History of Earth s Climate Earth formed

History of Earth’s Climate • • Earth formed ~4. 6 billion years ago Originally very hot Sun’s energy output only 70% of present Liquid water present ~4. 3 billion years

History of Earth’s Climate • Life appeared ~3. 8 billion years ago • Photosynthesis began 3. 5 -2. 5 billion years ago – Produced oxygen and removed carbon dioxide and methane (greenhouse gases) – Earth went through periods of cooling (“Snowball Earth”) and warming • Earth began cycles of glacial and interglacial periods ~3 million years ago

Earth’s Temperature Solar Sun Energy Solar Energy The temperature of the earth is directly related to the energy input from the Sun. Some of the Sun’s energy is reflected by clouds. Other is reflected by ice. The remainder is absorbed by the earth.

Earth’s Temperature Sun Solar Energy Radiation Cooling If amount of solar energy absorbed by the earth is equal to the amount radiated back into space, the earth remains at a constant temperature.

Earth’s Temperature Sun Solar Energy Radiation Cooling if the amount of solar energy is greater than the amount radiated, then the earth heats up.

Earth’s Temperature Sun Solar Energy Radiation Cooling If the amount of solar energy is less than the amount radiated, then the earth cools down.

Sun Greenhouse Effect To a certain degree, the earth acts like a greenhouse. Energy from the Sun penetrates the glass of a greenhouse and warms the air and objects within the greenhouse. The same glass slows the heat from escaping, resulting in much higher temperatures within the greenhouse than outside it.

Earth’s Atmospheric Gases Nitrogen (N 2) Oxygen (O 2) Non. Greenhouse Gases 99% Water (H 2 O) Carbon Dioxide (CO 2) Methane (CH 4) Greenhouse Gases 1%

Recap and importance: The photochemical reactions produce ATP and NADH at sites in the stroma. The Dark Cycle (Calvin Cycle), or more descriptively, the carbon reactions of photosynthesis ~200 billion tons of CO 2 are converted to biomass each year The enzyme ribulose biphosphate carboxylase/oxygenase, Rubisco, that incorporates CO 2 is 40% of the protein in most leaves.

The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration Carboxylation of the CO 2 acceptor, ribulose-1, 5 -biphosphate, forming two molecules of 3 -phosphoglcerate. Rubisco – the enzyme ribulose biphosphate carboxylase/oxygenase Reduction of 3 -phosphoglycerate to form glyceraldehyde-3 -phosphate which can be used in formation of carbon compounds that are translocated. Regeneration of the CO 2 acceptor ribulose-1, 5 -biphosphate from glyceraldehyde-3 -phosphate

Ru. BP The affinity of Rubisco for CO 2 is sufficiently high to ensure rapid carboxylation at the low concentration of CO 2 found in photosynthesizing cells The negative change in free energy associated with carboxylation of Ru. BP is large so the forward reaction is favored. Rubisco will also take O 2 rather than CO 2 and oxygenate Ru. BP – called photorespiration. The rate of operation of the Calvin Cycle can be enhanced by increases in the concentration of its intermediates. That is the cycle is autocatalytic. Also, if there are insufficient intermediates available, for example when a plant is transferred from dark to light, then there is a lag, or induction period, before photosynthesis reaches the level that the light can sustain. (There can also be enzyme induction. ) Rubisco is notoriously inefficient as a catalyst for the carboxylation of Ru. BP and is subject to competitive inhibition by O 2, inactivation by loss of carbamylation, and dead-end inhibition by Ru. BP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Really?

Basics of foliage photosynthesis Light Reaction Limiting Dark Reaction Limiting Increasing CO 2 concentration in the atmosphere can increase the maximum rate of photosynthesis in the short term Saturation level. sometimes called photosynthetic capacity. Photosynthetic efficiency: Increase in photosynthesis per increase in irradiance 0 0 Any questions? Compensation point The irradiance at which CO 2 uptake is zero

275 ppm CO 2 73 ppm CO 2 In the presence of higher O 2 levels, photosynthesis rates are lower. The inhibition of photosynthesis by O 2 was first noticed by the German plant physiologist, Otto Warburg, in 1920, and called the "Warburg effect". It is believed that photorespiration in plants has increased over geologic time due to increasing atmospheric O 2 concentration -the product of photosynthetic organisms themselves.

