Abstract
Mass spectrometric techniques were used to study several aspects of the competition between O2 and species of inorganic carbon for photosynthetically generated reducing power in the green alga, Scenedesmus.
In contrast to wild type, no appreciable light-driven O2 uptake was observed in a mutant lacking photosystem I. It is concluded that the carbon cycle-independent reduction of O2 occurs at the expense of photosystem I-generated reducing equivalents.
The commonly observed differences between CO2-grown and air-grown Scenedesmus with respect to CO2 uptake and glycolate formation cannot be ascribed to differences in their capacity for light-driven O2 uptake. There were no intrinsic differences found in O2 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 O2 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 CO2, O2 uptake, and O2 evolution in illuminated algal suspensions at alkaline 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 equilibrium (with the other inorganic carbon species) during the course of illumination. The observed kinetics in air-grown and CO2-grown 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 HCO3−. Since (unlike C4 plants) these algae have no special CO2 pump, and thus have a relatively low affinity for CO2, HCO3− assimilation is the major carbon uptake process at alkaline pH even when the total CO2 is present in millimolar concentrations.
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Selected References
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- BISHOP N. I. MUTATIONS OF UNICELLULAR GREEN ALGAE AND THEIR APPLICATION TO STUDIES ON THE MECHANISM OF PHOTOSYNTHESIS. Rec Chem Prog. 1964 Sep;25:181–195. [PubMed] [Google Scholar]
- Cheniae G. M., Martin I. F. Site of manganese function in photosynthesis. Biochim Biophys Acta. 1968 May 28;153(4):819–837. doi: 10.1016/0005-2728(68)90009-1. [DOI] [PubMed] [Google Scholar]
- Graham D., Reed M. L. Carbonic anhydrase and the regulation of photosynthesis. Nat New Biol. 1971 May 19;231(20):81–83. doi: 10.1038/newbio231081a0. [DOI] [PubMed] [Google Scholar]
- Nelson E. B., Tolbert N. E. The regulation of glycolate metabolism in Chlamydomonas reinhardtii. Biochim Biophys Acta. 1969 Jul 30;184(2):263–270. doi: 10.1016/0304-4165(69)90028-2. [DOI] [PubMed] [Google Scholar]
- Radmer R. J., Kok B. Photoreduction of O(2) Primes and Replaces CO(2) Assimilation. Plant Physiol. 1976 Sep;58(3):336–340. doi: 10.1104/pp.58.3.336. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Radmer R., Kok B. A kinetic analysis of the oxidizing and reducing sides of the O2-evolving system of photosynthesis. Biochim Biophys Acta. 1973 Jul 26;314(1):28–41. doi: 10.1016/0005-2728(73)90061-3. [DOI] [PubMed] [Google Scholar]
- Radmer R., Kok B. Kinetic observation of the system II electron acceptor pool isolated by mercuric ion. Biochim Biophys Acta. 1974 Aug 23;357(2):177–180. doi: 10.1016/0005-2728(74)90058-9. [DOI] [PubMed] [Google Scholar]
- Radmer R., Kok B., Ollinger O. Kinetics and Apparent K(m) of Oxygen Cycle under Conditions of Limiting Carbon Dioxide Fixation. Plant Physiol. 1978 Jun;61(6):915–917. doi: 10.1104/pp.61.6.915. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Thorne S. W., Boardman N. K. The effect of temperature on the fluorescence kinetics of spinach chloroplasts. Biochim Biophys Acta. 1971 Apr 6;234(1):113–125. doi: 10.1016/0005-2728(71)90136-8. [DOI] [PubMed] [Google Scholar]