Oxygen photoreduction and variable fluorescence during a dark-to-light transition in Chlorella pyrenoidosa

Abstract

When dark-adapted (5 min in the dark) Chlorella cells were deposited on a bare platinum electrode, treated with DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and illuminated, O 2 was consumed after a lag time of about 250 ms. The comparison of the O 2 consumption kinetics with the fluorescence O-I-D-P-S transition (the fast change in chlorophyll fluorescence which occurs after the onset of illumination of dark-adapted algae and is over within 2 s) observed in untreated algae indicates that no O 2 is consumed during the fluorescence rise and that O 2 uptake is initiated approximately when the maximum level of fluorescence P is reached. Mass spectrometry measurements of O 2 exchange (using 18O 2) were performed during dark to light transition with DCMU-untreated Chlorella cells. Under these conditions, O 2 reduction began after a lag time (about 200–400 ms) and stopped after about 5 s of illumination. The above experiments clearly show that the reduction of O 2 starts nearly at the same time that the fluorescence P-S decline. On the other hand, we show that the reduction of CO 2 does not interfere in the fluorescence O-I-D-P-S transient. We found the same apparent affinity for O 2 (about 57 μM) for both the fluorescence P-S decline and the reduction of O 2. At least three consecutive short (2 μs) saturating flashes were required to affect the fluorescence transient significantly and also to induce a significant uptake of O 2. Moreover, parabenzoquinone, an artificial Photosystem I electron acceptor, inhibited both the fluorescence D-P rise and the 250 ms lag time observed in the reduction of O 2. We conclude from the above results that in the early stages of the illumination of dark-adapted algae, some Photosystem I electron acceptors are in an inactive form. In this form, the electron transport chain is unable to reduce either O 2 or CO 2. This would lead to the accumulation of electrons on the Photosystem II acceptors (principally Q − A and the plastoquinone pool) and therefore explains the fluorescence D-P rise. The light activation, probably achieved through the reduction of at least two electron acceptors, first allows the reduction of O 2, and therefore explains the P-S fluorescence decline. By accepting electrons before the site of regulation and mediating rapid O 2 reduction, parabenzoquinone avoids the accumulation of electrons and therefore inhibits the D-P fluorescence rise.