Salt Enhances Photosystem I Content and Cyclic Electron Flow via NAD(P)H Dehydrogenase in the Halotolerant Cyanobacterium Aphanothece halophytica

To uncover the adaptation mechanisms of photosystems for halotolerance, changes in stoichiometry and activity of photosystems in response to changes of salinities were examined in a halotolerant cyanobacterium, Aphanothece halophytica. Photosynthetic O2 evolution was high even at high salinities. O2 evolution activity increased with increasing external concentration of NaCl, reached a maximum at 1.5 M NaCl, and then decreased. Similar salt dependence was observed for photosystem II activity. On the other hand, photosystem I activity increased concomitantly with increase in salinity. Photoacoustic measurements indicated that appreciable energy storage by photosystem I mediated cyclic electron flow at high salinities. Significant electron donation to photosystem I reaction centres through NAD(P)H-dehydrogenase complexes was observed in high salt media. The contents of cytochrome b6/f and photosystem II were almost constant under various salinity conditions, whereas the levels of chlorophyll α, photosystem I, soluble cytochrome c-553, and NAD(P)H-dehydrogenase increased in the cells grown with high salinities. These results indicate that salt specifically induces an increase of protein levels involving cyclic electron flow around photosystem I that may entail an important role for adaptation of Aphanothece halophytica cells to high salinities.