Photophosphorylation in Scenedesmus in vivo: O2 evolution, ATP pools and transients, and phosphate binding during photoreduction of NO3−, NO2− and CO2

Abstract

AbstractThe 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.