Corals are animals, not rocks. Each polyp is a small, soft-bodied creature that builds a hard skeleton around itself using minerals it takes from seawater. That construction work depends on a fine chemical balance in the ocean, and rising carbon dioxide is shifting it.

The process is called ocean acidification. It is one of the quieter pressures on reefs, which is part of the problem: bleached coral makes the news, a slightly less alkaline ocean rarely does. Here is what is happening, why a change that sounds tiny matters to coral growth, and what it means for the cold-water reefs, maerl beds and shellfish around the UK.

What ocean acidification actually is

The ocean absorbs a large share of the carbon dioxide released into the atmosphere — roughly a quarter of human emissions in a typical year. When CO₂ dissolves in seawater it does not sit there passively. It reacts with water to form carbonic acid, which releases hydrogen ions — and more hydrogen ions means a lower pH.

The word acidification is slightly misleading. Seawater is not turning into acid; it is alkaline, and it is becoming less alkaline. Direction of travel is what matters. Because pH is a logarithmic scale, small-looking numbers hide large changes: average surface ocean pH has fallen from roughly 8.2 to roughly 8.1 since the industrial revolution, which works out at about a 30 per cent increase in hydrogen ion concentration.

There is a second effect, and for shell-builders it matters more. The extra hydrogen ions combine with carbonate ions, leaving less free carbonate in the water. Corals, molluscs and coralline algae all need carbonate — in the form of aragonite or calcite — to build hard parts. Chemists call the result a fall in the saturation state: the water becomes less willing to supply the minerals a skeleton is made of. In some colder, deeper or naturally more acidic waters, aragonite saturation is already low enough that shells and skeletons begin to dissolve rather than form.

Why coral growth gets harder

A coral polyp does not simply pull ready-made rock out of the sea. It pumps ions across its tissue, actively concentrating the raw materials for calcium carbonate, and it spends energy doing so. The less carbonate there is in the surrounding water, the harder that pumping becomes.

Acidification does not usually kill a coral outright. It makes the coral pay more for the skeleton it already builds — and a reef that grows more slowly is a reef that recovers more slowly.

That distinction between death and slow decline is important. A reef building at a normal rate can repair storm damage, outgrow the seaweed and algae competing for space, and keep pace with a slowly rising sea level. Trim the growth rate and those jobs become harder. Studies on Australia's Great Barrier Reef have found calcification rates on some reefs falling by around 14 per cent since 1990, and comparable declines have been measured in the Caribbean and the Red Sea. None of those corals look obviously sick. They are simply building less.

There is a second, sneakier effect. Reef frameworks are held together not only by coral skeletons but by coralline algae — pink and purple crusts that cement the structure and give coral larvae somewhere to settle. Coralline algae are among the most sensitive organisms to falling saturation state, so the glue that holds a reef together can weaken at the same time as the coral building it.

Acidification does not act alone

Readers often meet acidification alongside coral bleaching, and it is worth separating the two.

• Bleaching is a heat response. When water gets too warm, corals expel the microscopic algae living inside their tissue. That is why they turn white — and it can kill them within weeks.
• Acidification is a chemistry response. Temperature is not the trigger; carbon dioxide is. The damage is slower and far less visible.

The two do, however, work together. A coral stressed by heat is more vulnerable to acidification, and a reef already growing slowly has less capacity to bounce back after a bleaching event. Add polluted runoff, sediment, overfishing of the herbivores that keep algae in check, and stronger storms, and the pressures compound rather than simply add up.

What it means around the UK

This is not only a tropical story. The UK sits beside cold waters that are naturally richer in CO₂ and less saturated in carbonate than the tropics, which makes some of its shell-builders comparatively sensitive.

• Cold-water coral reefs. Scottish waters hold reefs of Lophelia pertusa, including the Mingulay Reef Complex west of the Outer Hebrides. These build slowly, grow in the dark, and have no bleaching story at all — acidification is the main chemistry threat they face.
• Maerl beds. Unattached coralline algae form gravel-like beds in Scotland, Cornwall and elsewhere. They are nursery grounds for scallops and other species, and they are built from exactly the carbonate chemistry now at risk.
• Shellfish. Mussels, oysters and scallops all build shells from seawater carbonate. The clearest warning so far has come from commercial hatcheries on the Pacific coast of North America, where upwelling carried corrosive water into tanks and oyster larvae died in huge numbers; hatcheries now buffer their intake water simply to keep production going.

Long-term monitoring around the UK — including the Western Channel Observatory off Plymouth, home to one of the longest marine chemistry records in the world — has tracked the same downward pH trend seen globally, with plenty of local variation from season, depth and currents.

Why the small numbers are the ones to watch

A fall of 0.1 pH units sounds like a rounding error. It is not. It is a change in the basic chemistry of a habitat that has been broadly stable for millions of years, arriving over a couple of centuries — a pace that gives slow-growing corals, maerl and cold-water reefs almost no time to adapt or move elsewhere.

The only real long-term fix is the obvious one: reduce carbon dioxide emissions. But local action still matters, because acidification is not the only thing hurting reefs. Cutting nutrient pollution, sediment runoff and overfishing buys corals resilience. A slower-growing reef with clean water, healthy fish populations and intact coralline crusts is a reef with a better chance of weathering the chemistry it cannot escape.

Photo: Michael Pointner / Pexels