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The role of bicarbonate ion in the mechanism of photosynthesis

University of British Columbia

Three aspects of the role of HCO[sup -/sub 3] ion in photosynthesis were studied. The first part of the investigation involved the study of HCO[sup -/sub 3] ion as a substrate for photosynthesis in marine benthic algae. Net inorganic carbon assimilation by Sargassum muticum was recorded in the light up to pH 9.9 and at pCO₂ down to less than 5 ppm. Carbon uptake was measured on the basis of changes in the CO₂released by acid from 1 ml samples of the experimental sea water, and also calculated from pCO₂ and pH according to standard tables. The pCO₂ was monitored by infrared gas analysis. This alga assimilated HCO[sup -/sub 3] ion directly in photosynthesis. In a continuing study, carbon uptake was recorded in the light for the following marine algae: Alaria, Costaria, Desmarestia, Enteromorpha, Gigartina, Nereocystis, Porphyra, and Ulva. The change in total inorganic carbon in the experimental water was again determined by acid release. With the exceptions of Porphyra and Desmarestia, the algae used HCO[sup -/sub 3] ion as a substrate for photosynthesis. The relationship of rate of photosynthesis to total inorganic carbon present in seawater was determined for Ulva fenestrata, Iridaea cordata, and Sargassum mutlcum. The naturally occurring carbon concentration was found to be limiting for photosynthesis . In part two, the role of carbonic anhydrase and carbonic ;acid as a proton generating, system for. photophosphorylation was investigated. The compound Diamox (5-acetamido-1,3,4-thiadiazole-2- sulphonamide) is a specific inhibitor of carbonic anhydrase. Diamox inhibits carbon fixation in the light in whole plants and isolated chloroplasts. It also inhibits photosynthetic electron transport, both cyclic and non-cyclic. It was found that the light-induced pH shift in unbuffered chloroplasts was also affected, but that ATP-supported ¹⁴CO₂ fixation by isolated chloroplasts in the dark was not inhibited. These facts led to the conclusion that carbonic anhydrase plays its role in photosynthesis as a supplier of protons (from carbonic acid) to the hydrogen ion pump. Antimycin A stimulates ¹⁴CO₂ fixation by chloroplasts in saturating light. Diamox inhibition of ¹⁴CO₂ fixation in the light is overcome by Antimycin A at both saturating and non-saturating light. This interaction suggests that Antimycin A stimulates ¹⁴CO₂ fixation by changing the "energy charge" status of the chloroplast. During the first seconds of illumination and up to four or five minutes thereafter, the O₂ evolution associated with the beginning of photosynthesis shows a number of transients. The third part of this investigation describes some studies of these transients and the interaction of O₂ and inorganic carbon with regard to the various components. The results are discussed in relation to O₂ effects on photosynthesis in higher plants. A transient heretofore unknown in red algae is also reported.

Text

2011-03-21

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http://hdl.handle.net/2429/32658

Botany

University of British Columbia

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