Coral bleaching due to rising water temperatures is becoming increasingly common, leaving behind underwater gardens full of snow-white corals. While pictures of freshly bleached corals look almost beautiful, this is an extremely stressful condition for the corals, which they can only survive for a short time.

Corals are incredibly important for biodiversity in our oceans. Around 25% of marine species depend on corals in some way, for example, as habitat or through the food chain^{1, 2}. However, current predictions are that by 2030, 60% of the world's corals will be under severe threat from global warming and other human-made stressors such as overfishing and pollution^{3, 4}. So far, so depressing. But what is being done about it? Luckily, the answer is: a lot! In a race against time, many researchers around the world are trying to find ways to save the world's reefs. The approaches vary from breeding programs of particularly resilient corals to changes in the composition of microbes on the surface of corals, on which a promising study was published this year in Nature. However, to understand this study, we need some background knowledge about corals and their way of life.

The Coral Way of Life

Corals are actually not individual organisms, but colonies of many small cnidarians, a group of aquatic animals that include jellyfish and sea anemones. They are permanently attached to the ground, which is why many people mistake them for plants, even though they are actually animals. One of the fundamental survival strategies of corals is living in partnership with tiny algae. The corals provide protection, while the algae provide the corals with nutrients produced by photosynthesis. Interestingly, the bright colors we associate with corals are actually the colors of the algae and thus a sign of their presence in the corals’ tissue. However, if the water gets too warm, the algae produce harmful molecules called reactive oxygen species (ROS). These highly reactive molecules can cause major damage to coral tissue. To avoid this damage, corals expel the algae under these circumstances, causing them to lose their bright colors. This may sound like a good solution at first, but it also means that they lose their main source of nutrition. If the surrounding temperature does not drop quickly enough to allow the corals to take up the algae again, the corals will die of starvation.

The Coral Microbiome

And this is where the coral microbiome, the community of microorganisms living on the corals’ surface, comes into play. There are various microbes that can scavenge ROS as well as provide nutrients to corals and their partners. Researchers wondered if increasing these kinds of beneficial bacteria in the coral microbiome could support corals under heat stress. The idea is to treat corals with a probiotic, a cocktail full of beneficial bacteria, with the aim of increasing the abundance of these bacteria in the coral microbiome. A study from 2018 showed that treating corals of the genus Pocillopora with beneficial microbes reduced bleaching under heat stress in the lab⁵. While this is a promising result, it was still unclear whether probiotics would have a similar effect in the field. In addition, potential negative effects of the probiotic on the coral's environment would have to be investigated as well.

Probiotics for Corals

Researchers at the King Abdullah university of science and technology in Saudi Arabia dedicated themselves to this task and published their results this year in the scientific journal Nature⁶. As in the study mentioned above, they treated corals of the genus Pocillopora with a probiotic. However, this study was conducted in the Red Sea. They were able to show that bacteria in the probiotic were significantly increased in the microbiome of the corals, as long as the treatment was continuously applied. Surprisingly, the abundance of other beneficial microbes that were not present in the probiotic increased as well, while the presence of potential coral pathogens decreased.

Side Effects

Off-target effects - changes in the microbiome of the environment - were investigated as well. It is essential to look into this, as inadvertently damaging the environment through human intervention would be quite counterproductive. After all, the whole point of this approach is to save, not further damage. To investigate this, samples of the surrounding seawater and sediment were collected before the start and at the end of the treatment. Fortunately, no off-target effects were detected.

However, as there were no bleaching events during the study period, it was not possible to determine whether the probiotic had a positive effect on the corals' resilience to heat stress in the wild. An answer to this question is still pending and will probably be investigated in future studies. Nevertheless, these are exciting and promising results, bringing us one step closer to preserving coral reefs in a warming world. Meanwhile, the mission to save coral reefs continues so that future generations will get the chance to see these colorful reefs teeming with life, and not only in old documentaries.

Sources

1. Fisher, R., et al., Species Richness on Coral Reefs and the Pursuit of Convergent Global Estimates. Current Biology, 2015. 25(4): p. 500-505.
   

2. Hoegh-Guldberg, O., et al., Coral Reef Ecosystems under Climate Change and Ocean Acidification. Frontiers in Marine Science, 2017. 4.
   

3. Hughes, T.P., et al., Global warming transforms coral reef assemblages. Nature, 2018. 556(7702): p. 492-496.
   

4. Voolstra, C.R., R.S. Peixoto, and C. Ferrier-Pagès, Mitigating the ecological collapse of coral reef ecosystems: Effective strategies to preserve coral reef ecosystems: Effective strategies to preserve coral reef ecosystems. EMBO Rep, 2023. 24(4): p. e56826.
   

5. Rosado, P.M., et al., Marine probiotics: increasing coral resistance to bleaching through microbiome manipulation. The ISME Journal, 2018. 13(4): p. 921-936.

6. Delgadillo-Ordoñez, N., et al., Probiotics reshape the coral microbiome in situ without detectable off-target effects in the surrounding environment. Communications Biology, 2024. 7(1): p. 434.