FLUORESCENCE INDUCTION KINETICS AS A TOOL TO DETECT A CHLORORESPIRATORY ACTIVITY IN THE PRASINOPHYCEAN ALGA, MANTONIELLA-SQUAMATA

Pulse-modulated fluorescence measuring systems were used to study the fluorescence induction kinetics of the primitive prasinophycean alga, Mantoniella squamata. DCMU-treated cells show a very strong non-photochemical quenching which is insensitive to uncouplers and can not be attributed to state transitions or photoinhibition. In order to analyze the origin for this quenching, different inhibitors of the photosynthetic electron flow were applied. It was found that DCCD, an inhibitor of the ATP-synthase as well as of the cytochrome-c oxidase, enhances the fluorescence quenching, whereas antimycin A, which blocks the cyclic electron flow around PS I, stimulates it. The effect of both inhibitors can be abolished only under the condition of anaerobiosis, when all components of the electron transport chain were reduced. As a consequence of this, we postulate that this non-photochemical quenching is attributed to the redox state of the plastoquinone pool. In DCMU-treated cells it becomes oxidized under saturating light intensities, but will be reduced by the activity of an electron influx from NAD(P)H. This electron flow is considered to create a pH gradient in the dark, which is probably used for ATP synthesis under dark and extremely low incident light conditions. This chlororespiration requires the existence of an intrathylakoid bound cytochrome-c oxidase, which may be a conserved phylogenetic relict in primitive chloroplasts. © 1990.