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

2
(No Transcript)
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)
5
(No Transcript)
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
Stroma
Lumen
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
NPQ
Stimulated as DpH builds due to reduction of O2
H
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
Stroma
Lumen
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 gt superoxide.
Singlet oxygen originating from electron donation
at or near PS II
13
Pulsed Amplitude Modulated (PAM) fluorescence
analysis
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
NPQ
Stimulated as DpH builds due to reduction of O2
H
Zeaxanthin
PSII
DVE
Photosynth
Violaxanthin
NPQ - heat
Photochemistry versus heat
15
Porites cylindrica (June 1997)
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 doesnt 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
Stroma
Lumen
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
NPQ
Stimulated as DpH builds due to reduction of O2
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
Stroma
Lumen
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
NPQ
Stimulated as DpH builds due to reduction of O2
Diatoxanthin
Ascorbate Pool
DDE
Diadinoxanthin
23
New model (step 2) redox buildup
H2O O2 MDA
Stroma
Lumen
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
NPQ
Stimulated as DpH builds due to reduction of O2
Diatoxanthin
Ascorbate Pool
DDE
Diadinoxanthin
24
Zooxanthellae High light and Normal CO2 Fixation
H2O O2 MDA
Stroma
Lumen
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
NPQ
Stimulated as DpH builds due to reduction of O2
Diatoxanthin
Ascorbate Pool
DVE
SOD Superoxide dismutase APO Ascorbate
peroxidase DVE Violaxanthin deepoxidase
Diadinoxanthin
25
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
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.

28
(No Transcript)
29
(No Transcript)
30
(No Transcript)