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Place 01 · The Great Barrier Reef · Chapter 6 of 7
The Coral’s Bargain
There is a second living thing inside every reef coral. It pays rent in sugar.
About 15 minutes · 1 video · 1 virtual tour · 1 quick activity
Start here
The best business deal in the ocean
Building a skeleton costs energy, and a tiny polyp catching the odd passing plankton cannot pay for it. The answer is a partnership with algae living inside the coral’s own tissue. It explains where reefs grow, what color they are, and why they can turn white.
Watch this
See it move
BBC Earth — Fluorescent Coral Glows in the Depth of the Ocean. Corals glow. Scientists are still working out all the reasons why.
While you watch, look for
- Which colors the corals glow
- Whether the glowing corals are deep or shallow
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The tenant
Algae, living inside
Tiny single-celled algae called zooxanthellae live inside the coral’s tissue. They give reef corals most of their brown, green and gold color.
The rent
Sugar, and lots of it
The algae photosynthesize and hand over most of the food they make. In return the coral gives them a safe, sunlit home plus carbon dioxide and nutrients.
The catch
The tenants need light
It is not really the coral that needs sunlight. It is the algae. Every reef on Earth is shallow because of its lodgers.
Photos: Tom Fisk, Fuka jaz, QUI NGUYEN / Pexels
Play with it
Take a 360° tour of the reef
Another 360° video, this one from the Australian government. Drag to look around, and look closely at the colors of the corals.
Video: Great Barrier Reef 360 virtual tour, Australian Government Department of Climate Change, Energy, the Environment and Water, on YouTube.
Two questions to answer while you are in there
- Most corals here are brown, green or gold. Who is giving them that color?
- Find the shallowest, sunniest corals. Do they look healthier than the ones in shadow?
Try it yourself
Check yourself
Try it
Who Gives What?
Pick the true statement from each set to work out exactly what the coral and the algae trade.
Try it · interactive
Energy Budget Meter
Switch the algae on and off and see whether the coral can pay its energy bill.
Open the Energy Budget Meter in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Symbiosis
Two living things living together
Mutualism
A partnership where both come out ahead
Zooxanthellae
The algae living inside reef corals
Photosynthesis
Using light to make sugar
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Coral’s Bargain
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Tropical Marine Biology 1.6 — Symbiotic Relationships — LibreTexts · 8 minutes · the assigned reading
- MiraCosta 15.15 — Coral Reefs — LibreTexts · optional
Photo credits (Pexels): Cầu Đường Việt Nam, Jiří Mikoláš, Tom Fisk, Fuka jaz, QUI NGUYEN.
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All Places
- The Great Barrier Reef
- Monterey Bay
- The Galápagos
- The Ridge and the Trench — in development
- The Southern Ocean — in development
- The North Pacific Garbage Patch — in development