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

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.

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

Hoegh-Guldberg and Smith (1989)

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

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.

Net Photosynthetic Rate Classic photoinhibition. Photoinhibition Net Photosynthetic Rate Irradiance

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

Photoprotection against the effects of high irradiance. 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 Stimulated as DpH builds due to reduction of O2 NPQ H+ Zeaxanthin DVE Violaxanthin and Zeaxanthin are higher plant xanthophylls. In dinoflagellates, monoepoxide diadinoxanthin and diatoxanthin probably perform the same roles. Violaxanthin

Active oxygen Higher Plants: High light and photo-oxidation Lumen Chronic photoinhibition Stroma Active oxygen H+ POOL O2 - O2 Mehler NADP Rubisco LHC PSI CO2 + ATP NADPH e- Organic C (Dark Reactions) 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

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

Photochemistry versus heat 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 Stimulated as DpH builds due to reduction of O2 NPQ H+ Zeaxanthin PSII DVE Photosynth Violaxanthin NPQ - heat Photochemistry versus heat

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

Yes Sunrise Sunset

Dynamic Photoinhibition

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.

Stylophora pistillata One Tree Island Feb ‘97 Normal (28oC) Normal 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.

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).

Zooxanthellae: High light and Normal CO2 Fixation H2O + O2+ MDA Lumen Stroma APO H2O2 (Stromal SOD and APO not shown) H+ POOL O2 - SOD O2 O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH e- Organic C (Dark Reactions) 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 Diadinoxanthin SOD = Superoxide dismutase APO = Ascorbate peroxidase DVE = Violaxanthin deepoxidase

New model (step 1): reduced CO2 Fixation H2O + O2+ MDA Lumen Stroma APO H2O2 O2 - (Stromal SOD and APO not shown) H+ POOL O2 - SOD O2 O2 - O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH e- Organic C (Dark Reactions) 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

New model (step 2): redox buildup H2O + O2+ MDA Lumen Stroma APO H2O2 O2 - (Stromal SOD and APO not shown) H+ POOL O2 - SOD O2 O2 - O2 - Mehler O2 - 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

Zooxanthellae: High light and Normal CO2 Fixation H2O + O2+ MDA Lumen Stroma APO H2O2 (Stromal SOD and APO not shown) H+ POOL O2 - SOD O2 O2 - Mehler NADP Rubisco PSI LHC CO2 + ATP NADPH e- Organic C (Dark Reactions) 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 Diadinoxanthin SOD = Superoxide dismutase APO = Ascorbate peroxidase DVE = Violaxanthin deepoxidase

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

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

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.