When it comes to corals and climate change, it is well established that increasing ocean temperatures are harming coral colonies across the world. In fact, the United Nations has released a docket that highlights the relationship between warming seas and mass bleaching events documented in the past decade. This report also stated that ocean temperatures could rise by 1 – 3 C in the next few years, causing more devastation. 

In the wake of these findings, scientists are making a push to try and protect corals against this imminent threat. Through repopulation efforts, research and documentation, researchers across the world are hoping to secure corals through a troubling period and possibly ensure their prolonged survival as it is integral to ocean flora and fauna health. 

However, a recent finding might shed light on some variations that could help scientists understand the impact of cold temperature on corals. Northern Red Corals have adapted themselves to live in the northern Red Sea and Gulf of Aqaba. The corals that live in this region have shown a very high degree of heat tolerance. This particular strip of water has a high salt content and is also very reactive to climate changes. 

The increasing water temperature in this region (due to a range of factors) has created an environment of constant change and increasing temperatures. The corals in the region have been studied and have shown remarkable ability to withstand rising water temperature. But findings by a research team show an alarming susceptibility to colder water temperature. 

Though these corals are resistant to increasing temperatures, they exhibit highly sensitive reactions to colder waters and show physiological stress similar to other corals in warm waters. 

The Findings

The team of researchers from Bar-Ilan University, israel have found that variable weather patterns expected with global warming (fluctuating environmental temperatures between heat and cold) can cause periods of ‘extreme cold.’ This potential drop in temperature could prove extremely detrimental to corals in the area as the findings show that “even 1 degree Celsius cooler than average results in a physiological stress response similar to that seen in other corals under heat stress,” according to a press release by the university. 

The findings,published in PeerJ magazine, show that certain common reef-building species in the region like the Stylophora pistillata and Acropora eurystoma exhibited bleaching like symptoms including “declines in photosynthetic indices (maximum quantum yield, electron transport rate, saturation irradiance, and photochemical efficiency) and chlorophyll concentration.”

The team also tested this finding in an ex situ experiment by reducing environment temperatures to 1 or 3 °C below average winter temperatures in the area. It was found that at 3 °C below winter minima (approx 18.6 °C), both species showed massive changes in physiology.

Previous research on corals in this area focussed primarily on their impressive heat tolerance. When these species were exposed to higher than average temperature water, they showed no signs of stress. Other coral would have easily bleached or showed signs of bleaching with such a sharp spike but these corals were robust enough to withstand this change.

“Whilst 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 in the press release. “Indeed we found that exposure to cold water periods causes a physiological response akin to bleaching.”

Significance of Findings

The team were shocked to find how close to their cold threshold these corals were living in. Even the slightest fluctuations could set off a cold-water bleaching event in these corals that could prove extremely detrimental to their day-to-day functioning and long-term survival in the area.

The most detrimental impact and the main reason that coral bleaching is so harmful is the loss of essential algae that the coral rely on for food. These ‘algal symbionts’ embed themselves in the coral tissue and become a crucial part of a coral’s life cycle. The algae, being photosynthetic, make their own food and also allow the algae to extract energy from sunlight, especially in clear and shallow water. When coral bleaching like symptoms are displayed, the algae is affected and eventually dies off.

The fact that cold water induces coral bleaching-like symptoms in the coral in the gulf and that the temperature constantly hovers around threatening levels in the winter, immediate action must be taken to understand and examine the physiological changes in the coral and also causes of colder winters in the region. This will help the team prepare for a potentially catastrophic winter by possibly creating a bank of indigenous coral and also using mitigation methods like 3-D printed blocks, that help shelter the coral roots from cold water. 

Positive Takeaways

Though the team observed changes in the coral physiology with a drop in temperature, they also found that the effects are not permanent. The corals used for the study bounced back once the temperatures returned back to optimal levels. Also, the team were pleased to find that the corals did not become heat susceptible after undergoing a bleaching event due to colder water temperatures. This is incredible because this gives these corals a higher chance of survival as the region has longer summers than winters. 

Though water temperatures drop during the winter, the corals are not prone to death. Once they bounce back, the summer becomes a breeze, making this a hardy coral capable of spawning future genetic research on the viability of transferring the heat tolerant genes to other species. 

“It was an important discovery for us to understand that even those individuals that suffered the cold winter stress, still did not bleach at the high temperatures,” said researcher Dr. Jessica Bellworthy in the media release. This means that corals in the Gulf of Aqaba would not undergo two bleaching events in the same year with fluctuating temperatures. This makes them more resilient than species situated in other populous reef beds like the great barrier reef and the North American reefs.


This crucial finding shows that corals across the world have adapted differently to their environment and the range and variety they offer is astounding. We are still learning about their lives and discoveries like this open our eyes to two things – how detrimental human activity is to nature and how resilient nature can be to these effects. This finding could pave the way for future research and possible genetic selection that could help make other species of coral resistant to heat stress too.