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Pierre Cardol
Physiologie Membranaire et Moléculaire du Chloroplaste Institut de Biologie Physico-Chimique
13, rue Pierre et Marie Curie, F-75005 Paris PS07, Glasgow 22-27 july 2007
Photosynthesis in the smallest free-living
eukaryotic organism: light utilisation and
capture in the picoeukaryote Ostreococcus
~ 1 µm Plasma membrane (cell-wall less) Nucleus Chloroplast Starch Mitochondria
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Ostreococcus : pigments and LHCP antenna
Order of Mamiellales :
Ostreococcus , Micromonas, Bathycoccus
0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0 5 10 15 20 25
Temps de tétention (min) 1 23 456 7 89 10 11 12 13
Retention time (min)
O D 1. Mg-DVP 2. Uriolide 3. Neoxanthophyll 4. Prasinoxanthin 5. Violaxanthin 6. Micromonal 7 .Antheraxanthin 8.Zeaxanthin 9. Dihydrolutein 10. Chlorophyll b 11. Chlorophyll a 12. Carotene 13. ββββ-Carotene (Six et al., 2005
Mol. Biol. Evol. 22, 2217-2230) 5 0 0 6 0 0 7 0 0 0 , 0 0 , 5 1 , 0 O D O T H 9 5 s p in a c h λ (nm) LHCP No LHCII
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Ecotype localisation
>10 coastal ecotypes
1 oceanic ecotype
Strains used in this work
O. tauri (OTH95)
Courties et al. 1994 Nature 370, 255.
-Coastal strain -Surface
-Fluctuating irradiance
Aim : compare light harvesting and electron transfer capacity to reveal adaptation to nutrient and light shortage
RCC809
Rodriguez, et al. 2005. Environ. Microbiol. 7, 853-859.
-Oceanic (tropical Atlantic) -Isolated at 100 m depth
-low light
-Nutrient starvation ?
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1. Light availability and changes in the absorption properties of Photosystem LHC
Increased absorption capacity of photosystem II
Constitutive adaptation of RCC809 to low light environment
0 10 20 30 40 50 0,6 0,8 1,0 F lu o re s c e n c e ( r. u .) Time (ms)
PSII antenna size (Low light + DCMU)
QA QA -0 2 4 6 8 10 12 14 16 18 20 OTH95 RCC809 C h lo ro p h y ll e b (% p im e n ts t o ta u x )
White light 100 µE m-2 s-1
Blue light 10 µE m-2 s-1
Chlorophyll b
-♦- RCC809
5 0 0,5 1 1,5 2
PSII cyt b6/f PSI
OTH95
2. Nutrient availability and changes
in stoichiometries of photosynthetic complexes
0 0,5 1 1,5 2
PSII cyt b6/f PSI
OTH95 RCC809
Diatoms
(Strzepek and Harrison, Nature 2004 431, 689-692)
Cyanobacteria
(Boekema et al. Nature 412: 745 (2001)
Oceanic photosynthetic organisms cope with iron limitation by
decreasing iron-rich photosynthetic pathway RCC809 constitutively shows a Fe starvation-like phenotype PSII (4 Fe) Cyt b6f PSI + Fdx (20 Fe) PQH2 PQ PC ½ O2+2H+ NADP++H+ H2O 2H + 2H+ NADPH
6 PQ PQH2 PSII Cytb6f 2H+ ½ O2 + 2H+ H20 NADP+ + H+ NADPH PSI 2H+
PSII Light absorption increases PSI/cytb6f decreases
Impact on photosynthesis? RCC809
7 0 5 10 15 Time (min) O x y g e n ( r. u .) rcc809 0 100 200 300 2000 0 50 100 150 200 250 300 light intensity (µE m-2 s-1) µ mo le s O 2 h -1 mg c h l -1 OTH 95 RCC 809
Consequences of reduced PSI and cyt b6f complexes Oxygen Evolution and Photosystem II activity
PSII might transfer
electrons to another
acceptor : oxygen ?