C 4 Photosynthesis The first product of CO 2 fixation is malate (C 4) in mesophyll cells, not PGA as it is in C 3 plants. This is transported to bundle sheath cells CO 2 is released from malate in bundle sheath cells, where it is fixed again by Rubisco and the Calvin cycle proceeds. PEP is recycled back to mesophyll cells. Decarboxylation of malate (CO 2 release) creates a higher concentration of CO 2 in bundle sheath cells than found in photosynthetic cells of C 3 plants. This enables C 4 plants to sustain higher rates of photosynthesis. And, because the concentration of CO 2 relative to O 2 in bundle sheath cells is higher, photorespiration rates are lower.

Crassulacean Acid Metabolism (CAM) First discovered in succulents of the Crassulacea: e. g. , sedums Uses C 4 pathways, but segregates CO 2 assimilation and Calvin cycle between day and night CAM plants open their stomates at night. This conserves H 2 O. CO 2 is assimilated into malic acid and stored in high concentrations in cell vacuoles During the day, stomates close, and the stored malic acid is gradually recycled to release CO 2 to the Calvin cycle

C 3, majority of C 4, e. g. , sugar CAM, cane, corn e. g. , cacti Bundle sheath cells lack chloroplasts Bundle sheath cells have chloroplasts Mesophyll cells have large vacuoles Can be sun or shade plants Ineffective in shade CO 2 capture at night Requires relatively moist habitats Arid or tropical regions Arid environments Moderate High Low 15 -25 o. C 30 -40 o. C 35 o. C species Leaf structure Efficiency in light Typical habitat characteristics Productivity Optimum Temperature

ISOTOPES AND LAND PLANT ECOLOGY C 3 vs. C 4 vs. CAM

Cerling et al. 97 Nature δ 13 C Cool season grass most trees and shrubs Warm season grass Arid adapted dicots

εp = δa - δf = εt + (Ci/Ca)(εf-εt) When Ci ≈ Ca (low rate of photosynthesis, open stomata), then εp ≈ εf. Large fractionation, low plant δ 13 C values. When Ci << Ca (high rate of photosynthesis, closed stomata), then εp ≈ εt. Small fractionation, high plant δ 13 C values.

Plant δ 13 C (if δa = -8‰) δi εf εp = εt = +4. 4‰ δ 1 -12. 4‰ δf -27‰ εp = εf = +27‰ -35‰ 0 0. 5 1. 0 Fraction C leaked (φ3/φ1 ∝ Ci/Ca) εp = δa - δf = εt + (Ci/Ca)(εf-εt) Ca, δa φ1, δ 1, εt φ3, δ 3, εt Ci, δi Inside leaf Ca, δa Cf, δf φ2, δ 2, εf

εp = εta+[εPEP-7. 9+L(εf-εtw)-εta](Ci/Ca) εp = 4. 4+[-10. 1+L(26. 3)](Ci/Ca) Ci/Ca In C 4, L is ~ 0. 3, so εp is insensitive to Ci/Ca, typically with values less than those for εta. Under arid conditions, succulent CAM plants use PEP to fix CO 2 to malate at night and then use RUBISCO for final C fixation during the daytime. The L value for this is typically higher than 0. 38. Under more humid conditions, they will directly fix CO 2 during the day using RUBISCO. As a consequence, they have higher, and more variable, εp values.

Δ 13 C fraction-whole plant

δ 13 C varies with environment within C 3 plants

C 3 plants Quantum Yield (moles C fixed per photons absorbed) Crossover Temperature C 4 plants Today (360 ppm) 3 6 9 12 15 18 Temperature (°C) 21 24 27 30

What happens when p. CO 2 changes? C 3 decreases in efficiency because of Photorespiration Ehleringer et al. 1997 Oecologia

C 3 plants Crossover Temperature Quantum Yield C 4 plants (moles C fixed per photon absorbed) Today (360 ppm) LGM (180 ppm) 3 6 9 12 15 18 21 Temperature (°C) 24 27 30

