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

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Mass spectrometric techniques were used to study several aspects of the competition between 02 and species of inorganic carbon for photosyntheticafly generated reducing power in the green alga, Scenedesmus.In contrast to wild type, no appreciable light-driven 02 uptake was observed in a mutant lacking photosystem I. It is concluded that the carbon cycle-independent reduction of 02 occurs at the expense of photosystem Igenerated reducing equivalents.The commonly observed differences between C02-grown and air-grown Scenedesmus with respect to CO2 uptake and glycolate formation cannot be ascribed to differences in their capacity for light-driven 02 uptake. There were no intrinsic differences found in 02 uptake capacity between the two physiological types under conditions in which CO2 was saturating or CO2 uptake was inhibited. It was only under CO2-limited conditions that pronounced differences between the two physiological types were observed. This fact suggests that differences in 02 metabolism and sensitivity between the two types really reflect differences in their capacity to assimilate inorganic carbon; in this respect they are analogous to C3 and C4 plants.The hypothesis that air-grown Scenedesmus can assimilate HCO3 by directly monitoring the time course of dissolved C02, 02 uptake, and 02 evolution in illuminated algal suspensions at alkalne pH was tested.Inasmuch as the measuring technique employed was fast compared to the nonenzymic equilibration of the inorganic carbon species, it was possible to determine the degree to which the CO2 concentration deviated from equilbrium (with the other inorganic carbon species) during the course of illumination. Tbe observed kinetics in air-grown and COrgrown algae in the presence and absence of carbonic anhydrase, and a comparison of these kinetics with theoretical (computer-generated) time courses, support the idea that air-adapted algae are able to assimilate HCO3-actively at a high rate. The data suggest that these algae preferentially assimilate CO2 and supply the balance of their needs by taking up HC03-. Since takes place by a process that is independent of carbon cycle (18,19).In the experiments described here, we examined several aspects of this C02-02 competition. By monitoring the time course of U., E., and Uc, we were also able to infer the uptake of HCO3 , and the degree to which this process supplemented U,.MATERIALS AND METHODS The mass spectrometer inlet system, data acquisition and processing procedures, and assay techniques used in these experiments were similar to those used in previous experiments (18,20).Autotrophic cultures of Scenedesmus obliquus (Gaffron strain D3) were maintained in Roux bottles containing 500 ml inorganic medium (14) Although we observed some striking differences between the autotrophically grown and heterotrophically grown algae (for example, a 4-to 5-fold difference in Chl content per cell volume), there was little apparent difference in photosynthetic capacity on a Chl basis. Thus, the lack of light-driven UO ...