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

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, O₂ was consumed after a lag time of about 250 ms. The comparison of the O₂ 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 O₂ is consumed during the fluorescence rise and that O₂ uptake is initiated approximately when the maximum level of fluorescence P is reached. Mass spectrometry measurements of O₂ exchange (using ¹⁸O₂) were performed during dark to light transition with DCMU-untreated Chlorella cells. Under these conditions, O₂ 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 O₂ starts nearly at the same time that the fluorescence P-S decline. On the other hand, we show that the reduction of CO₂ does not interfere in the fluorescence O-I-D-P-S transient. We found the same apparent affinity for O₂ (about 57 μM) for both the fluorescence P-S decline and the reduction of O₂. 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 O₂. 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 O₂. 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 O₂ or CO₂. 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 O₂, and therefore explains the P-S fluorescence decline. By accepting electrons before the site of regulation and mediating rapid O₂ reduction, parabenzoquinone avoids the accumulation of electrons and therefore inhibits the D-P fluorescence rise.