Like a forest under water

Global warming and ocean acidification are endangering the coral reef habitat that is home to many species. However, stony corals are able to react to climate change and put up defences against it.

	Die tropischen Korallenriffe erstrecken sich kilometerweit entlang des Äquators und bieten unzähligen Tieren und Pflanzen einen Lebensraum.</div>
The tropical coral reefs stretch over many miles along the equator and provide a habitat for countless animals and plants. 
© Fabian Gösser

Yellow, green, blue, purple, pink – the miniature coral reef in the aquarium lab run by Professor Ralph Tollrian’s Department of Animal Ecology, Evolution and Biodiversity is reminiscent of a colourful tropical rainforest underwater. Here, PhD student Fabian Gösser, together with Dr. Maximilian Schweinsberg, is researching the reactions of stony corals to environmental changes, such as the rise in sea temperature. Gösser is primarily interested in the phenomenon of polyp bailout, i.e. the expulsion of small, bud-like individual corals, the so-called polyps. Under stress, the colony of polyps that form a stony coral dissolves. The individual polyps detach, settle elsewhere and can form new coral colonies. The phenomenon of polyp bailout has been little studied so far – and neither has the potential of this reaction for the survival of the reefs.

The coral reef habitat

Stony corals are members of the cnidarians and can form calcareous skeletons. “This is how they are able to set up massive structures and enormous reefs,” explains RUB biologist Fabian Gösser. Probably the best-known reef is the Great Barrier Reef, which stretches some 2,300 kilometres along the Australian coast. Such reefs provide a unique marine habitat for countless species. “This place is bursting with life. The corals form a highly interconnected community with many other living creatures, with many direct interdependencies and short nutrient cycles,” as the PhD student explains what makes coral reefs so fascinating. The symbiosis between algae and corals is particularly impressive. For example, small, unicellular algae from the dinoflagellate group live in the stony corals and help them secrete their calcium carbonate skeleton. “The corals offer these algae not only a protected habitat, but also CO2 and nutrients. In return, the algae supply the corals with photosynthesis products such as sugar and lipids,” as Gösser outlines the symbiotic interaction.

Coral bleaching

The fact that the marine habitat is likewise affected by climate change has long been established. In the summer months, marine biologists are already observing how the rising water temperature leads to global coral bleaching. If temperatures remain high, corals die. “The rise in temperature disrupts the symbiosis between corals and algae. When the algae responsible for the colouration of the corals die, the corals take on a white, pale appearance. The corals can’t survive this in the long run. If they die, other reef organisms die with them,” as Gösser elaborates the consequences of climate change.

Ocean acidification leads to coral bleaching.
© Fabian Gösser

But it is not only the heat that worries scientists. For example, the increase in CO2 levels in the ocean renders calcification more difficult. “The absorption of atmospheric CO2 has led to acidification of the oceans. The pH value of the oceans has decreased measurably in recent years,” explains the RUB researcher. This process also affects the corals. “It’s more difficult for them to secrete calcium carbonate, which dissolves much more easily at lower pH values. Researchers have observed how the skeletons of some coral species have become more porous and growth rates have decreased,” Gösser continues. The diversity of coral species on certain reefs has also decreased. “We very much expect to lose coral reefs in their current form if global warming continues at this rate,” says the biologist, citing the prevailing expert opinion. “If stony corals die out, many reef organisms, such as richly coloured fish, will also disappear. This is already evident on reefs suffering from this phenomenon.”

Corals under stress

The Bochum-based research team wants to understand the corals’ reactions to climate change in detail. To this end, they are conducting experiments in their research tanks by varying temperature, CO2, partial pressure and salinity.

Original publication

Maximilian Schweinsberg, Fabian Gösser, Ralph Tollrian: The history, biological relevance, and potential applications for polyp bailout in corals, in: Ecology and Evolution, 2021, DOI: 10.1002/ece3.7740

Ruhr-Universität Bochum (RUB), 12 November 2021. Press release.

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