Ocean Science · Research Status: Completed

Ocean Acidification: How Does Absorbed Carbon Dioxide Change Seawater Chemistry, and Which Organisms Are Most Sensitive?

Written and published by Kavya Butani

Founder of The Ocean Lens · Student Researcher, Goa, India

A student-led educational research article based on peer-reviewed studies and recognised scientific organisations.

“The ocean absorbs around one-quarter of the carbon dioxide released into the atmosphere each year. While this helps slow climate change, it also changes the chemistry of seawater in ways that many marine organisms struggle to tolerate.”

The ocean is often called Earth’s largest carbon sink because it absorbs vast amounts of carbon dioxide (CO₂) from the atmosphere. This natural process plays a vital role in reducing global warming by preventing some greenhouse gases from remaining in the air.

However, this service comes at a cost.

As more carbon dioxide dissolves into seawater, it triggers a series of chemical reactions that gradually change the ocean’s chemistry. This process is known as ocean acidification. Although these changes cannot be seen with the naked eye, they affect many marine organisms, particularly those that build shells and skeletons from calcium carbonate.

Scientists now consider ocean acidification one of the most significant long-term threats to marine ecosystems because it acts alongside warming oceans, pollution and habitat loss.

What Is Ocean Acidification?

Ocean acidification is the gradual decrease in the pH of seawater caused by the absorption of carbon dioxide from the atmosphere.

Seawater is naturally slightly alkaline, with an average pH of about 8.1. Since the Industrial Revolution, the average surface ocean pH has fallen by approximately 0.1 units, representing about a 30% increase in acidity because the pH scale is logarithmic.

Although the ocean remains alkaline, even small changes in pH can influence important chemical reactions that marine organisms rely on. (IPCC)

How Does Carbon Dioxide Change Seawater Chemistry?

When carbon dioxide enters the ocean, it reacts with water to form carbonic acid.

Carbonic acid then breaks apart into hydrogen ions and bicarbonate ions.

The increase in hydrogen ions lowers seawater pH and reduces the amount of carbonate ions available in the water.

Carbonate ions are essential because many marine organisms combine them with calcium to build shells and skeletons made of calcium carbonate.

A simplified process is:

From atmosphere to shell formation

01Atmospheric CO₂
02Dissolves into seawater
03Carbonic acid forms
04More hydrogen ions
05Fewer carbonate ions
06Harder to build shells and skeletons

(NOAA)

Which Organisms Are Most Sensitive?

Different marine species respond differently to ocean acidification. Organisms that rely heavily on calcium carbonate are generally considered the most vulnerable.

Coral Reefs

Reef-building corals use calcium carbonate to construct the skeletons that form coral reefs.

As carbonate ions become less available, corals may grow more slowly and produce weaker skeletons. This can reduce a reef’s ability to recover from storms, bleaching events and other disturbances.

Ocean acidification does not directly cause coral bleaching, but it can make already-stressed corals less resilient.

Shellfish

Oysters, mussels, clams and scallops all depend on calcium carbonate shells for protection.

Young shellfish are especially vulnerable because they must build shells rapidly during early development. Studies have shown that more acidic conditions may slow shell formation, reduce growth and lower survival rates in some species.

These impacts could affect both natural ecosystems and shellfish farming.

Plankton

Some microscopic plankton, including pteropods or “sea butterflies,” produce delicate calcium carbonate shells.

Although tiny, these animals are an important food source for fish, seabirds and whales.

A decline in pteropod populations could affect many organisms higher in the marine food web.

Sea Urchins and Other Invertebrates

Sea urchins, sea stars and some marine snails may also experience changes in growth, reproduction and larval development.

Scientists have found that early life stages are often more sensitive than adults, although responses vary between species.

How Does Ocean Acidification Affect Marine Ecosystems?

Ocean acidification influences more than individual species.

If shell-forming organisms become less abundant, predators that depend on them may also be affected.

Coral reefs, which provide habitat for roughly 25% of known marine species, may become less capable of supporting diverse communities if reef growth slows.

Changes in plankton populations could also alter food webs, fisheries and nutrient cycling throughout the ocean.

Can Marine Species Adapt?

Scientists are investigating whether marine organisms can adapt to changing ocean chemistry.

Some species appear more tolerant than others, particularly those living in naturally variable coastal environments.

However, adaptation takes time, and many marine organisms are already facing multiple pressures including:

Ocean warming
Marine heatwaves
Pollution
Habitat destruction
Overfishing
Ocean acidification

The combined effects of these stressors may reduce the ability of ecosystems to recover.

What Is Still Unknown?

Although ocean acidification has been widely studied, important questions remain.

Researchers continue to investigate:

Which species are most capable of adapting over multiple generations.
How ocean acidification interacts with warming oceans and declining oxygen levels.
Whether different regions will experience different rates of change.
How fisheries and seafood production may be affected in the future.
How changing seawater chemistry influences entire marine food webs over long periods.

Long-term monitoring will be essential for understanding these changes.

Summary

Ocean acidification occurs when the ocean absorbs carbon dioxide from the atmosphere, changing seawater chemistry by lowering pH and reducing the availability of carbonate ions used by many marine organisms to build shells and skeletons.

Corals, shellfish, pteropods and several other calcifying organisms are among the most sensitive groups. Because these species play important roles within marine ecosystems, changes to their populations may affect entire food webs.

While scientists continue studying how marine life may adapt, reducing carbon dioxide emissions remains the most effective long-term solution for slowing ocean acidification and protecting ocean biodiversity.

The ocean has helped protect our climate by absorbing carbon dioxide. Protecting the ocean now means reducing the emissions that are changing its chemistry.

About This Article

This article is part of The Ocean Lens – Research & Learning collection. It aims to make marine science accessible through evidence-based research written from a student perspective using peer-reviewed studies and recognised scientific organisations.

Written and Published by: Kavya Butani

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