In a 10-month Hawaii experiment, scientists raised CO₂ to future levels; coralline algae covered 4% of acidified tanks versus 25% of controls

What happens to a coral reef when the ocean around it becomes more acidic? The answer may not be limited to slower-growing corals. Some of the smallest organisms on a reef can also play an important role in holding its structure together, and changes in their abundance could alter how the ecosystem develops over time.

The importance of coralline algae goes beyond the amount of space they occupy. Unlike soft-bodied algae, these organisms incorporate calcium carbonate into their structures. They can therefore contribute to the accumulation and stabilization of reef material.

A reduction in their abundance could create a ripple effect. If fewer coralline algae grows on exposed surfaces, fewer areas may be available for some coral larvae to settle. At the same time, slower calcification means less carbonate is being added to the reef framework.

It does not mean that all reefs will react the same way to future ocean changes. Reefs can vary in their species, water chemical composition, temperatures, and other features of the local ecosystem. Because the experiment was done in mesocosms rather than on an entire reef, it cannot be read as a precise forecast for reefs worldwide.

However, the experiment did show the system’s vulnerability. Higher concentrations of CO₂ affected not just the water but also the development of organisms that help build reef structure. As oceans continue to absorb more human-produced carbon dioxide, the chemistry surrounding coral reefs is changing alongside the climate.

The Hawaii tanks offered a glimpse of what that shift could mean: not just corals struggling to build their skeletons, but the gradual weakening of some of the organisms that help reefs build and hold themselves together.

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