Before corals bleach, they do not show many other signs of feeling stressed. So, if we want to understand a coral's health, we have to study its cells. Inside cells we have a lot of information, including DNA, RNA, and proteins. These molecules can help us find clues about the communication between the coral and the algae. But also, these molecules can teach us how to know when corals are stressed.
When an organism is stressed, every cell in its body will react. Everything will do its best to survive! In response to stress, the cell will use its DNA to make RNA, so that it can then make proteins that will fight off the stress. If an organism has been stressed before, it can respond to the stress faster and better. Think of it like visiting a city: the first time you visit, you will need a map to find your hotel. The more often you visit the city, the less you will need the map because you will remember, and you will get back to the hotel faster.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00028