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30 - 10 ANGERT ET AL.: BIOLOGICAL EFFECTS ON THREE O 2 ISOTOPESthis value in the numerical model, we found that thisdeviation from steady state will cause an increase of 4per meg in the value of 17 D, when it reached a plateau.Thus, 17 D W is 211 per meg instead of the 215 per megcalculated based on the steady state assumption. Thiscorrection for 17 D W is small with respect to other uncertaintiesand does not significantly change the value calculatedfor q 2 (0.511).[54] Another nonsteady state effect results from the diurnalillumination cycle in the terrarium. Photosynthesis tookplace in the few hours of illumination, while the uptakethrough dark respiration continued all day, and as a resultthe O 2 concentration fluctuated. By modeling this conditionin the numerical model, we found that it will cause the 17 Dvalue to fluctuate around the value 17 D BSS . The amplitudeof the fluctuations in 17 D depends on the amplitude of thefluctuations in O 2 concentration. For interval ‘‘High,’’ wecan calculate from the dark periods that dark respirationconsumed about 9% of the O 2 reservoir of the terrarium perday. Using this value and the value of 0.9 for uptake toproduction ratio, the calculated magnitude of the 17 Dfluctuations resulting from the light-dark cycle is ±2 permeg. Again, this value is considerably smaller than theanalytical uncertainty, and therefore this effect can be alsoneglected.5.5. Dependence of 18 E on Illumination and [CO 2 ]:Implication for the Dole Effect[55] The weighted-average 18 e of all the processes in theterrarium can be estimated from the terrarium equivalent ofthe global Dole Effect (equation (14)), which is the value ofLn 18 O of ‘‘BSS’’ versus ‘‘W.’’ The Ln 18 O values of theterrarium air versus the value of the substrate water arepresented in Figure 4b. In the three intervals in which the17 D of the terrarium was constant, the Ln 18 O was almostconstant.[56] In interval ‘‘High,’’ the fractionation (e) in theterrarium according to equation (14) is 18.4 ± 1.8%, ininterval ‘‘Low’’ 23.9 ± 0.5%, and in interval ‘‘Variable’’21.7 ± 0.2%. The fractionation in interval ‘‘High’’ is inagreement with the known fractionation for the COX,18% [Guy et al., 1989], and with the fractionation thatwas calculated for the dark periods 16.7%. This agreementindicates that, as was assumed in section 5.3., theuptake in the terrarium was dominated by COX in interval‘‘High’’ in which the CO 2 concentration was high.[57] The fractionation by COX is 18%, in photorespirationit is 21.7%, and that of AOX is about30% [Ribas-Carbo et al., 2000]. Thus, photorespirationand COX alone cannot explain the high Ln 18 O valuesmeasured in interval ‘‘Low’’ and interval ‘‘Variable.’’These high values seem to indicate that a considerableportion of the uptake was through the AOX. Since ininterval ‘‘Variable’’ there was net production that introducedoxygen with light isotopic composition, the fractionationmust have been even stronger than that calculatedabove for the same interval according to the steady stateassumption ( 21.7%). The fractionation in interval‘‘Low’’ was extremely strong. The relative rate of uptakethrough the AOX in this interval was estimated from theobserved Ln 18 O values in section 5.6. as 41–31% ofgross production.[58] Some enrichment of the terrarium leaf water byevapotranspiration might have contributed to the highLn 18 O. However, since the relative humidity in the terrariumwas 100% this effect was probably very small. In fact,no enrichment was found when we compared the d 18 Oofthe terrarium free water and the terrarium leaf water (datanot shown). However, this result might originate frommeasuring total leaf water, which includes depleted veinwater. Even if we assume that the enrichment at the site ofphotosynthesis was as high as 1%, our main conclusionswill remain the same. The relative rate of the AOX ininterval ‘‘Low’’ will be 19–29%, still a very high figure,and the correction to q P (see section 5.6) will be muchsmaller than the other uncertainties.[59] The strong measured fractionation indicates that theAOX was activated in the same conditions that favorhigh rate of photorespiration-illumination and low CO 2 .This finding is in agreement with the indication for highAOX rates in the light inferred from in situ measurementsin a lake [Luz et al., 2002]. The CO 2 concentrationin the terrarium were very low (150 ppm), much lowerthan in most natural environments. However, since therelative humidity in terrarium was 100% stomatal conductancemust have been high. Consequently, the internalCO 2 concentration in the leaves was similar to that ofmidday in many natural environments. Since strongfractionation occurred not only with the 24 h d 1 illuminationbut also with the 10 h d 1 illumination, which iscloser to the natural cycle, we conclude that the engagementof the AOX in the light is likely also in manynatural systems.[60] In previous models of the Dole Effect, the global rateof the AOX was assumed to be very low and was neglected[Bender et al., 1994; Malaize et al., 1999]. This low rate isbased on measurements of the AOX activity in the dark.However, if the AOX activation is enhanced in illuminatedleaves in natural systems, then its global rate should beconsiderably higher. This higher rate may help to close thegap between the calculated value of the Dole Effect (20.8%[Bender et al., 1994]) and the measured one (23.5%[Kroopnick and Craig, 1972]), and compensate for theweak fractionation recently reported for soil respiration[Angert et al., 2001]. The connection between photorespirationand AOX might also explain past changes in the DoleEffect. Increased rate of photorespiration will be coupledwith an increased rate of AOX. Drier and hotter climate isexpected to cause an increased rate of photorespiration, aswell as more evapotranspiration that will result in 18 Oenriched leaf water. Thus, such climate will cause anincreased Dole Effect by both heavier composition of leafwater and increased rate of photorespiration and the AOX,two processes that have high fractionation relative to that ofCOX.5.6. Estimating the Q of Photorespiration[61] The average q in the terrarium in interval ‘‘Low’’ was0.511. This value and the values found for the AOX andCOX, can be used to estimate the q associated with photo-
