Isotopes as tracers of biogeochemical processes Scott Saleska, 2/11/11 Outline 1. Isotope Definitions and terms a) Isotopes and isotope ratios. b) Kinetic fractionation; thermodynamic fractionation c) Simple illustration with the water cycle 2. CO2 isotopes in photosynthesis a) Photosynthetic discrimination in C3 plants b) C3 vs C4 photosynthesis and the distinction in isotopes c) Measuring isotopic composition of a flux i. Flux composition ii is not the same as concentration composition ii. Keeling plots 1
Radioactive (not stable) 2
Nominal abundances and standards 3
Simple Example Rainfall (note: evaporation prefers the lighter isotope, condensation the heavier) In a closed system, isotope fractionation generates a Raleigh fractionation curve Accumulated product note: based on conservation of mass, the accumulated end product must have exactly the same isotopic content as the initial reactant 4
Raleigh fractionation/distillation curve for water vapor, with changing temperature drops to maintain saturation as vapor content of atmosphere drops Note: direction of curve here is opposite (trending down) from previous slide because condensation prefers heavy isotope Global d18o IAEA 5
Other examples from Biogeochemistry Methanogenesis CH 3 COOH CO 2 + CH 4 C = 25 35 13 C acetate ( 28 ) 13 C CH4 ( 50 60 ) 6
Isotope discrimination by photosynthesis Atmospheric CO 2 stomate 7
diffusion Net biochemical fixation 8
Consider two extremes Atmospheric CO 2 Atmospheric CO 2 Stomates open to atm (c i c a ) : = a + (b a) *1 = b = 27 Stomates closed (c i 0) : = a + (b a) *0 = a = 4.4 diffusion Net biochemical fixation 9
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Photosynthetic Pathway Variation C3 CO 2 fixation by Rubisco; PGA (phosphoglycerate a 3 carbon sugar) is the product of initial carboxylation C4 CO 2 fixation by PEP carboxylase to produce OAA (oxaloacetate a 4 carbon acid); C4 then transported within the leaf, decarboxylation and re-fixation by Rubisco. CAM (crassulacean acid metabolism) CO 2 fixation by PEP carboxylase to OAA, but results in the production of malic acid in the vacuole during the day and decarboxylation and fixation at night Why C4 plants? Problems with C3 photosynthesis Increase in photorespiration - in hot dry conditions - C3 plants conserve water by closing stomates, decreasing intercellular [CO2] - Competition between O2 and CO2 for Rubisco binding site - Photorespiration increases when [O 2 ] / [CO 2 ] inside the leaf increases 11
Why C4 plants? CO2 limitation -- The Compensation Point Compensation points for C3 and C4 differ Net Assimilation = gross photosynthesis - respiration http://www.steve.gb.com/science/photosynthesis_and_respiration.html Light-use efficiency At temperature optima C 4 Leaf Photosynthetic ra ate C 3 C 3 in the absence of photorespiration Photosynthetic Photon Flux Density (PPFD) 12
Light-use efficiency QUANT TUM YIELD OF PHOTO OSYNTHESIS C 3 where photorespiration is negligible C 4 C 3 TEMPERATURE (10 TO 40 o C) C4 Photosynthesis: a 2-step biochemical process What are consequences for isotopes? Stomate CO 2 HCO 3 - Mesophyll CO 2 concentration 100 mol mol -1 C 3 PEP PEP Carboxylase Bundle Sheath CO 2 concentration 2000 mol mol -1 C 3 Rubisco PCR CO 2 (Calvin Cycle) C 4 (OOA) PEP carboxylation step added by C4 pathway C 4 CHO cost benefit TRADE-OFF 2 extra ATP molecules Higher [CO2] at Rubisco site 13
Long-term trends in atmospheric CO2 The rise of vascular plants with roots Ratio of ancient to modern pre- industrial CO2 Amt of carbon in plant biomass Continuous downward trend over last 50 Ma Berner, 1997 Evolution of C4 plants 14
You are what you eat (isotopically) Example from Beer You are what you eat (isotopically) Example from Beer Brooks et al (2002) 15
You are what you eat (isotopically) Example from Beer Brooks et al (2002) Apparent synchronicity in rise in C4 at sites around the world. lant Paleo-Indicators of C4 pl prevalence Teeth of Herbivores Soil carbon Teeth of Herbivores Ehleringer & Monson (1993) 16
Size and 13 C (vs PDB) of Earth s Carbon Reservoirs Rocks (carbonate) = 0 Rocks (organic) -25 Volcanic CO 2 = -5 Atmospheric 13 C Suess Effect [Francey et al., 1999] 17