https://phys.org/news/2018-12-algae-poor-providers-corals.html

The algae are photosynthetic—meaning capable of converting the Sun's energy into chemical energy for food, just like plants. And the exchange of nutrients between the coral and the algae is essential for healthy reef communities. The coral provides the algae with carbon dioxide, nitrogen, and other compounds that they need to survive and perform photosynthesis. The algae, in turn, can stimulate the growth of the coral by providing them with sugars and fats, which are created via photosynthesis.

An international research team based in New Zealand and including Carnegie's Arthur Grossman set out to determine how the abundance and diversity of sugars and other carbon compounds shared with the coral varies between species of algae and what this could mean for a coral's ability to survive under stressful conditions caused by climate change. They did this by studying the anemone Aiptasia—a cnidarian like the coral—which can also host symbiotic dinoflagellates, but it grows much faster and is easier to study than corals.