Kinetics and Apparent K_m of Oxygen Cycle under Conditions of Limiting Carbon Dioxide Fixation

Abstract

A mass spectrometer with a membrane inlet was used to monitor lightdriven 02 evolution, 02 uptake, and CO2 uptake in suspensions of algae (Scenedesmus obHlquus MATERIALS AND METHODSThe mass spectrometer inlet system, data acquisition system, and assay procedures used in these experiments were similar to those described earlier (7,8). Algae were suspended in 0.1 M phosphate buffer (pH 6). The pertinent cell densities are given in the figure legends.The values for 02 uptake (Uo) and evolution (Eo) were obtained from the expressions: Cultures of Scenedesmus obliquus (Gaffron strain D3) were grown under the conditions described previously in this laboratory (2). Ten mm NH4' was used as the nitrogen source to avoid interference due to nitrate reduction. Due to an unavoidable mass spectrometer background at m/e = 44 ('2CO2), measurements of low CO2 concentrations necessitated the use of 13C-labeled algae.To this end, Scenedesmus cells were grown in a closed culture vessel containing 13CO2 generated from 13C-labeled barium carbonate (the initial concentration of CO2 in the gas phase was 5%). Chl concentrations were determined by the methods described in (2). RESULTS AND DISCUSSIONSubstrate Affinity of 03 Cycle (Apparent Kin). As described earlier (7), in the presence of the Calvin cycle inhibitor, iodoacetamide, 02 uptake replaces CO2 uptake and balances 02 evolution, so that there is no net change in 02 concentration. This inhibitor offers the opportunity to study the 02 uptake reaction unemcumbered by the varying and complicating effects of CO2. Figure 1, which is a compilation of a series of experiments similar to those of Figure 2 in reference 7, shows the net rates of 02 uptake (and 02 evolution, the two are mirror images) by Scenedesmus D3 in the presence of iodoacetamide. Note that at 02 concentrations above 30% the rate is maximal and invariant within experimental error (in Fig. 1, Vm. = 33.5 cell vol/hr). Half this rate is observed at an atmospheric abundance of 8% (indicated as Ki, Fig. 1). Five repetitions ofthis set ofexperiments (all made at room temperature and at intensities inducing greater than half-maximal rates) gave similar results.The linear plot (v versus v/02) in the inset of Figure I

Photosynthetic O2 Exchange Kinetics in Isolated Soybean Cells

“…Experi- [30].) This 02 affimity is similar to that previously determined for algae (22,26), but considerably lower than that reported from experiments with broken chloroplasts (2,12). The inherent rate of 02 reduction in soybean cells is considerably lower than some algae, e.g.…”

“…Although widely varied estimates of the apparent affinity of photorespiratory 02 uptake for 02 have been reported, most studies (3,7,14) suggest a relatively low affinity for 02 (>20%). In contrast, measurements of 02 uptake associated with 02 reduction suggest that this process has a higher affinity for 02 (s8-10%) (2,17,22,26).…”

Light dependence of quantum yields for PSII charge separation and oxygen evolution in eucaryotic algae

“…Mehler reaction activity has been observed in a wide range of organisms, including cyanobacteria (Kana 1992(Kana , 1993, Scenedesmus spp. (Radmer and Kok 1976;Radmer and Ollinger 1980;Radmer et al 1978), and higher plants (e.g. Canvin et al 1980;Neubauer and Yamamoto 1994).…”

Hydrogen Peroxide Synthesis in Isolated Spinach Chloroplast Lamellae

“…These reactions were investigated in the presence or absence of catalase, providing a tool to estimate the reduction of 02 to H202 (Mehler reaction) concomitant with NADP reduction. In the presence of 250 micromolar 02, 02 photoreduction, simultaneous with NADP photoreduction, was dependent upon light intensity, ferredoxin, Mn2", NADP, and the extent of coupling of phosphorylation to electron flow.In the presence of an uncoupUng concentration of NH4', saturating Ught intensity (>500 watts/square meter), saturating ferredoxin (10 micromolarity) rate-limiting to saturating NADP (0.2-0.9 milimolarity), and Mn2+ (50-1000 micromolarity), the maxium rates of 02 reduction were [13][14][15][16][17][18][19][20][21][22][23][24][25] H202 (18, 21, 30) and this reductive reaction is associated with PSI activity coupled to noncyclic electron transport (10,15,18,21,23,30). In preparations of NADP-deficient chloroplast lamellae, it has been established that 02 photoreduction results in the forma- (Mehler reaction) (4,(11)(12)(13)19).…”