Sarah Vitak: This is Scientific American’s 60-Second Science. I’m Sarah Vitak. 

Charles Darwin’s famous trip on the HMS Beagle is primarily known for bringing us the concept of evolution. But  Darwin also investigated coral reefs and their formation. One thing about reefs in particular really confused him—that conundrum became known as Darwin’s reef paradox. The paradox is this:

Voolstra: How can you find this lush, teeming of life in the otherwise, nutrient depleted ocean.

Vitak: That’s Dr. Christian Voolstra, a professor at the University of Konstanz in Germany. 

Voolstra: And the trick is symbiosis. Corals are basically sessile organisms or animals, so they basically pick a place and they sit and then they cannot move. So the way they make a living is that they team up with micro algae inside the tissues. And this is essentially tiny plants that do photosynthesis. And this photosynthesis generates sugars. And these sugars will be essentially delivered to the coral animals. 

Vitak: These little critters across the globe are in great peril a lot of danger. According to a report from the Global Coral Reef Monitoring Network published in October 2021, we lost 14% of the world’s coral reefs in the last decade. This was mostly due to large scale bleaching events. Coral bleaching is triggered by changes in the coral's environment—including increased temperature, sunlight, or pollutants. But what exactly does it mean for coral to be bleached?

Voolstra: The color of corals comes from the photosynthetic pigments of the algae. So the minute these algae are out, the coral looks white. So what happens in bleaching is that these symbiotic algae or tiny plant cells are getting expelled out of the coral tissue.

If the environmental conditions actually become better again, they can actually take up their algae again and they are fine. So this is a transitory state. But In actuality, if the environmental conditions persist, the coral literally starves to death.

Vitak: Dr. Voolstra and his team were really interested in research that took a new approach to helping coral cope with increased ocean temperatures: treating them with probiotics. [Christian R. Voolstra et al., Extending the natural adaptive capacity of coral holobionts]

Voolstra: The general consumer knows that you can buy yogurt with probiotic cultures, right? There are some bacteria that are good for your gut.

Vitak: Just like humans, coral have a microbiome; a community of microorganisms that live on or inside them. Previous work had shown that bolstering the coral’s bacterial microbiome by giving them doses of probiotics helped them survive challenging conditions. The process is similar to how we work with microbiomes and probiotics in people.

Voolstra: You extract microbes from these very resilient individuals, and then you literally transplant them or offer them to less resilient individuals of the same coral species. So it's not that you're putting something there that wasn't there, but it's really like this human fecal transplants. You have a healthy donor, and you offer these bacteria to an affected recipient.

Vitak: This had already been shown as a proof of concept in previous research. But Dr. Voolstra and his team wanted to drill down and understand it a bit better. 

To do the experiment, they worked in what they call “mesochosms”—sort of a sweet spot between a sterile isolated lab setting and a totally wild reef setting. Basically, they had aquariums with multiple types of corals and some other critters. This allowed them to control conditions but also get a slightly more real-world result. 

One very convenient thing about working with coral is that they are colonial organisms.

Voolstra: Which means that they consist of repetition of the same building blocks. From one colony, you can generate many, many fragments or pieces that all have the exact same genotype with this exact same environmental history. And then you can put them into different conditions. 

Vitak: Once they had their fragments they treated some of them with a mixture of bacteria that they had carefully isolated, selected, and grown from resilient coral—and, of course, they wanted to have a control for their experiment as well,  so they gave some a placebo saline solution.

Finally, they slowly cranked up the heat to simulate ocean warming. 

Voolstra: And this was a very long experiment that essentially lasted over 75 days. 

Vitak: What they found was fascinating. All the coral showed signs of bleaching as the temperature increased, but the coral treated with probiotics recovered faster. And they were 40% more likely to survive.

Voolstra: Okay, like as a coral biologist, or as a biologist, in general, I think you're usually very happy if you have a 5% effect or something observable that you can count with reasonable numbers. This is massive. I mean, if you almost double the survivorship, this is huge.

Vitak: The team also looked at how  adding this probiotic cocktail changed the coral’s microbiome and—how it changed the coral itself. Adding the probiotic changed the composition of the coral’s microbiome. 

Voolstra: It also instigated a change in the expression of certain genes in the coral host. And those genes, were really kind of the go-to genes that you would bet on if this is for increased recovery. 

Vitak: So basically—things like repair genes, immunity genes, and stress response genes. 

Voolstra:  So this is kind of the cliffhanger of this study, you actually change stuff in the host. And in the correlate host, and we don't know how long these changes will stay on. Of course, if those changes can be kept long term, you would not need to keep this probiotic treatment going on and on, right? 

Vitak: Which would be amazing in terms of translating this to the real world. 

Voolstra: I mean, there’s 300,000 square kilometers of coral reef. There's billions of coral. So if you want to bring a little magic potion underwater and inoculate each coral, this becomes unmanageable. No organism makes a living in isolation. And I think we're just getting better at understanding this.

Vitak: Thanks for listening. For Scientific American’s 60 Second Science, I’m Sarah Vitak. 

[The above text is a transcript of this podcast.]