Transcript www.reef.edu.au

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.
Stylophora
pistillata
Seriatopora
hystrix
Hoegh-Guldberg and Smith (1989)
Hoegh-Guldberg and Smith (1989)
Photosynthetic rate
S. pistillata S. hystrix
Respiratory rate
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.
Classic photoinhibition.
100
80
Photoinhibition
60
40
20
0
0
-20
-40
500
1000
1500
Irradiance
2000
2500
Higher Plants: Low light and Normal CO2 Fixation
Lumen
Stroma
ADP
H+
POOL
ATP
DARK REACTIONS
NADP
PSI
O2
H2 O
e-
PSII
NADPH
LHC
Thylakoid membrane
H+
+ ATP
CO2
Organic C
(Dark Reactions)
Photoprotection
against the effects
of high irradiance.
H2O + O2+ MDA
H2 O 2
H+
O2
POOL
O2
H2 O
Stimulated
as DpH builds
due to
reduction of O2
Violaxanthin and Zeaxanthin are higher
plant xanthophylls. In dinoflagellates,
monoepoxide diadinoxanthin and
diatoxanthin probably perform the
same roles.
O2 -
Mehler
PSI
e-
NADP
NADPH
PSII
NPQ
LHC
Zeaxanthin
DVE
Violaxanthin
H+
Higher Plants: High light and photo-oxidation
Lumen
Stroma
Chronic photoinhibition
Active oxygen
H+
POOL
O2
O2
H2 O
Singlet oxygen
originating from
electron donation at
or near PS II
O2 Mehler
PSI
e-
PSII
LHC
NADP
NADPH
+ ATP
CO2
Organic C
(Dark Reactions)
H+
Mehler Reaction:
Electrons donated
from near PS I to
oxygen => superoxide.
Pulsed Amplitude Modulated (PAM)
fluorescence analysis
qP
Fo
Ft
FV’/FM
qN
Chl a (PSII)
Time (min)
Pulsed amplitude modulated
(PAM) Fluorometry
PAM Fluorometer
measures efficiency
of light utilisation of
Photosystem II
H2O + O2+ MDA
H2 O 2
H+
O2
POOL
O2
PSII
Photosynth
H2 O
NPQ - heat
Stimulated
as DpH builds
due to
reduction of O2
O2 -
Mehler
PSI
e-
NADPH
PSII
NPQ
LHC
Zeaxanthin
DVE
Violaxanthin
Photochemistry versus heat
NADP
H+
Porites cylindrica (June 1997)
qP
Fo
Ft
FV’/FM
qN
1
Dark
2
800 umol m-2 s-1
3
4
Pink Sp
Yield versus time of day
Yes
Brow n Sp
Sunrise
Porites
Sunset
0.800
0.750
Yield
0.700
0.650
0.600
0.550
0.500
0.450
0.400
0
5
10
15
Tim e
20
25
Yield versus time of day
Dynamic Photoinhibition
Pink Sp
Brow n Sp
Porites
0.800
Yield
0.700
0.600
0.500
0.400
0
20
40
Tim e
60
80
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.
Normal (28oC)
Heat stressed
(32oC, 4 h)
measured in
presence
of oxygen.
Heat stressed
(32oC, 4 h)
measured in
absence
of oxygen.
Stylophora pistillata
One Tree Island
Feb ‘97
Calvin cycle
doesn’t turn on.
qN has to continue
dumping heat.
Electron flow in
heat stressed
zooxanthellae
is oxygen
dependent
(I.e. MP
pathway)
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
H2 O 2
H+
POOL
O2
Stroma
(Stromal SOD and APO not shown)
O2 O
2
Mehler
NADP
PSI
NADPH
e-
NPQ reduces
O2
PSII
possibility of
H2 O
LHC
singlet oxygen
NPQ
Stimulated
formation.
as DpH builds
Diatoxanthin
Singlet oxygen
due to
reduction of O2
promotes
degradation of
DVE
D1 protein and
Diadinoxanthin
damage to reaction
centre components
+ ATP
CO2
Organic C
(Dark Reactions)
H+
Ascorbate
Pool SOD = Superoxide dismutase
APO = Ascorbate peroxidase
DVE = Violaxanthin deepoxidase
New model (step 1): reduced CO2 Fixation
H2O + O2+ MDA
Lumen
H+
POOL
Capacity of NPQ
to quench PS II
exceeded.
O2
O2
H2 O
PSII
O 2 - H2 O 2
O2 -
(Stromal SOD and APO not shown)
O
2
Mehler
NADP
PSI
NADPH
eO2
LHC
NPQ
Stimulated
as DpH builds
Diatoxanthin
due to
reduction of O2
DDE
Diadinoxanthin
Stroma
O2 + ATP
H+
Ascorbate
Pool
CO2
Organic C
(Dark Reactions)
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
New model (step 2): redox buildup
H2O + O2+ MDA
Lumen
H+
POOL
O2
O 2 - H2 O 2
O2 -
(Stromal SOD and APO not shown)
O2 -
Mehler O2
O2 PSI
eO2 -
NPQ reduces
O2
PSII 1
possibility of
1O
O2
2
H
O
2
LHC
singlet oxygen
NPQ
Stimulated
formation.
as DpH builds
Diatoxanthin
Singlet oxygen
due to
reduction of O2
promotes
degradation of
DDE
D1 protein and
Diadinoxanthin
damage to reaction
centre components
-
Stroma
O2 -
O2 -
O2 -
O2 -
O2 O2 O2 -
4. MP pathway cannot
H+
absorb all singlet oxygen
5. Singlet oxygen and
superoxide builds up
and photosystems and
Ascorbate
general p/syn components
Pool
are damaged
6. Oxygen evolution reduced
Zooxanthellae: High light and Normal CO2 Fixation
H2O + O2+ MDA
Lumen
H2 O 2
H+
POOL
O2
Stroma
(Stromal SOD and APO not shown)
O2 O
2
Mehler
NADP
PSI
NADPH
e-
NPQ reduces
O2
PSII
possibility of
H2 O
LHC
singlet oxygen
NPQ
Stimulated
formation.
as DpH builds
Diatoxanthin
Singlet oxygen
due to
reduction of O2
promotes
degradation of
DVE
D1 protein and
Diadinoxanthin
damage to reaction
centre components
+ ATP
CO2
Organic C
(Dark Reactions)
H+
Ascorbate
Pool SOD = Superoxide dismutase
APO = Ascorbate peroxidase
DVE = Violaxanthin deepoxidase
800
(a) 28ºC
600
400
dark-adapted Fv/Fm
The interaction of light and
temperature may explain
some of the differences
in bleaching on the upper
versus lower surfaces
of corals.
200
0
0
50
200
800
1600
Upper surface Lower surface
800
(b) 33ºC
Lower
surface
600
400
Upper
surface
200
0
0
50
200
800
-2
1600
-1
PAR (µmol quanta m s )
H+
POOL
O2
O2
H2 O
PSII
NPQ
O 2 - H2 O 2
O2 O
2
Mehler
NADP
PSI
NADPH
eO2
O2 -
X
LHC
+ ATP
H+
Diatoxanthin
DDE
Diadinoxanthin
Ascorbate
Pool
CO2
Organic C
(Dark Reactions)
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.