They are a symbol of the beauty and diversity of the underwater world: coral reefs. The brightly colored warm-water corals in tropical waters in particular attract diving fans and researchers alike. But even in deeper, colder and darker regions of the oceans, so-called cold-water corals form huge and no less beautiful reefs. They are all home to a variety of different animal species and form fascinating ecosystems.

At the same time, the sensitive reef constructions and their master builders, the corals, exemplify the threat to the oceans. Rising water temperatures, polluted seas, intensive use of coastal waters and fishing are putting them under pressure. In addition, the world's oceans store large amounts of carbon dioxide. In this way, they slow down global warming to some extent. However, the additional carbon dioxide causes a chain of reactions in the water that make the oceans more acidic. This makes it more difficult for corals to form their calcareous skeletons.

Marine researchers in Kiel have been studying various aspects of coral reefs for many years, from fundamental questions of coral physiology and their ability to react to environmental changes to ways of reading past climate and environmental conditions from the corals.

Corals are valuable ecosystems and natural coastal protection systems. But their future is uncertain. Among other things, they are threatened by rising water temperatures, which repeatedly cause extensive coral bleaching. One of these events in 2010, for example, damaged half of all coral reefs in Thailand. Dr. Marlene Wall and Dr. Anna Roik from GEOMAR's Marine Ecology research department are investigating how tropical corals react to such events and whether they can regenerate and adapt. "Around the Andaman archipelago in the Indian Ocean, we found that corals that are regularly flooded by cold water from the deep are more resilient than those that live in constant water temperatures," explains Dr. Wall.

In parallel, Anna Roik, marine biologist at GEOMAR until 2021, pursued another approach: What role does the microbiome, i.e. the entirety of the microorganisms living on and on the corals, play in the resilience and fitness of the corals? "By sampling the corals off Thailand and with the help of DNA analyses, we want to learn more about the microbial communities of the corals," reports Roik, "Ultimately, knowledge about the microbiome could lead us to develop novel strategies that enable coral populations to be protected or even made fitter for the future."

Parallel to the studies on tropical corals, researchers in Kiel also began to take an interest in cold-water corals in the mid-1990s. As the reefs exist at much greater depths, for a long time they were only known to fishermen who occasionally had torn coral sticks in their nets. So it was a matter of discovery: where do cold-water coral reefs actually occur? Today, reefs are known from the coast of Mauritania along the European shelf edge to northern Norway, but also in the Gulf of Mexico. Some of them form enormous structures on the shelf edges. In the Porcupine Seabight, a submarine bay west of Ireland, carbonate mounds formed by cold-water corals reach a diameter of 100 to 1,800 meters at their base and a height of up to 350 meters above the sea floor. The Røst Reef off Norway even covers an area of 130 square kilometers.

For a long time, it was completely unclear why cold-water corals only grow in certain places and at certain depths. We now know that they settle in a special dense layer of water that provides them with sufficient nutrients. Over millions of years, the ups and downs of this comfort zone determined whether the corals could grow or not. As water temperatures play a role in density, a warming of the seawater could have a significant impact on coral growth in the future.