Coral bleaching and thermal stress: The first step is not damage to Photosystem II.

1. Coral bleaching and thermal stress: The first step is not damage to Photosystem II. Ove Hoegh-Guldberg School of Biological Sciences University of Sydney

3. Outline • Elevated temperature and coral-dinoflagellate symbioses. • Light, photoinhibition and photoprotection • Primary effects of temperature: A model • Take home message: Bleaching is due to a lowering of the threshold for photoinhibition.

4. Seriatopora hystrix Stylophora pistillata Hoegh-Guldberg and Smith (1989)

6. Hoegh-Guldberg and Smith (1989)

7. Photosynthetic rate Respiratory rate S. pistillata S. hystrix S. pistillata S. hystrix Hoegh-Guldberg and Smith (1989)

8. Outline • Elevated temperature and coral-dinoflagellate symbioses. • Light, photoinhibition and photoprotection • Primary effects of temperature: A model • Take home message: Bleaching is due to a lowering of the threshold for photoinhibition.

9. Classic photoinhibition. Photoinhibition Net Photosynthetic Rate Irradiance

10. Higher Plants: Low light and Normal CO2 Fixation Lumen Stroma ADP H+ POOL ATP DARK REACTIONS NADP Rubisco LHC PSI CO2 + ATP NADPH Organic C (Dark Reactions) e- O2 PSII H2O H+ Thylakoid membrane

11. H2O + O2+ MDA Photoprotection against the effects of high irradiance. APO H2O2 H+ POOL O2 - SOD O2 Mehler NADP PSI LHC NADPH e- O2 PSII H2O H+ Stimulated as DpH builds due to reduction of O2 NPQ Zeaxanthin DVE Violaxanthin and Zeaxanthin are higher plant xanthophylls. In dinoflagellates, monoepoxide diadinoxanthin and diatoxanthin probably perform the same roles. Violaxanthin

12. Higher Plants: High light and photo-oxidation Lumen Stroma Chronic photoinhibition Active oxygen H+ POOL O2 - O2 Mehler NADP Rubisco LHC PSI CO2 + ATP NADPH Organic C (Dark Reactions) e- O2 PSII H2O H+ Mehler Reaction: Electrons donated from near PS I to oxygen => superoxide. Singlet oxygen originating from electron donation at or near PS II

13. Pulsed Amplitude Modulated (PAM) fluorescence analysis qP Fo Ft FV’/FM qN Pulsed amplitude modulated (PAM) Fluorometry Chl a (PSII) Time (min)

14. H2O + O2+ MDA PAM Fluorometer measures efficiency of light utilisation of Photosystem II APO H2O2 H+ POOL O2 - SOD O2 Mehler NADP PSI LHC NADPH e- O2 PSII H2O H+ Stimulated as DpH builds due to reduction of O2 NPQ Zeaxanthin PSII DVE Photosynth Violaxanthin NPQ - heat Photochemistry versus heat

15. Porites cylindrica (June 1997) qP Fo Ft FV’/FM qN 1 2 3 4 Dark 800 umol m-2 s-1

16. Yes Sunrise Sunset

17. Dynamic Photoinhibition

18. Outline • Elevated temperature and coral-dinoflagellate symbioses. • Light, photoinhibition and photoprotection • Primary effects of temperature: A model • Take home message: Bleaching is due to a lowering of the threshold for photoinhibition.

19. Stylophora pistillata One Tree Island Feb ‘97 Normal Normal (28oC) Calvin cycle doesn’t turn on. qN has to continue dumping heat. Heat stressed (32oC, 4 h) measured in presence of oxygen. Electron flow in heat stressed zooxanthellae is oxygen dependent (I.e. MP pathway) Heat stressed (32oC, 4 h) measured in absence of oxygen.

20. Effects of heat on zooxanthellae: • Primary effect is to stall the flow of electrons to dark reactions of photosynthesis. The first step is not to disrupt the light harvesting reactions as previously thought. • Photoprotection is induced as dark reactions fail through oxygen dependent electron flow (Mehler Reactions).

21. Zooxanthellae: High light and Normal CO2 Fixation H2O + O2+ MDA Lumen Stroma APO H2O2 (Stromal SOD and APO not shown) H+ POOL SOD O2 - O2 O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH Organic C (Dark Reactions) e- NPQ reduces possibility of singlet oxygen formation. Singlet oxygen promotes degradation of D1 protein and damage to reaction centre components O2 PSII H2O H+ Stimulated as DpH builds due to reduction of O2 NPQ Diatoxanthin Ascorbate Pool DVE SOD = Superoxide dismutase APO = Ascorbate peroxidase DVE = Violaxanthin deepoxidase Diadinoxanthin

22. New model (step 1): reduced CO2 Fixation H2O + O2+ MDA Lumen Stroma APO H2O2 O2 - (Stromal SOD and APO not shown) H+ POOL SOD O2 - O2 O2 - O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH Organic C (Dark Reactions) e- O2 - Capacity of NPQ to quench PS II exceeded. O2 PSII H2O 1. Calvin cycle begins to fail Electron pressure builds. 2. NPQ then MP pathways come into play 3. Quantum yield still in tact as electrons are passed to oxygen H+ Stimulated as DpH builds due to reduction of O2 NPQ Diatoxanthin Ascorbate Pool DDE Diadinoxanthin

23. New model (step 2): redox buildup H2O + O2+ MDA Lumen Stroma APO H2O2 O2 - (Stromal SOD and APO not shown) H+ POOL SOD O2 - O2 O2 - O2 - O2 - Mehler O2 - Rubisco PSI LHC O2 - O2 - O2 - e- O2 - O2 - NPQ reduces possibility of singlet oxygen formation. Singlet oxygen promotes degradation of D1 protein and damage to reaction centre components O2 PSII O2 - O2 - 1O2 1O2 4. MP pathway cannot absorb all singlet oxygen 5. Singlet oxygen and superoxide builds up and photosystems and general p/syn components are damaged 6. Oxygen evolution reduced H2O H+ Stimulated as DpH builds due to reduction of O2 NPQ Diatoxanthin Ascorbate Pool DDE Diadinoxanthin

24. Zooxanthellae: High light and Normal CO2 Fixation H2O + O2+ MDA Lumen Stroma APO H2O2 (Stromal SOD and APO not shown) H+ POOL SOD O2 - O2 O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH Organic C (Dark Reactions) e- NPQ reduces possibility of singlet oxygen formation. Singlet oxygen promotes degradation of D1 protein and damage to reaction centre components O2 PSII H2O H+ Stimulated as DpH builds due to reduction of O2 NPQ Diatoxanthin Ascorbate Pool DVE SOD = Superoxide dismutase APO = Ascorbate peroxidase DVE = Violaxanthin deepoxidase Diadinoxanthin

25. Light The interaction of light and temperature may explain some of the differences in bleaching on the upper versus lower surfaces of corals. Lower surface Upper surface

26. APO H2O2 O2 - H+ POOL SOD O2 - O2 O2 - O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH Organic C (Dark Reactions) e- O2 - O2 PSII H2O H+ NPQ Diatoxanthin Ascorbate Pool DDE Diadinoxanthin X

27. Outline • Elevated temperature and coral-dinoflagellate symbioses. • Light, photoinhibition and photoprotection • Primary effects of temperature: A model • Take home message: Bleaching is due to a lowering of the threshold for photoinhibition.