Lesson 4 of 10 / Climate change, food and disease vectors
Coral reefs: a partnership under stress
Why does a coral turn white in hot water?
I can explain the coral and zooxanthellae mutualism, the calcium carbonate skeleton, and how warming, ocean acidification and heavy rainfall stress coral reefs.
I can explain physical climate effects, fresh-water stress, range shifts, coastal ecosystem stress and thaw feedbacks.Syllabus
Syllabus 8876, Extension A(b). Explain physical climate effects, fresh-water stress, range shifts, coastal ecosystem stress and thaw feedbacks.
About 6 min
Make a guess. You are not marked.
A coral turns white during a marine heatwave. Is it dead?
Show the answer
It has lost its algae but can recover.
Bleaching means losing zooxanthellae. The coral animal is still alive and can recover if conditions return to normal soon enough.
The key idea
Reef-building corals get most of their sugars from zooxanthellae, algae living in their tissue. Heat and low salinity break this mutualism and cause bleaching; ocean acidification makes it harder to build the calcium carbonate skeleton.
Explore the idea
Follow a coral partnership under stress
Zooxanthellae photosynthesise inside the polyp and pass most of the sugars they make to the coral. The coral gives them shelter, carbon dioxide and nitrogen-containing waste. Energy from the sugars helps the coral build its calcium carbonate skeleton.
Explanation
A reef-building coral is a colony of small animals called polyps. Each polyp lays down a hard skeleton of calcium carbonate beneath its soft tissue. Over many years these skeletons build up the reef, which gives shelter and food to many other species.
Single-celled algae called zooxanthellae live inside the cells of the polyp. Coral and zooxanthellae are an example of mutualism. Mutualism is a close relationship between two species in which both species benefit. The zooxanthellae photosynthesise and pass most of the sugars they make to the coral. In return, the coral gives the algae a sheltered place in sunlit water, and supplies carbon dioxide and nitrogen-containing wastes that the algae use. Energy from the sugars also helps the coral build its calcium carbonate skeleton.
Coral bleaching is the loss of zooxanthellae, or of their pigments, from coral tissue. When the sea stays warmer than usual for several weeks, the partnership breaks down and the coral expels its algae. The tissue becomes almost clear, so the white skeleton shows through. A bleached coral is still alive, but it has lost its main supply of sugars. If the water cools soon, algae can return. If the heat lasts, the coral can starve and die.
Ocean acidification is the fall in the pH of seawater as it absorbs extra carbon dioxide from the air. Carbon dioxide reacts with water to form carbonic acid, which releases hydrogen ions. Many of these hydrogen ions combine with carbonate ions, so fewer carbonate ions are left for corals to build calcium carbonate. Skeletons grow more slowly, and existing skeleton can start to dissolve. Acidification is caused by the carbon dioxide itself, not by warmer water.
Climate change also makes heavy rainfall more intense in many regions. Heavy rain pours fresh water onto shallow reefs and lowers the salinity. Low salinity disturbs the water balance of coral cells by osmosis, and this stress can also cause bleaching. Rain run-off from land carries sediment that makes the water cloudy. Less light reaches the zooxanthellae, so they photosynthesise less, and settling sediment can smother polyps.
Because so many species shelter and feed on a reef, losing the living coral harms the whole community. A dead reef stops growing and is slowly worn away, so the habitat itself disappears.
Step by step
- 1
Name both partners
Coral polyp and zooxanthellae. State what each gives the other.
- 2
Name the stress and its first effect
Heat or low salinity: algae expelled. Acidification: fewer carbonate ions. Sediment: less light.
- 3
Link it to the coral and the reef
Less sugar, so the coral can starve. Slower skeleton growth, so the reef weakens and habitat is lost.
Worked example
Pale corals near a river mouth
After a week of very heavy rain, divers find pale corals near a river mouth, while corals further offshore look normal. Suggest two reasons.
One way to explain it
Near the river mouth, fresh water from the rain lowered the salinity. This stressed the corals, which expelled their zooxanthellae and bleached. The run-off also carried sediment that clouded the water, so less light reached the zooxanthellae and they photosynthesised less. Offshore water was less diluted and clearer.
Why this answer works
- Use the location: the river mouth receives the most run-off.
- Give one salinity reason and one light reason.
- Name the zooxanthellae, not just the coral.
Watch out for this
A student says: "A bleached coral is dead." What is wrong with this?
Show the answer
Bleaching means the coral has lost its zooxanthellae or their pigment. The coral animal is still alive and can recover if conditions return to normal soon. It dies only if the stress lasts long enough for it to starve.
In the real world
The Great Barrier Reef, 2016
Record sea temperatures in 2015-2016 caused mass coral bleaching around the tropics. On the Great Barrier Reef, the 2016 bleaching was the most severe of the three events recorded since 1998, and the pattern of bleaching followed the pattern of hot water. Cleaner water and less fishing made little difference to how badly reefs bleached, so the researchers concluded that local protection gives little resistance to extreme heat.
Source: Hughes and others (2017), Global warming and recurrent mass bleaching of corals, Nature 543, 373-377
Check your understanding
Why can a long marine heatwave kill a reef-building coral?
Show the answer
The coral expels its zooxanthellae and starves without their sugars.
Heat breaks the mutualism. Without zooxanthellae the coral loses most of its sugar supply, and it can starve if the heat lasts.
Exam-style questions
Check your understanding
Which can cause coral bleaching? 1: A long period of high sea temperature. 2: Low salinity after heavy rain. 3: Clear, calm water. 4: Loss of zooxanthellae from coral tissue.
Show the answer
1, 2 and 4 only
Heat and low salinity stress corals, and bleaching is the loss of zooxanthellae. Clear, calm water is not a stress.
Quick recall
Why does a coral turn white in hot water?