布朗大学同位素地球化学Carbonisotope.ppt

,*,Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Carbon isotopePart I,Topics for this section,Carbon isotopic fractionation in natural water systems,Carbon isotope variation during photosynthesis in land plants(C,3,C,4,CAM plants),Objectives,Understand the,systematics,of carbon isotopic fractionation in inorganic and organic systems,Know the basis of carbon isotope application in study of carbon cycling and plant ecology,Characteristics of Carbon,Carbon has two stable isotopes:,12,C=98.89%;,13,C=1.11%,Carbon plays a key role in the biosphere,inorganic/organic carbon also exists in a diversity of compounds with different oxidation states,CH,4,:-4,CH,3,-CH,3,:-3,Diamond/graphite:0,CO:+2,CO,2,H,2,CO,3,HCO,3,-,+4,CO,3,2-,CaCO,3,CaMg(CO,3,),2,The,d,13,C values often show similar sequence as the oxidation states:,d,13,C,carbonate,d,13,C,graphite,d,13,C,CH4,Carbon Isotope standards,Standards,13,C,VPDB(primary std),Carbonate,0,NBS-16,CO,2,-41.48,NBS-17,CO,2,-4.41,NBS-18,Carbonate,-5.00,NBS-19,Marble,+1.92,NBS-20,Limestone,-1.06,NBS-21,Graphite,-28.10,NBS-22,Oil,-29.63,Natural variations in,d,13,C values,Excluding carbonate in carbonaceous,chondrite,the terrestrial range of,d,13,C values exceed 100,The Global Carbon Cycle,Carbonate system,CO,2,(g),CO,2,(aq)H,2,CO,3,*,H,2,CO,2,(aq)H,+,+HCO,3,-,(aq),HCO,3,-,H,+,+CO,3,2-,(aq),Bjerrum,plot showing the activities of inorganic carbon species as a function of pH for a value of total inorganic carbon of 10,-3,mol L,-1,.,In most natural waters,bicarbonate ion is the dominant charged,carbonate species in the alkalinity definition,Dissolved inorganic carbon(DIC),In equilibrium with atmospheric CO,2,d,13,C values of DIC vary with pH,as carbonate speciation changes,because different carbonate species(CO,2,(g),CO,2,(aq),HCO,3,-,CO,3,2-,)have different,d,13,C values,Carbonate,13,C fractionation at different temperature,Kinetic C isotope fractionation during photosynthesis,CO,2,+H,2,O,CH,2,O+O,2,(large kinetic fractionation),Two key enzymes,Ribulose-1,5-biphosphate,carboxylase/oxygenase,(,Rubisco,)for C,3,plants(trees,aquatic plants and algae):“,Rubisco,”for short,Phospoenol,pyrurate,carboxylase,(PEP,Carboxylase,)for C,4,plants(tropical grasses,sedges etc),Key isotopic effects of steps leading to CO,2,fixation in C,3,plants,Process,Isotope effect(,),Discrimination,(),Symbol,Diffusion of CO,2,in air through the,stomatal,/pore,1.0044,4.4,a,Net C,3,fixation with respect to,C,i,/C,a,1.02,7,2,7,b,Stomata,Closed,Open,Photosynthetic discrimination(,D,)in C,3,plants(,Farquhar,equation),13,D,=a+(,b-a),C,i,/C,a,a:Fractionation of diffusion,b:Fractionation of,Rubisco,C,i,:,Substomatal,CO,2,C,a,:Atmospheric,CO,2,When,C,i,=0(not realistic for plants),13,D,=a,diffusion only,When,C,i,=C,a,(infinite conductance),13,D,=b,Farquhar,et al.(1982),Aust,.J.Plant,Physiol,.9,121-137,Inside/outside CO,2,ratio and,D,C,i,/C,a,in different tree species and their,d,13,C value,Mean,d,13,C value of C,3,plants,-27,Air CO,2,d,13,C=-7.8,13,D,=27 7.8 19,19=4.4+(27-4.4)C,i,/C,a,C,i,/C,a,=0.64(this is the mean,stomatal,conductance),Photosynthesis rate vs.,C,i,13,D,and C,3,plant water use efficiency,In order for plants to grow,they must fix carbon.,Carbon enters the leaves as carbon dioxide,diffusing through pores in the epidermis called stomata,Increased,stomatal,conductance(g)of leaves causes an increase in partial pressure of CO,2,inside the leaves,C,i,.This usually cases an increase in the rate of CO,2,assimilation,Such an action by plants is a gamble,because while it increases the likelihood of growth and reproductive success,it also increases the probability of desiccation and death(Cowan,1986),Water stressand,d,13,C