Why are some corals less susceptible to the dreaded bleaching than others? A study suggests that a ventilation system may work particularly effectively for them. According to this, at least some corals can apparently influence the flow conditions in their environment with astonishing precision using tiny cilia. In this way, they can protect themselves from harmful oxygen concentrations, which are considered a key factor in causing coral bleaching, the researchers explain.
They are fascinatingly beautiful and play an important role in the complex communities of life in the oceans. But the world's coral gardens are under serious threat: Above all, what is known as bleaching is increasingly covering the previously lush underwater landscapes like a shroud. The phenomenon is caused by damage to their tiny master builders - the coral polyps, which are responsible for the formation of the calcareous structures. These cnidarians catch plankton from the water, but their diet is also based on a close partnership: unicellular algae that carry out photosynthesis live in certain cells of the polyps. They gain energy from sunlight and build up carbon compounds, much of which they give to the coral polyps. In return, the algae get the protection and nutrients they need to live from the polyps.
Targeting coral bleaching
But this friendship is sensitive to heat: Studies show that increased water temperatures lead to stress reactions that lead to the algae being expelled from the polyps. As a result, the corals increasingly lose their color and eventually die. Due to the rising water temperatures in the course of global warming, these menacing bleaching waves are now occurring more and more frequently in the coral gardens of the world. This problem is the focus of scientists who are investigating the causes and looking for ways to counteract the problem. The team led by Cesar Pacherres from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) in Bremerhaven is on the trail of a special aspect: “Not all corals on an affected reef fall victim to bleaching. Some bleach quickly, others not at all,” says Pacherres. He and his colleagues are researching what might be behind the difference.
Specifically, they are investigating the possible role of oxygen radicals. These harmful substances are caused by the high oxygen production of the algae during photosynthesis and, as is known, can lead to the disruption of the symbiosis. “When there is a lot of sunlight, corals have a problem getting rid of the excess oxygen. Low water movement and high temperatures promote this so-called oxidative stress, which is believed to be the main cause of coral bleaching,” explains Pacherres. In their study, the researchers have now taken a closer look at the cohabitation between the hard coral Porites lutea and its green tenants. They recorded the flow of oxygen by tracking oxygen-sensitive nanoparticles. The focus was also on the effect of the so-called cilia. These are microscopic cell extensions of the polyps that can flap like eyelashes.
Ventilation system revealed
As the researchers report, it first became clear during their investigations that the oxygen-producing algae are by no means evenly distributed in the corals examined. In some areas they are therefore much more densely gathered than in others. "Actually, we expected that the highest oxygen concentrations in the water would also be found above these photosynthetic hotspots," says co-author Soeren Ahmerkamp from the Max Planck Institute for Marine Microbiology in Bremen. "Surprisingly, exactly the opposite was the case." According to the researchers, the only possible explanation for the phenomenon was that the corals actively transported the oxygen-rich water elsewhere.
The analytical look at the function of the cilia then clarified the background: "The trick is that the cilia on the surface of the coral create small vortices through coordinated beating," says Ahmerkamp. In this way, the polyps can apparently influence the flow in such a way that the areas with many algae are aerated with low-oxygen water in a targeted manner. With the help of a computer model, the researchers were finally able to illustrate in detail the interaction between diffusion and the blink of an eye on the coral surface. The eddies mean that the hard coral can reduce critical oxygen loads by about half.
"The stubborn corals are therefore not at the mercy of the marine environment, as was previously thought," sums up co-author Moritz Holtappels from the AWI. Effectively influencing the water flow could be of great importance, particularly in ocean regions with little current. However, the researchers say that this sophisticated ventilation system is probably not equally well developed in all corals. So this is an interesting lead. To what extent this is actually connected to the differences in susceptibility to bleaching and other questions must first be clarified by further investigations.
Source: Max Planck Institute for Marine Microbiology, Article: Current Biology, doi: 10.1016/j.cub.2022.07.071