Cyclic electron transport around photosystem I and its relationship to non-photochemical quenching in the unicellular green alga Dunaliella salina under nitrogen deficiency

Electron transport in photosystem II (PSII) and photosystem I (PSI) was estimated in terms of chlorophyll fluorescence and changes in P700 redox, respectively, in the unicellular green alga Dunaliella salina in the presence or absence of a nitrogen source in the culture medium. In a nitrogen-containing medium, the quantum yield of PSII (Φ II ) and that in PSI (Φ I ) were at the same level in low light, but cyclic electron transport around photosystem I (CET-PSI) was induced under high light as estimated from an increase in Φ I /Φ II . High light might further enhance the rate of electron transport in PSI by inducing the state 2 transition, in which the distribution of light energy is shifted to PSI at the expense of PSII. Nitrogen deficiency resulted in a decrease in Φ II and an increase in Φ I . As a consequence, the rate of CET-PSI was expected to increase. The high CET-PSI under N deficiency was probably associated with a high level of energy quenching ( q E) formation in PSII.