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

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.Representative image of an underwater reef experiment showing differences in coralline algae growth under varying ocean acidification conditions. ( AI-generated/ ChatGPT)According to the U.S.

Geological Survey, a 10-month controlled experiment in Hawaii exposed reef communities to elevated carbon dioxide levels designed to represent conditions expected later this century. In the acidified tanks, crustose coralline algae covered just 4% of the available surface, compared with 25% in control tanks.

The difference represented an 86% relative reduction in coverage.What ocean acidification changesThe experiment was conducted in continuous-flow mesocosms at the Hawaii Institute of Marine Biology on Coconut Island, using unfiltered seawater from Kāneʻohe Bay on Oahu. The tanks were kept outdoors in full sunlight, allowing them to experience daily and seasonal changes in temperature and seawater chemistry similar to those on the nearby reef.

Researchers increased the partial pressure of carbon dioxide, or pCO₂, in treatment tanks. At midday, CO₂ levels in the acidified mesocosms exceeded those in the control tanks by an average of about 365 microatmospheres, with considerable variation around that average.

The treatment was intended to simulate elevated conditions expected during this century.Deep DiveOcean acidification is caused by the ocean’s absorption of carbon dioxide. The extra carbon dioxide initiates chemical reactions leading to an increase in hydrogen ions and a decrease in the amount of carbonate ions.

Since carbonate ions form the foundation of calcium carbonate skeletons, calcifying organisms have a harder time building their shells and skeletons under acidic conditions. This highlights the significance of crustose coralline algae.

The algae are hard and encrusting, and they form calcium carbonate structures on reefs.A sharp decline in reef-building algaeThe contrast between the experimental tanks was striking. At the end of the 10 months, crustose coralline algae had developed around 25% cover in the control mesocosms but only about 4% in those exposed to elevated CO₂.

The researchers calculated this as an 86% relative reduction. The effect was not limited to the algae covering the tank surfaces.

Rhodoliths, free-living structures made from coralline algae, grew in the control conditions at a rate of about 0.6 grams of buoyant weight per year. Under elevated CO₂, they instead lost about 0.9 grams per year.Authors also warned that changes of this magnitude could affect the ability of coral reefs to grow and remain structurally stable.

The experiment also found that coral calcification declined by 15% to 20% under acidified conditions, while linear extension fell by 14% in one experiment. These findings were consistent with a separate Hawaiian mesocosm study published in Nature Geoscience, which found that elevated CO₂ strongly inhibited the recruitment and growth of crustose coralline algae.

Together, the findings suggest that ocean acidification could affect both reef-building processes and the balance of organisms competing for space on reef surfaces.Why a small algae matters to an entire reefThe 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.The Hindustan Times US Desk brings you the latest news and developments from across the United States. The team covers everything from breaking news and politics to sports, business, entertainment and stories that are trending across the country.

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