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Self-cleaning corals: Hope after an oil spill

By Leandro Santos

English edit by Katyanne Shoemaker

Illustration by Joana Ho.

Brazilian corals are known for their strength and resilience, but I wish that they didn't need to prove themselves so often these days. Although the Northeastern coast of Brazil has warm, inviting waters, the water is also more turbid than other places where corals occur around the world. Cloudy water makes development more difficult for most coral species (read more about coral biology here). These environmental conditions allow for a very special assemblage of shallow-water coral species – almost half of which are only found in Brazil!

2019 was a difficult time for the Northeastern corals. Due to El Niño, the water warmed above the ideal for these animals for too long. These adverse conditions generate stress, which leads to bleaching events for coral colonies. When bleaching persists for an extended time, it can cause the death of the corals. In some places, like in Recife de Fora (Porto Seguro – BA), this bleaching event was responsible for around 90% mortality of the Fire Coral (Millepora alcicornis).

Fire Coral colonies bleached in March 2019.

After the warm summer and fall had passed, we were able to see an improvement in the colonies. However, near the beginning of the spring, we started hearing news of a mysterious oil spill disaster that impacted the Northeast. For those of us living and working with coral in Porto Seguro (an area of Abrolhos Bank which hosts the majority of the South Atlantic coral biodiversity), the worries began to creep in. How would our lives be impacted in the short and long term? What would be the impacts on the ecosystems reached? Just thinking of what was to come created restless nights, dreams of catastrophic outcomes, and difficult discussions with peers and collaborators about possible actions and outcomes. The Projeto Coral Vivo, an NGO that works on research and conservation of corals, designed scientific experiments to test the different degrees and types of contact with the oil spill. The project was also interested in investigating the consequences of this crime, the potential economic impacts, and in producing data to assist in future decision making.

Researcher preparing an experiment with oil on coral.

Although researchers have long known of the resilience of Brazilian corals, one observation surprised us: these animals’ ability to remove the oil touching them. In many trials we found that corals ended up clean even when we completely covered them with oil. Then we planned an experiment just to confirm that this really occurs.

The experiment consisted of putting living corals next to (dead) coral skeletons and covering them both in oil. If the oil stayed on the skeleton but not on the living coral, it would be evidence that the coral had actively removed it rather than just floating away. For this study we used the species Mussismilia harttii, more commonly known as cauliflower coral.

The coral responded very quickly. Within 5 minutes we noted the production of large amounts of mucous. Corals continuously produce mucus, but in this case a remarkable amount of mucus and bubbles formed rapidly. Additionally, the oil-covered coral began a stress response of removing some of its own internal gut filaments.

The result of all this movement was that within two hours, all of the corals had removed the oil covering them. This oil floated off together with the coral's mucus. The skeletons however remained intact and full of oil. This was a truly amazing observation of the ability of these animals. Not only can they remove sediment that has landed on them, they have a powerful enough stress response to remove a thick coat of oil.

Figure showing how well living corals can remove oil from themselves. On the left is the living coral, and on the right is just the skeleton.

This experiment teaches us an important lesson in science: answers seldom come without creating more questions. We could see the removal of the oil, but still don’t know exactly which mechanisms within the corals are responsible for the reaction. We have an idea that it may be an interaction between the mucous production, the movement of flagella around the mount, and other actions, but this needs to be further investigated.

We can’t always predict what will happen. Although some experiments have been done previously to measure the potential impact of oil spills, much research is still needed to understand the bigger picture. Moments of crisis demand a lot of knowledge, which means research needs to be done sooner rather than later. With further research, we can make better policy decisions and hopefully prevent further disasters from happening. The value of traditional experimentation must be recognized in this time as well, as they can often provide more information than theoretical models alone.

Corals are incredible animals that have learned survival skills from millennia of evolution. Until now, oil hasn’t seemed to cause much direct harm to corals, but non-direct harm could still be caused through some yet unknown cascade effect. One clear threat to corals is climate change, as we saw in 2019. In 2020 other regions from the Northeast showed bleaching events and high mortality. The South Atlantic is a refuge for our special fauna, but with all the increasing attacks on them, it is becoming harder to defend themselves, even with their surprising arsenal of tricks.

The crime of the oil spill that reached the Brazilian coast, and the magnitude of it ( 3000 km of coastline!) was an example of what can occur when the political powers don’t act fast or accurately, and the communication among scientists and policy managers is inefficient. Few actions were taken based on previous knowledge about oceanographic factors. If the damage was controlled, it was through the action of volunteers, traditional communities, and local organizations. The true impacts of the oil spill will remain a mystery for years to come. It was only one chapter from a series of disasters that are still to come.


References and suggested reading:

Santos HF, Santos LFA, Jesus HE, Lacerda CHF, Mies M. In Press. The South Atlantic coral Mussismilia harttii actively and quickly removes heavy crude oil from its surface. Bulletin of Marine Science.


About the author:

An ever-learning biologist that graduated from the University of São Paulo. I have experience in marine invertebrate zoology, ecology of consolidated substrates, environmental education, and aquaculture. I am currently a master’s degree student at the Federal University of Southern Bahia, where I integrate the Research and Outreach Nucleus on Environmental Education (NUPPEA/UFSB). My project involves environmental education in conservation units along the Costa do Descobrimento of Brazil. When I’m able to, I like to be caught by some waves (sometimes they win!) and learn about the thousand art forms of capoeira. I believe in the urgency to change habits and that we can only build our future collectively. With all folks.

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