This is Scientific Americans' 60-Second Science. I am 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 in their formation.

One thing about reefs in particular really confused him.

That conundrum became known as Darwin's reef paradox. The paradox is this.

How can you find this lush teeming of life in the otherwise nutrient depleted ocean?

That's Dr. Christian Volstra, a professor at the University of Constance in Germany.

And the trick is symbiosis. Corals are basically SSI 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 their 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.

These little critters across the globe are in 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.

That 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?

The color of corals comes from the photosynthetic pigments of the algae.

So the minute these algae are out, the coral looks white.

What happens in bleaching is that these symbiotic algae or tiny plant cells are getting expelled out of the coral tissue.

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