May 11, 2021


Photos of corals in the Gulf of Aqaba courtesy Prof. Maoz Fine, Bar-Ilan University

Sea corals – which are living creatures and not plants, even though they look like vegetation, are extremely sensitive to changes in their environment. Coral reefs form within defined environmental boundaries, and reef-building corals are limited mostly to the tropics as low winter water temperature limits the expansion of coral reefs to higher latitudes.

Prof. Maoz Fine of the Faculty of Life Sciences at Bar-Ilan University (BIU) in Ramat Gan near Tel Aviv has repeatedly shown that corals in the Gulf of Aqaba can tolerate very high temperatures. With some experts predicting that the Red Sea region is now entering a cooling phase, Fine and the team have now discovered that even a slight cold spell may cause bleaching.

Coral reefs are one of the most biologically diverse ecosystems on earth. The northern Red Sea and Gulf of Aqaba corals also have an exceptionally high tolerance to increasing seawater temperatures, now occurring as a consequence of global warming. This characteristic led coral reef scientists to designate this region as a potential coral reef refuge in the face of climate change – a reef where corals may survive longer than others that are being lost at an alarming rate due to human pressures. 

Photos of corals in the Gulf of Aqaba courtesy Prof. Maoz Fine, Bar-Ilan University

But global climate change will also result in more variable weather patterns, including extreme cold periods. With predictions that the region will be exposed to cooler weather, the BIU researchers and colleagues from the Interuniversity Institute for Marine Science in Eilat conducted an experiment to investigate the effect of an unusually cold winter on corals from the Gulf of Aqaba. 

In a paper recently published in the journal PeerJ under the title “Warming resistant corals from the Gulf of Aqaba live close to their cold-water bleaching threshold,” they show that winter even one degree Celsius cooler than average results in a physiological stress response similar to that seen in other corals under heat stress. This result shows for the first time how dangerously close Gulf of Aqaba corals live to their lower temperature threshold.

 Previous studies conducted within Fine’s research group have used controlled experiments in the Red Sea Simulator System to expose locally abundant corals to increased seawater temperatures expected to occur within and beyond this century. Multiple experiments found that corals from this region have an extraordinary tolerance to high temperatures that kill corals elsewhere in the world. The team and other groups have tested multiple coral species and at different phases of their life cycle including the reproductive and larval phases, which are typically more sensitive to environmental change; all show similar tolerance. 

 “While we have repeatedly demonstrated the high-temperature tolerance of corals on the shallow reefs in Eilat, we wanted to test the possibility that this exceptional heat tolerance comes with the trade-off of being cold-sensitive,” said Dr. Jessica Bellworthy, who conducted her doctoral research in Fine’s lab. “Indeed, we found that exposure to cold water periods causes a physiological response akin to bleaching.”

Better known as a response to high water temperatures, coral bleaching is the loss of algal symbionts that must be present within the coral tissue to provide the coral with energy. Without the symbionts, the risk of coral death is high. Coral bleaching is the leading cause of the coral decline in the world today. So, while the corals of the Gulf of Aqaba can tolerate very high temperatures, even an acute cold spell may cause bleaching in this population, they warned. 

Bellworthy and Fine’s cold stressed corals of the Stylophora pistillata and Acropora eurystoma strains did not die; they recovered once water temperatures returned to normal. Experimental corals were maintained at normal temperatures and then underwent a second test – an anomalously hot summer. “It was an important discovery for us to understand that even those corals that suffered the cold winter stress, still did not bleach at the high temperatures,” noted Bellworthy. This is good news for the corals in the Gulf of Aqaba; the high thermal tolerance is not lost and thereby corals should not undergo two bleaching events in the same year, a threat that faces others such as the Great Barrier Reef in Australia. 

 Since coral reefs are so sensitive to temperature change, they concluded, identifying those that respond differently to thermal stress aids in understanding the mechanisms of environmental adaptation in corals. In addition, researchers can focus their attention on conserving and studying such unique reefs.