Climate Change

Understanding the Demise and Recovery of Coral Reefs

Kishimoto, M., Baird, A. H., Maruyama, S., Minagawa, J., & Takahashi, S. (2020).“Loss of symbiont infectivity following thermal stress can be a factor limiting recovery from bleaching in cnidarians”.The ISME Journal, 1-4.

When it comes to finding food, corals have a couple of tricks up their tentacles. Like jellyfish, their scarier-looking relatives, corals have stinging cells to catch tiny organisms floating in the water column. But the majority of their food come from a much more peaceful interaction — a symbiotic relation with colorful single-celled algae called zooxanthellae. Corals provide a home near the surface where algae can do photosynthesis, while zooxanthellae provide food for corals. Every species that depend on corals reefs, including our own, relies on this partnership. They are the foundation of one of the most biodiverse ecosystems in the world. 

Climate change, however, is putting this partnership and corals reefs around the world in peril. Corals expel their symbiotic algae from their tissues when ocean temperatures rise in an event called coral bleaching. As ocean temperatures continue to rise and heat waves intensify, coral bleaching events are becoming more common. According to the IUCN, the coral bleaching event of 2016 and 2017 killed roughly 50% of the corals found in the Greater Coral Reef in Australia. Corals can gain their symbiotic algae back once temperatures return to normal, but this rarely happens. As a consequence, most corals die of starvation.

Bleached coral reef (image credit: wikimedia commons)
Bleached coral reef (image credit: wikimedia commons)









Until recently scientists did not know why most corals failed to regain their symbiotic algae after a coral bleaching event. That was until August of 2020 when scientists at Tohuku University in Japan and James Cook University in Australia published their research findings, which suggests that algae, not corals, lose their ability to form a symbiotic relationship with corals after a bleaching event. 

In a laboratory setting, scientists mimicked normal and bleaching conditions by pre-exposing both single-celled algae and sea anemones, which are closely related to corals, to temperatures of 25° C and 32° C. Sea anemones pre-exposed to ocean temperatures of 32° C did not lose their ability to host single-celled algae. However, single-celled algae pre-exposed to ocean temperatures of 32°C lost their ability to form a symbiotic relationship with corals.

Fluorescent pictures of corals and symbiotic algae. In the left image, neither the host or symbiont were pre-exposed to high temperatures. In the image to the right, the symbionts were pre-exposed to high temperatures. (Image Credit: National Institute of National Sciences).

In addition, scientists found that single-celled algae can regain their ability to form a symbiotic relationship with sea anemones. However, this recovery is dependent on the number of days that the algae are pre-exposed to bleaching temperatures. Algae pre-exposed to ocean temperatures of 32° C for 2 days regained their ability to form a symbiotic relationship with sea anemones within 5 days. Meanwhile, algae pre-exposed to bleaching conditions for 3 days regained their symbiotic abilities gradually, but never fully recovered. In the wild, bleaching events last more than three days; they usually last one or several weeks. Therefore, the recovery of symbiotic algae takes too long and corals die of starvation.

Surprisingly, not all algae species lost their ability to form a symbiotic relationship with sea anemones. Breviolum psygmophilum, a different species of algae used in this study, did not lose their ability to partner with anemones when they were pre-exposed to bleaching conditions. Unless climate change is addressed, these heat resilient algae species may become the lone survivors in an ecosystem that usually harbors a great diversity of algae species.

In order to create a sustainable world for future generations, we must understand the demise and resilience of coral reefs. Coral reefs provide $375 billion each year in ecosystem services such as coastal protection, food security, jobs and medical innovations. The dramatic loss of coral reefs is threatening millions of peoples’ livelihoods around the world. Understanding the constraints that hinder coral reefs recovery is an important step in creating conservation plans and preparing for the future. 

One thought on “Understanding the Demise and Recovery of Coral Reefs

  1. Thanks for sharing, Pablo! I’m a coral reef ecologist teaching marine biology at a community college, but I missed this piece of news. I always tell my students we don’t know if the coral kicks out the algae or the algae leaves on its own. This doesn’t quite answer that, but close! I’m adding the original article and your blog post to my list of potential sources for our Current Research Project!

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