The Role of Zooxanthellae in the thermal tolerance of corals

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

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

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

6. Experimental Design • Acroporamillepora –a widespread Indo-Pacific hard coral species • Three different populations of A. milleporawere 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

7. Geography of Study

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

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

11. Strengths/Weaknesses • Strengths • 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

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

13. References • http://data.aims.gov.au/staffcv/jsf/external/view.jspx;jsessionid=22A9E673BB4E173F062BBB20915F2B3E?partyId=100000290 • 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): 2305-312. PubMed. 8 June 2006. Web. 17 Feb. 2011. • http://www.aims.gov.au/docs/publications/waypoint/001/headlines-01.html