IB Biology Topic D4.3.7 Threats to Coral Reefs as an…

Coral reefs are collapsing as warming and acidification break the coral–zooxanthellae partnership

Coral reefs are among the most biologically diverse ecosystems on Earth, supporting about a quarter of all marine species.

The reef structure is built by coral polyps, tiny animals that lay down calcium carbonate skeletons.

Healthy reefs also support fisheries, provide coastal protection, and generate income through tourism.

Bleaching is the collapse of the coral–zooxanthellae symbiosis under stress

Coral bleaching happens when a coral expels the zooxanthellae, the photosynthetic algae living in its tissues.

Without these algae the coral loses its colour and its main source of energy, which is why the white skeleton shows through.

Triggers of bleaching.

Increased water temperature: even a rise of 1–2°C above normal for a few weeks can trigger bleaching.
Other stressors: pollution, overexposure to sunlight, and changes in salinity can also drive corals to expel their algae.

Why the coral expels the algae.

Under heat stress the zooxanthellae produce harmful reactive oxygen compounds, so the coral ejects them to protect its own tissue, breaking the partnership that feeds it.

Example

In 2016 the Great Barrier Reef suffered a severe bleaching event from a prolonged marine heatwave that affected over 90% of its corals.
Fish populations, invertebrates, and other species declined as the reef's biodiversity fell.

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IB Biology Topic D4.3.7: Threats to Coral Reefs as an Example of Potential Ecosystem Collapse Notes

IB Biology Topic D4.3.7: Threats to Coral Reefs as an Example of Potential Ecosystem Collapse Notes

Coral reefs are collapsing as warming and acidification break the coral–zooxanthellae partnership

Bleaching is the collapse of the coral–zooxanthellae symbiosis under stress

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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Coral reefs are collapsing as warming and acidification break the coral–zooxanthellae partnership

Bleaching is the collapse of the coral–zooxanthellae symbiosis under stress

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

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IB Biology Topic D4.3.7: Threats to Coral Reefs as an Example of Potential Ecosystem Collapse Notes

A - Unity and Diversity9 subtopics

B - Form and Function10 subtopics

C - Interaction and Interdependence9 subtopics

D - Continuity and Change12 subtopics

IB Biology Topic D4.3.7: Threats to Coral Reefs as an Example of Potential Ecosystem Collapse Notes

A - Unity and Diversity9 subtopics

B - Form and Function10 subtopics

C - Interaction and Interdependence9 subtopics

D - Continuity and Change12 subtopics

Coral reefs are collapsing as warming and acidification break the coral–zooxanthellae partnership

Bleaching is the collapse of the coral–zooxanthellae symbiosis under stress

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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Coral reefs are collapsing as warming and acidification break the coral–zooxanthellae partnership

Bleaching is the collapse of the coral–zooxanthellae symbiosis under stress

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

Unlock the rest of this chapter with a free account