Photophosphorylation in Scenedesmus in vivo: O2 evolution, ATP pools and transients, and phosphate binding during photoreduction of NO3−, NO2− and CO2 |
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Authors: | C.-M. LARSSON |
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Affiliation: | Institute of Botany, University of Stockholm, S-106 91 Stockholm, Sweden |
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Abstract: | The relation between light-induced electron transport with NO3?, NO2? or CO2 as acceptors, ATP pools and transients in dark-light-dark transitions, and phosphate uptake was examined in phosphorus-starved cells of Scenedesmus obtusiusculus Chod. Net O2 evolution at saturating light was around 6 μmol × (mg chlorophyll × h)?1 in the absence of any acceptor, but reached average rates of 21, 65 and 145 μmol × (mg chlorophyll × h)?1 upon additions of 5 mM KNO3, KNO2 and KHCO3, respectively. The apparent rate of photophosphorylation in transition experiments was only a few percent of the rate calculated from CO2-dependent O2 evolution. Blocking non-cyclic electron transport with DCMU inhibited phosphate assimilation, but acceleration of non-cyclic electron flow by addition of NO3? or NO2? did not stimulate phosphate assimilation as compared to the situation without an acceptor. A functional non-cyclic system might primarily be needed for an efficient shuttle transfer of ATP from the chloroplast to the cytoplasm. An inhibition of the non-cyclic system due to lack of reducible substrates accelerates the cyclic system and thus indicates a regulation mechanism between the two systems. |
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Keywords: | Algae ATP CO2 fixation O2 evolution nitrate nitrite phosphate |
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