Out of the blue: does coral color predict bleaching resistance?

Written By Danielle Moloney 

Introduction

Coral reefs are some of the most biodiverse ecosystems on the planet. They provide a habitat for hundreds of different marine species and are a critical part of many economic systems (such as tourism and the seafood industry). Those who are lucky enough to have visited a reef will point to another one of their benefits: their aesthetic beauty. 

With this in mind, scientists at the Okinawa Institute of Science and Technology Graduate University decided to dive into the beauty of colorful corals to assess whether coral color impacts reef survival under thermal stress. 

Scleractinian corals

Over time, many corals have developed an important symbiotic relationship with marine algae in order to survive. Scleractinian corals in particular, which are the “building blocks” of reefs, use this mutually-beneficial mechanism to live. Also known as stony corals, these corals get their name due to their structure- scleractinian corals produce hard skeletons made of calcium carbonate, which allow them to act as a foundation for the reef system. Algae called dinoflagellates or Symbiodinium live in corals and provide nutrients via photosynthesis to the coral in exchange for shelter. Coral stress may be triggered by heat waves or other anthropogenic causes, which can cause the symbiotic relationship between corals and their Symbiodinium to break down. The breakdown is caused by an increase in the energy demand of the coral host as well as a decrease in the output of nutrients by the Symbiodinium, both of which are instigated by heat stress. The algae leaves the coral, rendering it bleached, which increases mortality rates for the coral when they lose the nutrients provided by the algae. While lots of information has been published regarding causes of coral stress, scientists have struggled to determine the mechanism that causes coral bleaching for decades without finding the elusive cause. Equally as important to understanding the cause of bleaching is determining which factors make corals resilient to bleaching. Satoh et al.’s recent study did just that, by assessing how different color morphs among corals in Japan’s Okinawa prefecture responded to heat stress in 2017. 

Methods and findings In order to assess how coral color might play a role in bleaching resistance, Satoh and colleagues first identified three different color morphs of the family Acroporidae (a scleractinian coral family) along the Okinawa coast. The three morphs they identified were brown, yellow-green, and purple. Each category was based on the fluorescence emitted by the coral. Each type of fluorescence was classified through analysis of the coral’s genome, where genetic code signifies the presence of different types of fluorescent proteins. Armed with the ability to genetically code color morphs, Satoh et al. went on to study how each morph fared during the summer of 2017.

A panel from Satoh et al. showing different coral color morphs that can be observed off the coast of Okinawa, Japan.

They found that brown and purple morphs of coral experienced significantly more bleaching than yellow-green morphs when exposed to summer temperatures. Photosynthetic activity of symbiotic dinoflagellates was reduced in the brown and purple morphs, whereas the yellow-green morph’s dinoflagellates remained unaffected. In order to confirm that the difference in resilience was not due to different clades (i.e types) of dinoflagellates living in the corals, which may indicate differential thermal tolerance based on algae composition, the scientists tested each morph to find out which clade lived inside. They saw that each of the three coral morphs housed similar clades of Symbiodinium, thus eliminating clade type as a source for bleaching susceptibility in the brown and purple morphs. Satoh et. al then looked at the genes coding for the unique fluorescent wavelengths of each morph, where they found differences from one morph to another, as well as differences based on the time of year. The group determined that though more studies are required to understand the biological significance of the different color morphs, resistance to coral bleaching appears to be regulated via genetic mechanisms that coincidentally lead to the unique color morphs of corals. 

A figure from Satoh et al. showing the outcome of different color morphs in their study. Panel titles starting with N refer to brown morphs, G refer to green morphs, and P refer to purple morphs.

Conclusions 

Overall, yellow-green Acropora morphs were the most stable under heat stress whereas brown morphs were the most sensitive. However, brown morphs are the most common in the Okinawa research area. This suggests that brown morphs may be the wild type whereas yellow-green and purple morphs have developed more recently out of Acropora’s  need to survive increasing heat stress. Research focused on the specifics of how yellow-green color protects symbiosis under stress would help clarify how reefs may adapt to become more resilient in the future. 

Furthermore, exceptionally resilient corals known as “super corals” have been previously identified, but the mechanism that makes them “super” has not yet been fully understood. Color morph research conducted by Satoh et. al may provide some insight into how super corals become so resistant to stress. The findings in this study warrant further research into how coral color, and more specifically the fluorescent proteins that control color, might play a role in bleaching resistance or sensitivity. This information would help conservationists assess how to best help corals withstand the various stressors that plague them. Hopefully these studies will lead to even more colorful and healthier reefs in the future. 

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

Cover Photo courtesy of Scotty’s Action Sports Network.

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