Cyclic and Non-cyclic Photophosphorylation as Energy Sources for Active K Influx in Hydrodictyon africanum

Abstract

Under anaerobic conditions in the light, active K influx in Hydrodictyon africanum is supported by cyclic photophosphorylation. The use of selective inhibitors shows that, in the presence of CO2, a considerable portion of the ATP used by the K pump is supplied by noncyclic photophosphorylation. The rest of the ATP in these conditions comes from cyclic photophosphorylation. This is true under light-limiting as well as light-saturated conditions. If non-cyclic photophosphorylation is inhibited (by removal of carbon dioxide, by the addition of cyanide which interferes with the carboxylation reaction, or by inhibition of photosystem two with DCMU or supplying only far-red light), the K influx at low light intensities is stimulated, and its characteristics become those of a process powered by cyclic photophosphorylation alone. These results are interpreted in terms of a competition for ATP between K influx and CO2 fixation. Implicit in this explanation is a requirement for a switch of excitation energy absorbed by photosystem one from cyclic photophosphorylation to non-cyclic photophosphorylation whenever conditions (presence of CO2and photosystem two activity) allow CO2 fixation to occur. Further evidence for such a switch of excitation energy absorbed by photosystem one was obtained in experiments in which red and far-red light were applied separately and together. It was found that CO2 fixation showed the Emerson enhancement effect, while K influx (in the presence of CO2) shows a ‘de-enhancement’. This suggests that far-red light alone powers cyclic photophosphorylation; if red light is also present, some of the far-red quanta are diverted to non-cyclic photophosphorylation. The nature of the interaction between cyclic and non-cyclic photophosphorylation is discussed in relation to these and other published results.