8. Photosynthesis: “Dark Reactions” (continued) Read for this lesson: Biology of Plants 6th ed. pp. 126-153 7th ed. pp. 115-139 8th ed. pp. 122-149 (same as for last lesson) Extra reading: [Plant Physiology (Taiz & Zeiger) pp. 195-224]

An Overview of Photo-Synthesis Light Reactions “Dark” Reactions 5 5 1 4 3 CO2 Fixation 2

3 CO2 Rubisco 3 RuBP 6 PGA Calvin cycle 6 6 PGAL 5 TP 6 TP TP Sugars 4 6 ATP Calvin cycle 3 ATP 6 NADPH 6 6 PGAL 5 TP 6 TP TP Sugars

PHOTORESPIRATION Chloroplast Peroxisome NH3 Calvin cycle glyoxylate glycine O2 CO2 2 x PGA Rubisco O2 PGA + glycolate glycine NH3 serine + CO2 5 glycerate Mitochondrion

Repetition on the Calvin (C3) cycle: This is the only1 (biochemical) way in which high-energy organic compounds (sugars) can be formed from low-energy compounds (CO2 and water) - PGA (a C3 product) is formed as the first product of CO2 fixation; The enzyme that catalyzes CO2 fixation in the Calvin cycle (Rubisco) can also fix O2 – this leads to photorespiration; All plants1 have the Calvin (C3) cycle. Plants in which this cycle is the only2 cycle in which CO2 can be fixed are called C3 plants; (This2 indicates that there are plants that have CO2 fixation cycles ADDITIONAL to the Calvin sycle!) 5

Repetition on the Calvin (C3) cycle: This is the only (biochemical) way in which high-energy organic compounds (sugars) can be formed from low-energy compounds (CO2 and water) - PGA (a C3 product) is formed as the first product of CO2 fixation; The enzyme that catalyzes CO2 fixation in the Calvin cycle (Rubisco) can also fix O2 – this leads to photorespiration; All plants1 have the Calvin (C3) cycle. Plants in which this cycle is the only cycle in which CO2 can be fixed are called C3 plants; (This indicates that there are other cycles and plants other than C3 plants!) In the 70’s, Kortchak repeated Calvin’s experiment on terrestrial plants (Sugar cane) 4-carbon compounds (malate, aspartate), and not the 3-carbon compound PGA, were formed as first products of CO2 fixation! Plants that fix atmospheric CO2 into 4-carbon compounds are called C4 plants (However, 1 dictates that also C4 plants have the Calvin cycle!) 6

Why are most terrestrial plants (C3 plants) limited by the aerial CO2 concentration? 2) Why are some plants (C4 plants and CAM plants) not limited by the aerial CO2 concentration? 7

The special leaf anatomy of C4 plants 8

9

C4 Plants 10

Carboxylation into C4 acids CO2 C4 compounds PEP carboxylase a) HCO3- is fixed (not CO2), b) PEP carboxylase is NOT sensitive to O2 11

12

A Plant Cell 10 μM CO2 CO2 + H2O > H2CO3 > HCO3- + H+ > CO32- + H+ pH 8.2: 10 μM 2000 μM 300 μM

C4 metabolism is a CO2 concentrating mechanism (CCM) C4 Plants C4 metabolism is a CO2 concentrating mechanism (CCM) 14

Advantage of C4 metabolism: PEP carboxylase: high affinity for CO2 (uses HCO3-) No oxygenase activity (insensitive to O2) C4 plants show No Photorespiration! * C4 plants have a higher temperature optimum than C3 plants * C4 plants can function at very low CO2 concentrations and thus “save water” in arid regions! 15

C4 C3 16

Advantage of C4 metabolism: PEP carboxylase: high affinity for CO2 (uses HCO3-) No oxygenase activity (insensitive to O2) C4 plants show No Photorespiration! C4 plants have a higher temperature optimum than C3 plants * C4 plants can function at very low CO2 concentrations and thus “save water” in arid regions! 17

C4 C3 18

C4 plants can partly close their stomates and still photosynthesise 19

CAM plants (CAM = Crassulacean Acid Metabolism) CAM is also a CCM! 20

21

(WUE = CO2 fixed / water lost) Why are most terrestrial plants (C3 plants) limited by the aerial CO2 concentration? 2) Why are some plants (C4 and CAM plants) not limited by the aerial CO2 concentration? 3) How much better are C4 and, especially, CAM plants regarding water use efficiency (WUE) than C3 plants? (WUE = CO2 fixed / water lost) 22

