Coral bleaching

Question 1

What are key features of reef-building corals most affected by bleaching?

Answer 1

• Found in shallow tropical waters; mostly colonial calcifying Anthozoans.
• Anthozoa (~6,000 species): Polyps only, no medusae.
• Scleractinia: True stony corals forming aragonite corallum (colonial skeleton) with individual corallites for each polyp.
• Aragonite: A natural form of calcium carbonate essential for skeleton formation.

Question 2

What factors influence coral growth and structure?

Answer 2

• Slow growth atop previous calcification; Acropora corals can grow >100m annually.
• Large colonies reach several meters in size but have <1mm living tissue on the surface.
• Corals are superorganisms, similar to large sessile animals.
• Calcification rates are regulated by calicoblastic tissue (crystal formation) and zooxanthellate symbiotic photosynthesis.
• Most primary reef builders are zooxanthellate, containing algal symbionts that support growth.

Question 3

Explain the 3 roles of zooxanthellae

Answer 3

1) Influence calcification process
2) Provide nutritional material to their hosts
3) Take up host waste products

Question 4

Explain how zooxanthellae can influence calcification process

Answer 4

Through light-enhanced calcification
→ Light, photosynthesis, and calcification are intimately linked
- A cloudy day can reduce carbonate deposition by 50%
- Carbonate production are 3x higher during the day

Question 5

Explain how zooxanthellae can influence nutrition

Answer 5

• Photosynthesis: Zooxanthellae provide the host with amino acids, fatty acids, glucose, and glycerol.
• Autotrophy: When the photosynthesis-to-respiration (P:R) ratio >1, corals can meet all metabolic needs via photosynthesis.
• Variation: P:R varies with light, depth, season, and habitat.
• Heterotrophy: Corals also feed heterotrophically, boosting tissue growth and calcification rates.

Question 6

How do zooxanthellae influence waste uptake

Answer 6

• Nutrient uptake: Zooxanthellae absorb phosphorus and nitrogen from their hosts.
• Transform these into organic compounds and return them to the coral.
• Act as ‘metabolic kidneys’, removing toxic ammonium and nitrate waste products.

Question 7

What environmental changes are reef corals particularly sensitive to?

Answer 7

• Reduced salinity
• Prolonged low tides
• Disease
• Pollution
• Extreme temperatures (most significant stressor).

Question 8

Describe corals stress response

Answer 8

At threshold levels → inc mucus secretion, polyp withdrawal

Beyond threshold levels → zooxanthellae symbionts are expelled (coral bleaching)

Question 9

What happens to corals during severe bleaching?

Answer 9

• Results in the host tissue becoming transparent = reveals the bleached white aragonite skeleton beneath
• Severe bleaching = 60–90% loss of symbiont cells + a reduction in host tissue biomass.
• Bleaching affects more than photosynthetic capacity, impacting overall coral health.

Answer 10

• Sensitivity: Reef corals bleach with just 1–2°C increases above normal summer highs due to their ectothermic nature.
• Triggers: Long-term climate patterns like El Niño can warm oceans.
• Stress threshold: Sea temperatures above 30–32°C in the tropics can induce bleaching.

Question 11

What was the impact of the 1998 El Niño on coral reefs?

Answer 11

• Caused >90% coral loss in parts of the Indian Ocean.
• Similar devastation in the Pacific and Caribbean.
• 16% of global shallow water reefs were lost.
• Unprecedented in scope and severity at the time.
• Since then: Worse events, such as the 2016 mass bleaching

Question 12

What are the ecosystem effects of bleaching

Answer 12

Obvs bad for corals themselves. But also…
- Corals provide habitat for a huge diversity of other organisms
- Many sp entirely dependent on healthy corals for habitat and for food
- Coral cover and topographic complexity particularly important for fish
- Climate change is now a major threat for reef fish communities

Question 13

What reefs were most badly affected by the 2016 bleaching?

Answer 13

Across the tropics, particularly the Great Barrier Reef (GBR)

Question 14

What does the future hold for coral bleaching?

Answer 14

• Hughes et al (2018): Time between severe bleaching events is shortening, with a median return time of 6 years.
• Tropical conditions: Warmer during La Niña than past El Niño events.
• Annual bleaching may become the norm in the coming decades.