Questions: Coral Reef Ecosystems: Biology and Threats

Question 1

A coral reef is thriving in clear, nutrient-poor tropical water. How is this ecologically possible, given that most marine ecosystems require nutrient-rich water to support high biodiversity?

• A. Coral polyps are highly efficient filter feeders that extract trace nutrients from the water column
• B. The symbiotic zooxanthellae photosynthesize and transfer up to 90% of their sugars directly to the coral, providing an internal energy source independent of external nutrient levels
• C. Reef organisms have evolved to recycle nutrients within the ecosystem at near-100% efficiency
• D. Coral reefs actually do require nutrient-rich water — upwelling near coastlines provides the nutrients needed

Answer: (B)

The zooxanthellae symbiosis is the key to the paradox. Corals essentially carry their own internal food factory — the algae photosynthesize using sunlight and donate most of the organic carbon to the coral host. This decouples coral energy supply from external nutrient availability. Option C (near-perfect recycling) is partially true but not the primary driver; option A mischaracterizes corals as filter feeders, which is not how they get most of their energy.

Question 2

After a severe bleaching event, divers observe that coral skeletons appear white and the coral is not visibly decaying. What is the most accurate interpretation?

• A. The coral has died and the skeleton is being exposed by decomposition
• B. The coral has expelled its zooxanthellae and is severely stressed but may still be alive and capable of recovery
• C. The coral is photosynthesizing normally — white color indicates peak zooxanthellae activity
• D. The white color is caused by calcium carbonate bleaching from ocean acidification, not thermal stress

Answer: (B)

A bleached coral has expelled its zooxanthellae and lost its color source — coral tissue is nearly transparent, so without the algae's pigments, the white calcium carbonate skeleton shows through. The coral is alive but energetically starved. If thermal stress abates quickly, corals can reacquire zooxanthellae and recover. If not, they die. Option A confuses bleaching (which leaves intact tissue) with death and decomposition.

Question 3

Bleached corals are dead.

• T. True
• F. False

Answer: False

Bleached corals have expelled their zooxanthellae and are stressed, but they are not necessarily dead. They have lost their primary energy source and are in a weakened state with a window for recovery if water temperatures return to normal. The distinction matters for reef management: areas with bleached-but-living corals can recover with protection, while truly dead reefs cannot.

Question 4

Ocean acidification threatens coral reefs primarily by dissolving existing coral skeletons.

• T. True
• F. False

Answer: False

Ocean acidification's primary threat to corals is slowing calcification — it reduces the availability of carbonate ions that corals need to secrete new calcium carbonate skeleton. This weakens reef structures and slows reef growth. While severely acidic conditions could dissolve carbonates, the main mechanism at projected pH levels is impaired skeleton-building, not active dissolution of existing structure.

Question 5

Why do coral reefs support extraordinary biodiversity despite occupying less than 0.1% of the ocean floor, and what makes this productivity possible in typically nutrient-poor tropical waters?

Model answer: Coral reefs create enormous physical complexity (caves, crevices, channels) that generates diverse microhabitats. The foundation of this productivity is the coral-zooxanthellae mutualism: symbiotic algae inside coral tissue photosynthesize and provide up to 90% of the coral's energy, giving corals an internal food factory powered by sunlight rather than external nutrients. This bypasses the usual dependence on nutrient-rich water and supports a highly efficient, diverse community.

The reef paradox — maximum biodiversity in minimum-nutrient water — is explained by the symbiosis enabling primary productivity from within, and by the structural complexity of the reef itself. Understanding both components (the biological mechanism and the physical habitat it creates) is essential to understanding reef ecology and why they are so vulnerable to the combined stresses of thermal bleaching and ocean acidification.