Efficiency C3 CAM C4 C3 CAM C4 Close stomata At day time Grow slow C4 Tolerate higher Temp. & Drier conditions C3 CAM C4 C3 mol water lost / mol CO2 fixed 60 590 1,100 Water Use Efficiency (mol CO2 fixed / mol water lost) 0.016 0.0017 0.0009 23

C4 and CAM Plants Sugarcane Pineapple C4 : Maize, Sugercane Atriplex CAM: Cactus, Pineapple Many succulent desert plants C3: All trees, most crops, most temperate plants C4 : Maize, Sugercane Atriplex Many tropical grasses 24

C4 plants: Spatial division between primary fixation of atmospheric CO2 and its subsequent re-fixation and reduction in the Calvin cycle CAM: Temporal division between primary fixation of atmospheric CO2 and its subsequent re-fixation and reduction in the Calvin cycle 25

9. Marine Photosynthesis

Importance of Marine Photosynthesis: 1) Marine plant productivity “covers” some 72% of earth’s surface 2) Marine plants are responsible for about half of the world’s primary production (90% According to Encyclopedia Britannica) (mostly by Phytoplankton) 3) Benthic marine plants produce some 570 g C m-2 y-1 (as compared to 400 for terrestrial plants and 81 for phytoplankton) Marine benthic plants have many other roles apart from primary production! There can be a very good correlation between photosynthetic rates and growth (e.g. Lipkin et al. 1986, Aquat. Bot. 26: 129-1420),

Facts and Experiences by Photosynthesis in the Marine Environment Facts and Experiences by Sven Beer and John Beardall Prof. John Raven, FRS Prof. Paul Falkowski

Catalysed by carbonic anhydrase (CA) 0.035% = 350 ppm CO2 ~ 15 μM 10 μM CO2 CO2 + H2O <> H2CO3 <> HCO3- + H+ <> CO32- + H+ SLOW reaction Catalysed by carbonic anhydrase (CA) pH 8.2: 10 2000 (200) μM

Diffusion boundary layer Plasma membrane Cell wall Inorganic carbon

Species Km(CO2) Rubisco Author Giffordia 60 μM Weidner & Kuppers Bryopsis, 32 μM Yamada et al. (in Spatoglossum “ Johnston 1992) Palmaria 30 μM Cook & Colman Gracilaria 85 μM Israel & Beer Ulva 68 μM Beer et al. Average ~51 μM Terrestrial C3 ~10 μM

Air

There must be a CO2 concentrating mechanism (CCM)

Photosynthetic rates of many algae are NOT sensitive to O2, and CO2 compensation points are low…

C4 C3 35

= They have a CO2 concentrating mechanism (CCM). Photosynthetic rates of many algae are NOT sensitive to O2, and CO2 compensation points are low… = They have a CO2 concentrating mechanism (CCM). So, perhaps C4 photosynthesis? Or CAM?

MOST Algae, Most Seagrasses Plasma membrane Diffusion boundary layer Cell wall MOST Algae, Most Seagrasses

Ulva

SOME Algae (Ulva spp.) Plasma membrane Diffusion boundary layer Cell wall SOME Algae (Ulva spp.)

Red Blood Cell OH- HCO3- in Algae

HCO3- HCO3- SOME Seagrasses

2 1 HCO3- utilisation IS the CO2 Concentrating Mechanisms (CCM) in marine plants... 3

…that saturates Rubisco with CO2… Km (CO2)

How to measure gas exchange… “B” IRGA Air “C” Gas exchange column Seawater Seawater N2 Pump Acid ‘Stirrer’ “A” O2 metre O2 electrode

Photosynthesis (P) (mmol O2 m-2 s-1) Irradiance (I) (mmol photons m-2 s-1)

Photosynthesis (P) (mmol O2 m-2 s-1) Pgross Pnet Irradiance (I) (mmol photons m-2 s-1)

Pmax b a P (app) (O2 evolution or CO2 uptake) Icomp Ik Isat I (mmol photons m-2 s-1) R

Fig. 3.2 Pmax(true) b P(true) Pmax(app) b P(app) a P (O2 evolution or CO2 uptake) a Ik Isat 1,000 I (mmol photons m-2 s-1) R Icomp

P(app) (mmol O2 or CO2 m-2 s-1) Pmax(app) b a a High-light plant Pmax(app) P(app) (mmol O2 or CO2 m-2 s-1) b Low-light plant Ik Isat Isat 200 1,000 I (mmol photons m-2 s-1) Ik Icomp L H

Pmax(true) b a a High-light plant Pmax(true) P(true) (mmol O2 m-2 s-1) b Low-light plant Isat Isat 200 1,000 I (mmol photons m-2 s-1) L H

500 umol 100 umol 50 umol