how are calcifiers, organisms that build their own shells, affected by an increase in atmospheric carbon dioxide?

June 28, 2026

An increase in atmospheric carbon dioxide generally makes life harder for most calcifiers (shell‑builders), slowing their shell growth and in some cases even dissolving their shells, though a few species show neutral or even boosted shell production under certain conditions.

What happens chemically

When more CO₂ dissolves into the ocean, it changes seawater chemistry in ways that are bad for shell‑building.

CO₂ dissolves in seawater and forms carbonic acid, which releases hydrogen ions and lowers pH (ocean acidification).

Extra hydrogen ions react with carbonate ions, turning them into bicarbonate, so there is less carbonate available to make calcium carbonate shells.

As pH drops and carbonate falls, the saturation state of calcium carbonate minerals (aragonite and calcite) drops, making shells harder to build and easier to dissolve.

An easy way to picture it: shell‑builders are like bricklayers suddenly told that the supply of bricks has been cut and the bricks they already laid are starting to crumble.

Effects on different calcifiers

Plankton and microscopic calcifiers

Experiments on coccolithophores (tiny plankton that make chalky plates) show reduced calcite production and more malformed plates under higher CO₂.

This reduces the rain of calcium carbonate to the deep ocean and can alter carbon cycling and ocean–atmosphere CO₂ exchange.

Corals

Reef‑building corals generally calcify more slowly as CO₂ rises and pH drops; some studies project coral calcification could fall by up to about 40% at ~560 ppm CO₂.

At very high CO₂, aragonite (the form of CaCO₃ many corals use) can start to dissolve, weakening skeletons and reefs.

Shellfish and other invertebrates

Oysters, clams, and some snails often show thinner shells and slower growth in more acidic water, making them more vulnerable to predators and stress.

In extreme experiments (very high CO₂), shells of clams, snails, urchins, and other calcifiers actually began to dissolve, and prolonged exposure could strip shells completely.

Young life stages (larvae and juveniles) are usually more sensitive, struggling to form first shells in under‑saturated or low‑saturation conditions.

Mixed and species‑specific responses

Not all calcifiers react the same way, which complicates the story.

In a multi‑species experiment, 10 of 18 species reduced net calcification or even experienced net shell dissolution under elevated CO₂.

Seven species, including some crabs, shrimp, and lobsters, actually increased shell production at certain high CO₂ levels, likely by biochemically manipulating dissolved inorganic carbon at their site of calcification.

One species showed little change, highlighting that physiology, shell mineralogy (aragonite vs calcite), and protective organic layers all influence vulnerability.

So, while the overall trend is negative for many key groups (corals, pteropods, some shellfish), there are pockets of resilience or even short‑term advantage in a few species.

Broader ecosystem consequences

Changes in calcifiers ripple through marine ecosystems and the carbon cycle.

Weaker or slower‑growing coral skeletons undermine reef structures that protect coasts, support fisheries, and host high biodiversity.

Reduced shell strength or abundance in planktonic calcifiers and shellfish affects food webs, from small fish and invertebrates up to commercially important fish and marine mammals.

Shifts in the balance between calcification and dissolution alter how much carbon sinks to the deep ocean versus stays in surface waters and the atmosphere.

Some researchers note that, because calcification itself releases CO₂, a reduction in global marine calcification could act as a small negative feedback on atmospheric CO₂, but this does not offset the broader ecological risks.

Snapshot answer in plain terms

Higher atmospheric CO₂ → more CO₂ in seawater → lower pH and less carbonate.

Most calcifiers: harder to build shells, slower growth, thinner or dissolving shells, especially in larvae and in high‑CO₂ hotspots.

Some species: neutral or increased calcification due to special physiological tricks, but they are the exception, not the rule.

Ecosystems: coral reefs and shellfish beds become more fragile, food webs shift, and ocean carbon cycling changes.

Bottom line: Increasing atmospheric CO₂ pushes oceans toward conditions where many calcifiers struggle to build and keep their shells, with cascading effects on marine ecosystems and the global carbon system.

Information gathered from public forums or data available on the internet and portrayed here.