φ φ φ φPSII Light 1250 1750 2250 2750 3250 3750 0 20000 40000 60000 80000 100000 120000 OTH95 + nig Fm’ (QA-) F (QA) φPSII=(Fm’-F)/Fm’ Light8 1000 1500 2000 2500 3000 0 20000 40000 60000 80000 100000 120000 RCC809 + nPG 100 µM 1000 1500 2000 2500 3000 0 20000 40000 60000 80000 100000 120000 OTH95 + nPG 100 µM 0 20 40 60 80 100 0 200 400 600 800 1000 1200 I (µE m-2 s-1) RCC809 OTH95
nPG inhibition of relative PSII ETR (%)
Impact of n-propylgallate on PSII electron Transport rate
PQH2 PQ ½ O2+2H+ NADP++H+ H2O 2H+ 2H+ NADPH ½ O2 H2O
Plastid terminal oxidase (PTOX)
½ O2+2H+
H2O Mehler
PTOX acts as a valve
possible role in ∆pH homeostasis
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Estimation of
∆
µH+ (∆pH and ∆Ψ) in the light :1. Lower ∆pH in RCC809
2. “water to water” electron flow via PSII-PTOX activity supports ∆pH
homeostatis in RCC809
OTH95 RCC809 Relaxation of the electrochromic shift (ECS) signal at steady state
∆pH
∆pH
no
∆pH
10 Photoprotection Mechanisms Chl 1Chl* 1. Photochemistry 2. Fluorescence 3. Heat (NPQ)
i . qT : State transitions
No Fluorescence decrease during
an aerobiosis to anaerobiosis transition
ii . qI : Photoinhibition
occurs in RCC809
(indicated by the decrease in PMAX)
0 100 200 NPQ qI+qT qE Fm Fm’ (s) F lu o re s c e n c e Light Dark 0 1 0 0 2 0 0 3 0 0 2 0 0 0 0 5 0 1 0 0 1 5 0 lig h t in te n sity (µE m-2 s-1) µ m o le s O 2 h -1 m g c h l -1 R C C 8 0 9 -anaerobiosis -aerobiosis
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iii. qE :
thermal dissipation of energy at the antenna level
LI818, CP26, PSBS
Induced by lumen acidification
Psbs Protonation
Activation of Violaxanthin deepoxidase (Vx to Ax)
Photoprotection Mechanisms
NPQ machinery present in RCC809 but not quickly activated
300 400 500 600 >12 hours 100 µE 21-22% 0 1 2 3 4 5 0 1 2 3 OTH 95 + nig 10µM N P Q Time (min) 0 1 2 3 4 5 0 1 2 3 RCC 809 + nig 10µM 0 to 9% 0 to 1% Dark-adapted
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Conclusions
Ostreococcus tauri (coastal strain OTH95)
• Behavior similar to that of land plants • Equimolar PSI/Cytb6f/PSII ratio
• Fast and sustained photoprotective response
½ O2 + 2H+ PQ PQH2 PSII Cytb6f H20 NADP+ + H+ NADPH PSI 2H+ 2H+ NPQ
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Conclusions
Deep oceanic ecotype (RCC809)
• Constitutive iron starvation phenotype: Low b6f and PSI content
Plastid terminal oxidase alleviate PSII excitation and provides “extra” ∆pH • Constitutive adaptation to low light intensities
Larger PSII antenna size
No rapid photoprotective response (∆pH not optimum) photoinhibition NPQ PQ PQH2 Cytb6f PTO X H20 ½ O2 2H+ ½ O2 + 2H+ H20 NADP+ + H+ NADPH PSI PSII 2H+
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Acknowledgements
IBPC
UMR7141, Paris
Giovanni Finazzi
Fabrice Rappaport
Benjamin Bailleul
Cécile Breyton
Laboratoire Arago
UMR7628, Banyuls
Hervé Moreau
Evelyne Derelle
Photobiology
University of Liège
Fabrice Franck
Michèle Radoux
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Summary
RCC809
PQ PQH2 Cytb6f H20 ½ O2+ 2H+ NADP+ + H+ NADPH PSI 2H+ ½ O2 + 2H+ PQ PQH2 PSII Cytb6f H20 NADP+ + H+ NADPH PSIOstreococcus tauri (OTH95)
2H+ NPQ Cytb6f PSI PSII NPQ PQ PQH2 Cytb6f PTO X H20 ½ O2