ANGERT ET AL.: BIOLOGICAL EFFECTS ON THREE O 2 ISOTOPES 30 - 11respiration by assuming production uptake steady state, anda simple weighted average equation,GP ¼ U P þ U COX þ U AOXð20Þ0:511 ¼ ðq P U P þ 0:516U COX þ 0:514U AOX Þ=GP; ð21Þwhere GP is gross production in the terrarium, U stands foruptake, and the subscript P stands for photorespiration. Inorder to solve equation (20) for q P the rates of the differentuptake pathways in the terrarium relative to gross productionmust be estimated.[62] The gross oxygen production in the terrarium, atinterval ‘‘Low,’’ can be estimated from previous experimentsin the same terrarium [Luz et al., 1999]. In one ofthese experiments the d 18 O of the water was 6.6% (onSMOW scale). As a result, the d 18 O value in steady statewas very different from the initial atmospheric value. At thebeginning of the experiment (days 1–37, illumination of24 h d 1 , as at interval ‘‘Low’’), changes in the d 18 O anddO 2 /Ar of the terrarium were observed, when it approachedsteady state. The changes in the d 18 O and dO 2 /Ar weremodeled by the numerical model described in section 5.4.The best fit to the observed data was reached with uptake togross production ratio of 0.99 and gross production rate(GP) that is 19.2% of the terrarium O 2 reservoir per day.[63] The uptake rate measured in the dark periods was45% of the GP estimated above, and was probably mostlythrough the COX. When the terrarium was illuminated,considerable portion of the total dark respiration must havebeen through the AOX (section 5.5.); hence, the rate ofCOX in interval ‘‘Low’’ was lower than 45% of GP. If weassume a COX of 10–30% of GP, the rate of photorespiration(U P ) can be calculated from the known fractionations ofthe different pathways and the terrarium-fractionation ( 18 e)we evaluated for interval ‘‘Low’’ ( 24%) by equation (20)and a weighted average equation similar to equation (21),24 ¼ ðe P U P þ e COX U COX þ e AOX U AOX Þ=GP:ð22Þ[64] Using fractionation values of 30% for AOX[Ribas-Carbo et al., 2000], 18% for COX [Guy et al.,1989], and 21.8% for photorespiration U P was calculatedas 29–59% of GP. The corresponding rate of AOX is 41–31% of GP. According to these rates and the q values of theCOX and the AOX, the q associated with photorespiration iscalculated as 0.506 ± 0.003. The error margin is based onlyon the uncertainty in the rate of the COX, and neglects theuncertainty in the fractionations of the different pathways.The uncertainty in the fractionation in the COX and photorespirationis small [Guy et al., 1989, 1993]; however theuncertainty in the fractionation of the AOX is larger, about±2% for green tissue [Ribas-Carbo et al., 2000; Robinsonet al., 1992]. Including this uncertainty the q associated withphotorespiration is 0.506 ± 0.005.[65] In photorespiration, O 2 is consumed by two enzymes,Rubisco and glycolate-oxidase. Hence, the q value of photorespirationrepresents the weighted average of the two consumptionprocesses. Further study is necessary in order toderive the q values associated with each of the two enzymes.Figure 6. Analysis of triple isotopic composition (not toscale) of the Last Glacial Maximum (LGM). The point thatrepresents the LGM atmosphere (after correction for lowerrate of stratospheric photochemistry) lies 12 per meg lowerthan the present atmosphere. A change in the global averageq (q b ) will affect the triple isotopic composition of theatmosphere in a way that is similar to the way a change in qaffected the terrarium air (Figure 5). Hence, a change in q bwill cause a change in the atmospheric 17 D with amagnitude of the Ln 18 O difference between atmosphericO 2 and photosynthetic oxygen times the difference in q b .Thus, a change of 0.001 in q b will result in a change ofabout 19 per meg in the atmospheric 17 D. The higher globalrates of photorespiration (PR) in the LGM (27% of globalproduction instead of 24% today) caused lowering of q b thatcan explain 5 per meg of the 12 per meg difference. The restof this difference can be attributed to lower global grossproduction.[66] The q value of 0.506 ± 0.005 we report here forphotorespiration is considerably lower than that of both darkrespiration pathways. In the next section, we will use thisvalue to estimate the effect of photorespiration on the tripleisotopic composition of the atmosphere.5.7. Implication for the Triple Isotopic Composition ofAtmospheric O 2[67] In order to estimate how a change in the globalaverage q of biological processes will affect the tripleisotopecomposition of the atmosphere, we can analyze itin a graphical approach presented in Figure 6, or with arigorous analytical model. In this model, the troposphere(which is well mixed relatively to the lifetime of oxygen init) is represented by one box, and O 2 is exchanged betweenthe biosphere, the troposphere, and the stratosphere. Insteady state we can write the following equation:y Sy T þ y Py U ¼ 0;ð23Þwhere y S is the stratosphere to troposphere flux and y Trepresent the flux from the troposphere to the stratosphere
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