The reefs of the earth threaten to disappear – the ghost of coral bleaching is looming. Two genetic studies now shed light on why corals lose their symbiotic algae partners and die when temperatures are too high. One suggests that the activity of symbiosis-supporting genes is disturbed by heat stress. In the other, the researchers were able to identify a possible protective gene against temperature stress by using the Crispr / Cas gene scissors. The insights could offer approaches to coral protection, say the researchers.
They are not only fascinatingly beautiful, the coral gardens of the earth also form key elements in the complex communities of the oceans. They are therefore of great importance for the material cycle and thus also for us humans. But these precious habitats are seriously threatened: coral death is plaguing reefs around the world. The problem lies with the tiny builders of the underwater world: the coral polyps, which are responsible for the formation of the limestone framework. These cnidarians catch plankton from the water, but their diet is also based on a close partnership: unicellular algae live in certain cells of the polyps. There they generate energy from sunlight and build up carbon compounds, a large part of which they give to their hosts. In return, the algae get the protection and nutrients they need to live from the polyps.
Fragile coral-algae symbiosis
But this symbiosis is sensitive to heat, as studies have already shown: When the water temperature increases, the algae leave their coral polyps. This causes them to lose their color and eventually die. This so-called coral bleaching is now occurring more and more frequently in the coral reefs of the world due to the rising water temperatures in the course of global warming. Scientists are therefore looking for ways to counteract the problem. It is important to understand the genetic basis of the symbiosis and the effects of bleaching, say the experts. The researchers led by Phillip Cleves from Stanford University are also dedicated to this goal. You have now published two studies on the subject.
As part of their first study, they looked into the question of which genes in coral polyps play a role in the processes that lead to bleaching. They used a technique called RNAseq to do this. It can record how intensively certain genes are read in cell tissue. They used the model cnidarians Aiptasia for their studies. It is an anemone related to corals that also lives in symbiosis with algae. In their investigations, the researchers compared the gene activity of specimens that were in a symbiosis with those that did not carry any algae. They identified 337 genes, the expression of which in the symbiotic specimens was significantly increased compared to the algae-free ones. According to the researchers, some of these genetic makeup are probably involved in the function of the symbiosis. They then exposed some of the cnidarians to a water temperature of 34 degrees Celsius, which typically leads to a bleaching reaction, and examined the changes in gene expression. It was found that more than a quarter of the presumed symbiosis genes showed a sharp decrease in their activity during the first twelve hours of heat exposure. According to the researchers, this is an indication that the heat-induced decrease in the expression of symbiosis genes plays a role in the loss of algae partners in the context of bleaching.
A heat shock factor in sight
However, the study also showed the complexity of the system. While some genes were downregulated by the heat stress, the opposite was the case with others: Within the first three hours after the temperature increase, 524 hereditary factors showed a more than four-fold increase in normal activity levels. The researchers found indications that two so-called transcription factors are involved in this reaction, which regulate the activities of many genes. Interestingly, one of these is known as a heat shock factor. This led to the suspicion that HSF1 is responsible for the mobilization of protective gene processes in the cnidarians. HSF1 may thus also play a role in the tolerance of the coral polyps to temperature increases.
In order to investigate this possible function, the researchers used the genome editing technology Crispr / Cas in their second study. This method, recently awarded the Nobel Prize, makes it possible to change specific genes in a targeted manner, like a kind of scissors. “We have developed a Crispr / Cas method with which we were able to test gene function in corals for the first time,” says Cleves. The scientists used the system to switch off the gene for the heat shock factor in larvae of the model coral Acropora millepora. They wanted to check how this affects heat tolerance.
As Cleves and his colleagues report, coral larvae with deactivated HSF1 genes survived without any problems at normal water temperatures of 27 degrees Celsius. But if it was raised to 34 degrees, they died very quickly. In contrast, unmodified larvae could survive much better in the warm water, the experiments showed. “This result illustrates the potential key role HSF1 can play in coping with rising temperatures through the corals,” says Cleves. In addition to this concrete result, the researchers now see the successful application of the system as a “proof of concept”: It is a possibility to sound out the functions of the various genes that may be involved in the coral-algae symbiosis as well the system’s heat tolerance play a role.
According to the researchers, the insights gained in this way could ultimately serve to protect the coral reefs. There are projects that explore the extent to which the problem can be countered by breeding corals specifically for increased heat resistance or by identifying less susceptible species. They could then form reefs that are more able to cope with climate change, so the hope. “Understanding the genetic characteristics of corals’ heat tolerance is key to understanding not only the natural response of corals to climate change, but also weighing the benefits, opportunities and risks of novel management tools such as selective breeding and movement of corals between reefs” says co-author Line Bay in conclusion.
Source: Australian Institute of Marine Science, Article: PNAS, doi: 10.1073 / pnas.2015737117 and 10.1073 / pnas.1920779117