C 3 versus C 4 plants C 3 C 4 Photorespiration Yes Not detectable CO 2 compensation point (m. L CO 2 l-1) 20 – 100 0– 5 Temperature optimum (o. C) 20 – 25 30 – 45 Quantum yield as a function of temp. Declining Steady Transpiration ratio 500 – 1000 200 – 350 Light saturation (mmole photons m-2 s-1) 400 – 500 Does not saturate C 3 plants are favoured in environments where water is plentiful, temperature and light levels are moderate (temperate climates) C 4 plants are favoured in environments where water is limiting and light and temperatures are high (tropical / subtropical habitats)

CO 2 uptake rate C 3 C 4 250 350 Atmospheric CO 2 (ppm) 9/12/07 700 30

Three modes of photosynthesis C 3 pathway, aka Calvin cycle, most common. – Ribulose bisphosphate (Ru. BP, Rubisco) most abundant protein on Earth; enzyme captures CO 2 but also has high affinity for O 2. – Phosphoglyceric acid (PGA) is 3 -C sugar formed during CO 2 uptake. – Photorespiration makes photosynthesis less efficient but also protects cells from excess light energy. – At high CO 2: O 2 ratios, Rubisco is more efficient, thus C 3 plants respond more to elevated CO 2 than do C 4 plants – Most trees, shrubs, cool-season grasses 9/12/07 31

Calvin Cycle 9/12/07 32

Photorespiration • depends on light • “wastes” CO 2 • protects against light damage • favored by high O 2, low CO 2 and warm temperatures 9/12/07 33

Three modes of photosynthesis • C 4 pathway, aka Hatch-Slack, has additional enzyme, PEP carboxylase, with much higher affinity for CO 2. – Oxaloacetate (OAA) is 4 -C sugar formed during CO 2 uptake. – Rubisco concentrated in bundle sheath cells, where OAA delivers CO 2. – Photorespiration limited because CO 2: O 2 is much higher inside bundle sheath cells than in C 3’s. – Less Rubisco needed for psn means higher N-use efficiency. 9/12/07 34

9/12/07 35

Three modes of photosynthesis • C 4 pathway – Higher T optimum and light saturation. – High water use efficiency (C gained per H 2 O lost) because stomates can be partly closed. – Lower response to elevated CO 2 – Cost of C 4: additional ATP is needed for PEP cycle, which may limit C 4 growth at low light levels – 2000 species in 18 families; half of all grass (Poaceae) species (warm-season grasses) 9/12/07 36

• • There is a clear correlation between the amount of anthropogenic CO 2 released to the atmosphere and the increase in atmospheric CO 2 concentration during last decades. • Atmospheric oxygen is declining proportionately to CO 2 increase and fossil fuel combustion. • For the last half century, the CO 2 airborne fraction (AF) parameter remained consistent and averaged at 0. 55 (the AF parameter is the ratio of the increase in atmospheric CO 2 concentration to fossil fuel-derived CO 2 emissions). AF has been introduced to assess short- and long-term changes in the atmospheric carbon content; in particular, AF of 0. 55 indicates that the oceans and terrestrial ecosystems have cumulatively removed about 45 % of anthropogenic CO 2 from the atmosphere over the last half century [6]. • The isotopic signature of fossil fuels (e. g. , the lack of 14 C and the depleted level of 13 C carbon isotopes) is detected in atmospheric CO 2. • There exists an interhemispheric gradient in the atmospheric CO 2 concentrations in the Northern and Southern Hemispheres. In particular, the predominance of fossil-derived CO 2 emissions in more industrially developed Northern Hemisphere (compared to the Southern Hemisphere) causes the occurrence of the atmospheric CO 2 gradient in the amount of about 0. 5 ppm per Gt. C per year [6]. • There have been dramatic changes in RFCO 2 values over the last decades. For example, during 1995 – 2005, the RFCO 2 increased by about 0. 28 W/m 2 (or about 20 % increase), which represents the largest increase in RFCO 2 for any decade since the beginning of the industrial era. RFCO 2 in 2005 was estimated at RFCO 2=1. 66± 0. 17 W/m 2 (corresponding to the atmospheric CO 2 concentration of 379± 0. 65 ppm), which is the largest RF among all major forcing factors (The concept of radiative forcing (RF)) • The data show that the changes in the land use greatly contributed to the RFCO 2 value in the amount of about 0. 4 W/m 2 (since the beginning of the industrial era). This implies that the remaining three quarters of RFCO 2 can be attributed to burning fossil fuels, cement manufacturing, and other industrial CO 2 emitters [6].