value,Genetic variations in water use efficiency,The ratio of rate of carbon assimilation A,over the rate of transpiration,E,G,c,:Conductance to diffusion of CO,2,G,w,:Conductance to diffusion of H,2,O.In the air,G,w,/G,c,=1.6,:Difference in intercellular and atmospheric vapor pressure of water,e,i,-e,a,Water Use efficiency,Some carbon fixed during the day are respired by the leaf at night.Hence the net carbon gain is lower.The net water use efficiency,W is proportionate to P,i,/P,a,which can be conveniently determined for large number of plants from analyses of carbon isotope ratios,13,D,and WUE,Selecting crops,Carbon isotope ratios and agriculture,Photorespiration,As its name suggests,RUBISCO,catalyzes two different reactions:,adding CO,2,to,ribulose,bisphosphate,the,carboxylase,activity,adding O,2,to,ribulose,bisphosphate,the,oxygenase,activity,Which one predominates depends on the relative concentrations of O,2,and CO,2,with,high CO,2,low O,2,favoring the,carboxylase,action,high O,2,low CO,2,favoring the,oxygenase,action,The,light reactions of photosynthesis,liberate oxygen and more oxygen dissolves in the,cytosol,of the cell at higher temperatures.Therefore,high light intensities and,high temperatures(above 30C),favor the second reaction,The impact of photorespiration,The uptake of O,2,by RUBISCO undoes the good anabolic work of photosynthesis,reducing the,net productivity,of the plant.This process,uses,O,2,and,liberates,CO,2,as cellular respiration does which is why it is called,photorespiration,.,For this reason,much effort so far largely unsuccessful has gone into attempts to alter crop plants so that they carry on less photorespiration.,The problem may solve itself.,If atmospheric CO,2,concentrations continue to rise,perhaps this will enhance the net productivity of the worlds crops by reducing losses to photorespiration.,C,4,plants,Kranz,Anatomy,most common,but not required,(,Voznesenskaya,et al.,Nature,414,543-546,2001),C,3,and C,4,photosynthetic pathway,3-phosphoglyceric acid(PGA):a C3 compound,oxaloacetic,Acid:a C,4,compound,PEP,carboxylase,C,4,Plants,Over 8000 species of,angiosperms,scattered among 18 different families,have developed adaptations which minimize the losses to photorespiration,They all use a,supplementary,method of CO,2,uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the,Calvin cycle,.Hence these plants are called,C,4,plants.(Plants that have only the Calvin cycle are thus,C,3,plants.),C,4,Plants,C,4,plants have structural changes in their leaf anatomy so that,Their C,4,and C,3,pathways are separated in different parts of the leaf with,RUBISCO sequestered where the CO,2,level is high;the O,2,level low,Examples of C,4,plants:crabgrass;corn(maize);sugarcane;sorghum,Some C,4,plants called,CAM plants,separate their C,3,and C,4,cycles by,time,.More about CAM plants are discussed later,C,4,and CAM plants,Most CAM plants have similar,d,13,C values as C,4,plants,but some CAM plants can switch to C,3,pathway when conditions allow,Crabgrass,C4 plant:,Zea,Mays(corn),Poaceae,Gramineae,of warm regions(by Peter v.,Sengbusch,),C4 plant:,Saccharum,officinarum,(sugar cane),Poaceae,(by Peter v.,Sengbusch,),Spartina,alterniflora,(Smooth,cordgrass,),A salt marsh C,4,plants,Spartina,alterniflora,(Smooth,cordgrass,),C,4,plant fractionation,No carbon isotope fractionation occurs inside the bundle sheath cell,because all,the CO,2,is used,Kinetic isotopic fractionation caused by PEP,carboxylase,5.7,as compared with 22 to 35 by,Rubisco,Therefore,d,13,C values of C,4,plants are more than 15 higher than C,3,plants,Mean,13,C,C4 plant,=-12 (-10 to-16),Modeling C,4,plant