Corals and Zooxanthellae Endosymbiosis with Corals and Ocean Acidification

As you may already know, corals are some of the most productive and intricate organisms on Earth.  They are also at great risk of losing their structure and adaptability from stressors rooted in human activity.  One of the most impactful being ocean acidification.

What are Zooxanthellae?

Zooxanthellae are what give corals their color,  when a coral organism is bleached or paling, it’s in the process of losing its zooxanthellae.  The coral physically expels them as temperature increase and the environment worsens.  So what are exactly?  They are are single celled Eukaryotes, otherwise known as animal cells.  Specifically are photosynthetic dinoflagellate, meaning they can produce their own energy through sunlight and have a particular cell structure.  The easiest way to understand them is as algae.  They can be found living in endosymbiosis with corals, as well as jellyfish, sponges, and some sea snails.

How does the endosymbiosis work?

You can read more about the endosymbiotic relationship between Zooxanthellae and corals here, but I’ll provide a basic rundown again. Zooxanthellae are found embedded beneath the mesoglea and stinging cells of corals.  Zooxanthellae benefit from their endosymbiotic relationship with corals by being protected from predators as well as gaining various nutrients from coral waste—such as CO2, nitrates, and phosphate, and the coral benefits from the energy the zooxanthellae makes during photosynthesis as well as having its waste cycled.  This relationship leads to corals being one of the most productive organisms on earth and leads to their role as a keystone species in the tropical marine ecosystem.

How does ocean acidification play a role?

The energy that zooxanthellae produce for corals is essential for them to continue to build their intricate calcium shell.  As these algae are put under stress from acidification, the loss of these zooxanthellae causes reef building corals to undergo more stress—exacerbating their bleaching and eventual death.  Basically, the change in pH almost with temperature strongly stresses the zooxanthellae in myriad ways such as enzyme disruption and expulsion out of their coral symbiont as the calcium shell loses its intricacies in more acidic environments.

Like any organism, corals can bounce back and regain their coloration after a major bleaching event.  However, like a lot of organisms that are especially vulnerable to rapid environmental change, they don’t have enough time between bleaching events to reacquire their algae.  A study done by Sampayo and his associates titled “Bleaching susceptibility and mortality of corals are determined by fine-scale differences in symbiont type,” demonstrated as well as asserted that different species of coral and zooxanthellae are more capable of adapting to lower pH than others—and that only a few species come out on top.  They also assert than this ends up in a overall loss in diversity, showing that only a specific few microalgae come out on top.

Solutions

There are plenty of reef conservation efforts being made, and you can read more about them here (link to reef conservation).

Some specific organizations that aim to restore coral reefs that can be found here, and here

Author: Torin Hicks