1. The Role of Zooxanthellae in the thermal tolerance of corals
Emily Wanerka
Juliet Blass

2. Key Words
Acclimatization – an organism’s response to changes in its environment
Temperature change in our case
Zooxanthellae – photosynthetic algae that live in coral tissues forming a symbiosis with the coral
Coral Bleaching
Climate Change

3. About the Authors Ray Berkelmans
-Australian Institute of Marine Science (1999 – present)
-climate change and coral reefs
Madeleine J.H. van Oppen
- Australian Institute of Marine Science (program leader 2005 – present)
-Research scientist
-Marine molecular ecology
Berkelmans:
Research into climate change impacts and adaptation of coral reefs
-particularly thermal stress causing the breakdown of symbiosis between corals and zooxanthellae
Van Oppen
-understanding marine microbes and symbioses

4. Background Information
Projected increases in temperature due to global warming are a major threat to coral reefs.
Survival depends largely on the corals’ ability to acclimatize
Algal symbiosis between coral reefs and zooxanthellae
Heat stress results in bleaching of corals, and can lead to death of the coral
Algae provides the corals with its color and up to up to 90% of its nutrients
Heat causes the algae to be toxic to corals, so they eject the algae and appear to be white

5. Purpose
Investigate the role of zooxanthellae in the thermal tolerance of corals
Necessary to understand whether symbiont change will affect the level of thermal tolerance in corals, with anticipated increases in sea surface temperatures in near future
Average seawater temperatures predicted to increase 1-3°C over the next 100 years
-study in this field is becoming increasingly important based on projected temperature changes

6. Experimental Design
Acropora millepora –a widespread Indo-Pacific hard coral species
Three different populations of A. millepora were studied
22 colonies from North Keppel Island (coolest)
22 colonies from Davies Reef (middle temp)
22 colonies from Magnetic Island (warmest)
Location of Study: Magnetic Island
Control: Native colonies from each location
North Keppel – cool southern inshore reef
- coolest of temperatures
Davies Reef – cool central offshore reef
- temp is 1.3 degrees celsius warmer than Keppels
Magnetic Island – Central Barrier Reef
corals were kept here on mesh racks at the same depth that they were collected
data loggers record temperatures here every half hour
 temp is 0.9 degrees celsius warmer than that at Davies

7. Geography of Study Great Barrier Reef  off Eastern Coast of Australia
Davies much closer to magnetic island – possibly suggests its resisitance to change in zooxanthellae type
C2*  zooxanthellae of Davies differed slightly in ribosomal DNA from C2

8. Regional Temperatures
Figure 2. Average daily temperatures at Magnetic Island, Davies Reef and Halfway Island (approximately 15 km from North Keppel Island) for the warmest austral summer months. Data are averages of 48 readings per day over 15 years (Magnetic Island) and 10 years (Davies Rf and Halfway Is) and were averaged over the reef flat (0 m at LAT) and slope (5 m at LAT). A 10 day smoothing function is applied to indicate the general trend in summer temperatures. Temperatures differences between Halfway Island and Nor th Keppel Island are less than 0.1 8C based on an 18 month period when loggers were deployed at both sites (data not shown).

9. Obtained Results Magnetic Island colonies:
- most thermally tolerant population
-only contained D zooxanthellae
Keppels colonies:
-80% had C2 at transplantation, 20% had D present along with C2
-all bleached, 7 died (those that died all contained C)
-surviving colonies regained color, and only had D zooxanthellae present
Davies colonies:
- 13 suvived,(9 died)
- all remaining corals either completely or partially bleached
- The remaining corals contained C2 only

10. Results
Thermal tolerance among native populations was strongly linked with location
Relative thermal sensitivity was demonstrated by contrasting and significantly different patterns of zooxanthella density
Reduced algal density did not affect the thermal resistence of the individual corals associated
Increased tolerance only proven with type D
hopeful, but not enough to help populations cope w/ potential increases in average tropical sea temps
-type D  magnetic Island

11. Strengths/Weaknesses
Keppels results support adaptive bleaching hypothesis
Temperatures recorded in 3 areas every half hour
Transplants - highly regulated (depths etc)
Weaknesses
Method of symbiont change not certain (shuffling, switching)
Davies – Inconsistent with link between acclimatization and location
ABH Environment change  loss of zooxanthellae followed by new formation of zooxanthellae more suited to apparent conditions
depths remained constant
Davies did not exhibit take up of new zooxanthella in its transplanted colonies
Results in control and transplant were similar

12. Further Study Cell Biochemistry of the coral proteins
Temperature limits in sustaining coral life, and biological changes of coral in response to changing temperatures
Physiological characteristics of zooxanthella
Process of Symbiont Change
Zooxanthellae
– responses to temperature and light stress (as they are the weakest in the symbiont relationship)
Stability of the thylakoid membrane in chloroplasts
Symbiont change - shuffling of zooxanthellae or switching types?

13. References
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 Berkelmans, Ray, and Madeleine J.H. Van Oppen. "The Role of Zooxanthellae in the Thermal Tolerance of Corals: a ‘nugget of Hope’ for Coral Reefs in an Era of Climate Change." Proceedings of The Royal Society B (2006): PubMed. 8 June Web. 17 Feb
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