Microscopic Footage of Coral Reveals a Colorful Symbiotic Relationship

Recent footage of coral inspected under a microscope reveals not only gorgeous colors but also what are known as dinoflagellates, which have a fascinating symbiotic relationship with coral.

James Weiss of the YouTube channel Journey to the Microcosmos was given the opportunity to inspect coral under a microscope. That task in itself was difficult as coral doesn’t fit very easily under a microscope, but after taking slices of the creature, he was treated to footage of what are called dinoflagellates as they spilled out onto the surface.

Dinoflagellates are a monophyletic group of single-celled eukaryote organisms that are usually classified as a type of algae. These particular dinoflagellates are known as zooxanthellae because they live in a symbiotic relationship with creatures like coral.

They are referred to as creatures because while coral looks like a plant because of the form they take, they are actually animals, members of the phylum Cnidaria which makes them relatives to creatures like the hydra and sea anemone.

The footage shown by Journey to the Microcosmos explains that corals have an intricate relationship with zooxanthellae.

“The zooxanthellae live their algae lives within the coral, absorbing light and photosynthesizing it into nutrients,” Weiss explains. “But those nutrients aren’t really for them. About 90% of what the zooxanthellae makes gets senet to the coral instead.”

That tradeoff might not seem particularly fair, but the coral puts those nutrients to good use.

“Scientists studying stony corals found that when the zooxanthellae were removed from the corals, or when it was darker and the zooxanthellae couldn’t do their photosynthesis, the corals were much slower at making calcium carbonate. And while there are corals that don’t form symbiotic relationships with zooxanthellae, they’re also slower at making the calcium carbonate structures compared to symbiotic corals,” Weiss explains.

“In exchange, the zooxanthellae get the protection of their coral host. And they also get the waste that the coral produces, things like ammonia that are useless to the coral but nutrition to the zooxanthellae.”

It’s also thanks to the zooxanthellae that coral appears as colorful as it does in a healthy reef. But it’s a delicate relationship, and if the water becomes too warm or is polluted, the coral expels the zooxanthellae and as a result becomes white — this is known as coral bleaching.

“With our changing climate, these stresses are starkly rendered in our oceans, as corals turn white and face new challenges without their dinoflagellate inhabitants,” Weiss says. “The relationship they formed with zooxanthellae may have been one of the defining features of their past, but the loss of those relationships may be one of the defining features of their present.”