They fade and eventually die of a lack of nutrients – the hunger of the coral polyps was previously considered to be the result of the dreaded bleaching. But now there is also a triggering hunger effect: In connection with excessively high water temperatures, a nutrient deficiency occurs, which causes the loss of the symbiotic partner that supplies the corals with carbohydrates. The new insights into the basics of bleaching could now benefit the fight against global coral extinction, say the researchers.
The wonder worlds of the world’s coral gardens are ultimately based on a fascinating friendship: The tiny builders of the limestone framework – the coral polyps – live in a close symbiosis with tiny algae. They sit in certain cells of these cnidarians, where they gain energy from sunlight and build up carbon compounds, a large part of which they release to their partners. In return, the unicellular algae receive protection and mineral nutrients from the polyps that they need to live.
Many studies have already shown that this community is breaking up in the course of coral bleaching, which is increasingly plaguing reefs around the world. The basic cause is too high a water temperature, which occurs more and more frequently in the reefs in the wake of climate change: At the higher values, the algae leave the coral polyps. As a result, they lose their color and eventually starve to death because their partners no longer provide them with carbohydrates. The nutrient deficiency has thus far only been seen as a consequence of the loss of algae – as a late consequence in the context of coral bleaching.
Look at the beginnings
The researchers led by Nils Rädecker from the University of Konstanz have now devoted themselves to researching the processes that lead to bleaching. To do this, they simulated the environmental conditions in aquariums that trigger coral bleaching in the oceans. The widespread species Stylophora pistillata served them as a test coral. In order to track down the decisive processes in the unfavorable conditions, the scientists carried out studies of the gene activity in the tissues of the polyps as well as analyzes of the metabolic processes and nutrient flows within the framework of the symbiosis between corals and algae.
As they report, their results show that the increased water temperatures cause a critical nutrient deficiency: Both the coral polyps and the algae therefore need more nutrients for themselves at higher water temperatures and therefore stop sharing these nutrients with their symbiosis partners . “They become selfish, so to speak, and hold back the nutrients for themselves. As a result, the algae no longer feed their host and the symbiosis is disturbed, ”explains Rädecker. “So the problem doesn’t just begin with the coral repelling its algae. Even before that, there was no more nutrient exchange between them. “
Potential for coral protection
His colleague Christian Voolstra says: “It’s like asking about the chicken and the egg: is coral bleaching the cause or the effect of starving corals? We have now found clear indications that the nutrient deficiency does not only exist from the point in time when the coral repels its algae and bleaches. The corals are already starving before they become bleached, ”the scientist sums up. As he and his colleagues emphasize, this finding is now an important contribution to understanding the problem. “As a consequence, the results now underline: If we want to save corals, it is not enough to try to stabilize bleached coral reefs. The reefs need to be supported even before the corals bleach, ”says Voolstra.
The new insights into the connections between coral deaths have now also provided additional clues for identifying endangered coral reefs, the researchers say: “We now know very well the environmental parameters that lead to the disturbed symbiosis between coral and algae. Polluted waters with a high nitrogen content are particularly at risk, ”says Rädecker. As part of the study, a specific warning signal was also made clear: “Corals that are starving are releasing ammonium. An increased ammonium content in the water near or in the coral reefs themselves could therefore be a good indicator to identify endangered reefs before the corals bleach, ”says Rädecker.
Source: University of Konstanz, specialist article: PNAS, doi: 10.1073 / pnas.2022653118