fractionation,13,D,(C4)=a+(b,C4,+b,C3,a)P,i,/P,a,b,C4,:PEP,carboxylase,fractionation,5.7,b,C3,:,Rubisco,fractionation,29,:Fraction of CO,2,leak back from bundle sheath cells to,mesophyll,cells;,0.2,Farquhar,1983,Aust,.J.Plant,Physiol,.9,121-137.,Distribution of C,3,and C,4,plants,C4 plants tend to occur in hot,intense sunlight ecosystems,C,4,plants and environmental condition,Why should low,p,CO,2,matter?,Physiological effects,Ecological effects,Decreased carbon accumulation,Increased,stomatal,conductance,From,Maherali,et al.,2000 ESA,Leaf,CO,2,H,2,O,Why should low,p,CO,2,matter?,Physiological effects,Ecological effects,200,400,p,CO,2,Today:Closed Canopy,200,400,p,CO,2,LGM:Open Canopy,Gasp!,Gasp!,C,4,cells in C,3,plants,The ability to use the C,4,pathway has evolved repeatedly in different families of angiosperms.Perhaps the potential is in them all.,A report in the 24 January 2002 issue of,Nature,(by Julian M.Hibbard and W.Paul Quick)describes the discovery that tobacco,a C,3,plant,has cells capable of fixing carbon dioxide by the C,4,path.These cells are clustered around the veins(containing,xylem and phloem,)of the stems and also in the,petioles,of the leaves.In this location,they are far removed from the stomata that could provide atmospheric CO,2,.Instead,they get their CO,2,and/or the 4-carbon,malic,acid in the sap that has been brought up in the xylem from the roots.,If this turns out to be true of many C,3,plants,it would explain why it has been so easy for C,4,plants to evolve from C,3,ancestors.,C,4,Diatoms,On 26 October 2000,Nature,reported the discovery of both the C,3,and C,4,pathways in a marine,diatom,.In this,unicellular,organism,the two paths are kept separate by having the C,4,path in the,cytosol,and the C,3,path confined to the chloroplast.,The presence of a C,4,pathway probably reflects the frequent low concentrations of CO,2,in ocean waters?,CAM plants,These are also C,4,plants but instead of segregating the C,4,and C,3,pathways in different parts of the leaf,they separate them in,time,instead.(CAM stands for,c,rassulacean,a,cid,m,etabolism because it was first studied in members of the plant family,Crassulaceae,.)At night,CAM plants take in CO,2,through their open stomata(they tend to have reduced numbers of them).,The CO,2,joins with PEP to form the,4,-carbon,oxaloacetic,acid.,This is converted to,4,-carbon,malic,acid,that accumulates during the night in the,central vacuole,of the cells.In the morning,the stomata close(thus conserving moisture as well as reducing the inward diffusion of oxygen).,The accumulated,malic,acid leaves the vacuole and is broken down to release CO2.,The CO2 is taken up into the Calvin(C3)cycle.,C,4,and CAM plants,Most CAM plants have similar,d,13,C values as C,4,plants,but some CAM plants can switch to C,3,pathway when conditions allow,CAM plants,These adaptations also enable their owners to thrive in conditions of,high daytime temperatures,intense sunlight,low soil moisture.,Some examples of CAM plants:,cacti,Bryophyllum,the pineapple and all,epiphytic bromeliads,sedums,the ice plant that grows in sandy parts of the,scrub forest biome,Cactus:CAM plants,Switch of C,3,and CAM pathways,These plants only use CAM pathway when under water stress,Other environmental factors affecting abundance of C,4,and CAM,Both C,4,-and CAM plants have higher WUE(water-use efficiency)than C,3,-plants.Both can use light while restricting water loss.,C,4,-plants are about twice as water efficient as C,3,-plants.,Most summer annuals in the southwestern U.S.deserts are C,4,-plants,however.,Seasons are very important.C,4,-plants dont do as well inthe spring and fall.,Different components of plants,Carbon isotope variations in living organisms,Reversed TCA cycle,Less fractionation,