Coral reefs often lie like a protective bulwark off the coast and defy even severe storms. Scientists have now investigated what makes these reefs so stable. As they found out, certain red algae play a crucial role in this. Because their lime secretions act like a cement that strengthens the coral reefs and thus enables the growth of many reefs in the first place.
The reefs in the sea formed by corals are unique ecosystems. Tropical coral reefs such as the Great Barrier Reef, thanks to their three-dimensional framework, offer a habitat and shelter for countless animal species. At the same time, the coastal reefs act like a bulwark against the surging sea. The reefs are formed from the calcareous skeletons of millions of tiny creatures – the so-called coral polyps. However, the frameworks of the individual corals are often so fragile that they would hardly be able to withstand severe storms.
Do the reefs depend on red algae?
How do the reefs become so stable anyway? A research team led by Sebastian Teichert from the University of Erlangen-Nürnberg got to the bottom of this question. Scientists have long suspected that certain red algae play a role. These algae, which belong to the Corallinophycidae group, are also able to store lime and thereby build a stable skeleton. “These algae can form a ridge on the reef, which stabilizes it against waves and binds loose sediment,” the researchers explain. “That is why they are also considered the glue that holds coral reefs together.”
To find out whether the growth of coral reefs depends on these red algae, the scientists analyzed the growth of more than 700 individual fossil reefs. To do this, they used data from the “PaleoReefsDatabase” (PARED) – a comprehensive collection of geological and palaeontological data on coral reefs from 150 million years of geological history.
The more algae, the more reefs
The result: In the course of the earth’s history, the number of coral reefs increased when the occurrence of red algae increased. “We found a significant relationship between the proportion of reefs that contained coralline algae as secondary reef builders and the proportion of real reefs in the last 150 million years,” said Teichert and his team. In their analyzes, the researchers found that the size of the coral reefs supported by red algae fluctuated greatly over the millions of years – depending on how high the algae occurrence was. Although there were coral reefs without calcareous deposits of the red algae in all epochs, the data confirm that the algae had a significant influence on the growth of some coral reefs.
The researchers were also able to explain the strongly changing occurrence of red algae in the course of the earth’s history: The red algae have supported the reefs with their special cement for a very long time – but were subject to constant crises. “Over the millions of years, various crises have repeatedly restricted her in this function,” says Teichert. These crises include, for example, fluctuating sea levels, changing sea temperatures, the level of carbon dioxide in the water or the evolution of herbivorous marine animals. Sea urchins and parrot fish in particular have repeatedly decimated the coralline red algae over time because they fed on them. The latter occurred at least three times during the period investigated.
However, the algae also developed defense mechanisms such as special growth forms in order to assert themselves against their predators. “The algae have adapted so well that they are now even benefiting from the herbivores,” says the scientist. “The herbivores release the algae from harmful growth, for example green algae, so that they can grow unhindered.” As a result, coralline red algae support the reefs more successfully than ever before in the history of the earth.
Does climate change endanger the support function?
Despite their improved stability, many coral reefs nowadays suffer from climate change: due to increasing global warming, coral larvae are now increasingly colonizing cooler ocean areas in the subtropics. The rising water temperatures also repeatedly lead to devastating coral bleaching, in which the organisms reject their algae symbionts and become susceptible to diseases. The extent to which climate change also influences the supportive function of the coralline red algae has not yet been clarified. If living conditions continue to deteriorate, this will probably not only weaken the corals in the future, but will also affect reef inhabitants and even people – because coral reefs are important for coastal protection, as they weaken storm waves and thus protect densely populated coastal regions from flooding. They also provide a nursery for many popular edible fish and seafood.
Source: Friedrich-Alexander-Universität Erlangen-Nürnberg, Article: Scientific Reports, doi: s41598-020-73900-9