Variable fluorescence of photosystem I particles and its application to the study of the structure and function of photosystem I

Modulation of the fluorescence yield in heliobacterial cells by induction of charge recombination in the photosynthetic reaction center

Heliobacteria contain a very simple photosynthetic apparatus, consisting of a homodimeric type I reaction center (RC) without a peripheral antenna system and using the unique pigment bacteriochlorophyll (BChl) g. They are thought to use a light-driven cyclic electron transport pathway to pump protons, and thereby phosphorylate ADP, although some of the details of this cycle are yet to be worked out. We previously reported that the fluorescence emission from the heliobacterial RC in vivo was increased by exposure to actinic light, although this variable fluorescence phenomenon exhibited very different characteristics to that in oxygenic phototrophs (Collins et al. 2010). Here, we describe the underlying mechanism behind the variable fluorescence in heliobacterial cells. We find that the ability to stably photobleach P800, the primary donor of the RC, using brief flashes is inversely correlated to the variable fluorescence. Using pump-probe spectroscopy in the nanosecond timescale, we found that illumination of cells with bright light for a few seconds put them in a state in which a significant fraction of the RCs underwent charge recombination from P800 (+)A0 (-) with a time constant of ~20 ns. The fraction of RCs in the rapidly back-reacting state correlated very well with the variable fluorescence, indicating that nearly all of the increase in fluorescence could be explained by charge recombination of P800 (+)A0 (-), some of which regenerated the singlet excited state. This hypothesis was tested directly by time-resolved fluorescence studies in the ps and ns timescales. The major decay component in whole cells had a 20-ps decay time, representing trapping by the RC. Treatment of cells with dithionite resulted in the appearance of a ~18-ns decay component, which accounted for ~0.6 % of the decay, but was almost undetectable in the untreated cells. We conclude that strong illumination of heliobacterial cells can result in saturation of the electron acceptor pool, leading to reduction of the acceptor side of the RC and the creation of a back-reacting RC state that gives rise to delayed fluorescence.

Chlorophyll fluorescence transients from the diatom Phaeodactylum tricornutum: relative rates of cyclic phosphorylation and chlororespiration

In Phaeodactylum tricornutum cells kept 30 min in the dark, induction of fluorescence showed the well-known levels OIDPSMT. The decrease of MT was the most important when the intensity of excitation light was high. It was mainly due to the photochemical quenching. After addition of DCMU (2 to 20 μM), a quenching qE was still observed: this quenching, cancelled by NH4Cl (2 to 20 mM) is attributed to ΔpH. This qE was also inhibited by antimycin, an inhibitor of cyclic phosphorylation and may be of chlororespiration above plastoquinones. Anaerobiosis also decreased it. We can infer that chlororespiration also plays a part in the formation of the ΔpH in the presence of DCMU. After 30 mn of preillumination in red light, the levels P and M were lower and the quenching in presence of DCMU was no more observed: thus, neither the chlororespiration nor the cyclic phosphorylation were active, unless the activity of ATPase was much more important. So, in diatoms, one at least of the above cited phenomena can be modulated by light.