in hydrogen ion concentration. This rate of acidification is faster than any known natural ocean chemistry change in the past 300 million years.

Ocean acidification reduces the concentration of carbonate ions in seawater, which are essential building blocks for calcium carbonate structures such as shells, coral skeletons, and the tests of echinoderms. As carbonate ion availability decreases and acidity increases, organisms must expend more energy to build and maintain their structures, and existing shells may begin to dissolve.

Ocean acidification affects the entire water column including deep-sea environments. Cold, deep water naturally holds more dissolved CO2 and is already closer to corrosive conditions. Deep-sea and cold-water coral ecosystems are particularly vulnerable because they exist near or below the aragonite saturation horizon, the depth below which calcium carbonate dissolves spontaneously.

Organisms that build structures from calcium carbonate are most vulnerable to ocean acidification. Oysters and bivalves struggle to form shells in more acidic water. Coral reefs experience reduced calcification and skeletal dissolution. Pteropods are considered sentinel species for acidification because their thin aragonite shells dissolve rapidly under lowered pH conditions.

The aragonite saturation horizon is the depth below which the ocean is undersaturated with respect to aragonite, a form of calcium carbonate, meaning it dissolves faster than it forms. As ocean acidification progresses, this horizon is rising toward the surface, threatening shallower organisms including cold-water corals and pteropods that were previously above the dissolution zone.

Ocean acidification causes reduced calcification in corals and mollusks, making shells thinner and more fragile. Research shows altered behavior and impaired olfactory and auditory responses in some fish exposed to elevated CO2. Coral reef dissolution is projected under high acidification scenarios. While some algae benefit from elevated CO2, not all marine algae increase in growth uniformly.

The ocean absorbs approximately 25 to 30 percent of anthropogenic CO2 emissions annually, which reduces the concentration of greenhouse gases in the atmosphere and slows the rate of atmospheric warming. However, this dissolved CO2 reacts with seawater through well-understood carbonate chemistry to produce carbonic acid, driving the ocean acidification that threatens marine ecosystems worldwide.

Ocean acidification and ocean warming are both consequences of rising atmospheric CO2 and frequently act together to compound stress on marine ecosystems. Warmer water holds less dissolved oxygen and accelerates metabolic rates, while acidification compromises shell formation and calcification. Coral bleaching from warming and skeletal dissolution from acidification can together devastate reef systems more severely than either stressor alone.

The key chemistry of ocean acidification involves CO2 dissolving in seawater to form carbonic acid, which then partially dissociates into bicarbonate ions and hydrogen ions. The increased concentration of free hydrogen ions lowers the pH of the water. Simultaneously, hydrogen ions react with carbonate ions to form more bicarbonate, reducing carbonate availability for shell-building organisms.

Ocean acidification and coral bleaching are distinct but interacting stressors on reef ecosystems. Bleaching is triggered by thermal stress from warming ocean temperatures, which causes corals to expel their photosynthetic symbiotic algae, known as zooxanthellae. Acidification separately reduces calcification and weakens skeletal integrity. Together, these stressors reduce coral resilience and recovery capacity, threatening reef systems globally.

Addressing ocean acidification requires both global and local strategies. Reducing CO2 emissions through renewable energy transition is the only solution that targets the root cause. Ocean alkalinity enhancement using materials such as crushed limestone is being researched as a local buffer strategy. Shellfish hatcheries in regions like the Pacific Northwest already buffer seawater. Removing sensitive organisms is not a viable mitigation strategy.

Pteropods are small free-swimming snails with delicate aragonite shells that are highly sensitive to changes in ocean pH. Studies have shown measurable shell dissolution in pteropods collected from regions experiencing elevated CO2 and reduced pH. Because of their sensitivity and ecological importance as prey for fish and whales, pteropods are widely used as biological indicators of ocean acidification.

The biological pump transports carbon from the surface ocean to depth through the sinking of dead organic matter and the shells of calcifying organisms. Ocean acidification threatens this process by reducing the abundance and shell mass of key calcifying plankton such as coccolithophores and foraminifera. A weakened biological pump could reduce carbon export to the deep ocean, potentially leaving more CO2 in the surface ocean and atmosphere.