A new breakthrough in bleaching origins

Written By Danielle Moloney


Scientists from around the world have come together in a recent paper detailing a new and groundbreaking discovery about coral bleaching. Though coral bleaching has been described by scientists for decades, the mechanism underpinning this phenomenon has remained elusive. Despite extensive research, little has been discovered regarding whether it is initiated by coral or by the symbiotic zooxanthellae living in the coral. According to Rädecker et. al, the delicate balance of the metabolic interaction between coral and the symbiont becomes disrupted under stress and leads to bleaching- and it begins earlier than previously believed. 

Symbiosis for survival 

Most corals live in tropical, nutrient-poor environments. The lack of suspended nutrients is why the water around reefs is generally so clear. For this reason, corals need to look beyond the water column for nourishment- this is where algae comes in. Over time, coral and algae have evolved together to form a mutually beneficial relationship in which the coral provides shelter and carbon dioxide for the algae, and the algae in turn provides nutrients to the coral via it’s photosynthetic capabilities. Scientists have known that heat stress can cause this relationship to break down, leading to the algae vacating the coral, leaving it bleached white and starving. With global ocean temperatures on the rise, it is essential to determine the initial cause of bleaching in order to better protect against the breakdown of coral-algae symbiosis. 

Disrupted nutrient cycling is to blame 

Rädecker et al’s recent publication examines the time frame between the onset of heat stress and the onset of visible bleaching to find out what occurrences may precede a bleaching event. In order to study their hypothesis, the researchers collected coral samples from Abu Shosha reef in the Red Sea, off the coast of Saudia Arabia. They then brought the samples back to the lab where they could undergo heat stress exposure and subsequent analysis. 

Upon further investigation, they found that heat stress made the exchange of nutrients between coral and algae less stable. This breakdown occurred over many steps, which included an initial increase in the energy demand by the coral host in response to heat stress. Because the host didn’t have enough energy, the amino acids broke down, switching the coral from needing more nitrogen to needing more carbon- effectively starving the coral of carbon. Carbon is a key component of coral-algae symbiosis. Using photosynthesis, algae breaks down the carbon dioxide produced by coral cells during cellular respiration. It turns the CO2 into sugars that the coral then uses to build the proteins and calcium carbonate (among other things) that keep it alive.  Since carbon is one of the main elements of coral-algae symbiosis, the breakdown of the carbon cycling translated into the breakdown of the symbiotic relationship. Furthermore, they discovered that these detrimental steps toward symbiosis breakdown occur well in advance of visible bleaching.

A panel from Rädecker et. al detailing the mechanism through which coral-algae symbiosis is established, and then either stabilized or destabilized. C-limited refers to a carbon limited state whereas N-limited refers to a nitrogen limited state.

Thermal tolerance 

Rädecker et. al’s findings have important implications for how scientists study the thermal tolerance of corals. Corals in the Red Sea, the sampling site for this research, are known as some of the most thermally tolerant corals in the world. Thermal tolerance is one of the major components of coral conservation research- the more thermally tolerant a coral is, the more likely it is to survive heat stress events. For this reason, scientists have long sought the answer to what makes a coral thermally tolerant, with the hopes of using the answer to better protect the corals that already have it, as well as reinforce the ones that do not. 

Part of Rädecker et. al’s conclusions from their experiments include the idea that nutrient cycling stability is more predictive of thermal tolerance than other mechanisms such as the thermal bleaching threshold of symbionts by themselves. 

A panel from Radecker et. al comparing a control coral and a heat stressed coral 10 days after the onset of heat stress. On day 10, there was no significant difference between the algal symbiont densities or the chlorophyll a content between the two samples. Neither sample showed visible signs of bleaching. This supports the hypothesis that a breakdown in symbiosis occurs before bleaching becomes apparent. 

What’s next?

Though the mystery of coral bleaching has long haunted scientists, these breakthrough findings have widespread implications for future coral research and conservation. The stability of nutrient cycling is an important line of research to pursue in order to better understand thermal tolerance of symbiosis, especially in the face of a growing climate crisis. Rädecker et. al additionally note that due to the passive yet competitive relationship between the coral and symbiont, these findings may be applicable to other symbiotic relationships, both marine and terrestrial. This further reinforces the negative consequences of warming global temperatures. For now, this research provides a step in the right direction towards better understanding how coral symbiosis operates and how that symbiosis can be protected. 

If you are interested in learning more, especially about an in-depth explanation of the cellular mechanisms touched on in this article, please read Rädecker et. al’s publication here

Please contact the author with any questions: dmoloney@fandm.edu. 

Cover Photo courtesy of EcoWatch.

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