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 (qE) formation in PSII.