Excitation dynamics in Photosystem I from Chlamydomonas reinhardtii. Comparative studies of isolated complexes and whole cells

Abstract

Identical time-resolved fluorescence measurements with ~3.5-ps resolution were performed for three types of PSI preparations from the green alga, Chlamydomonas reinhardtii: isolated PSI cores, isolated PSI-LHCI complexes and PSI-LHCI complexes in whole living cells. Fluorescence decay in these types of PSI preparations has been previously investigated but never under the same experimental conditions. As a result we present consistent picture of excitation dynamics in algal PSI. Temporal evolution of fluorescence spectra can be generally described by three decay components with similar lifetimes in all samples (6-8ps, 25-30ps, 166-314ps). In the PSI cores, the fluorescence decay is dominated by the two fastest components (~90%), which can be assigned to excitation energy trapping in the reaction center by reversible primary charge separation. Excitation dynamics in the PSI-LHCI preparations is more complex because of the energy transfer between the LHCI antenna system and the core. The average trapping time of excitations created in the well coupled LHCI antenna system is about 12-15ps longer than excitations formed in the PSI core antenna. Excitation dynamics in PSI-LHCI complexes in whole living cells is very similar to that observed in isolated complexes. Our data support the view that chlorophylls responsible for the long-wavelength emission are located mostly in LHCI. We also compared in detail our results with the literature data obtained for plant PSI.

Comparison of excitation energy transfer in cyanobacterial photosystem I in solution and immobilized on conducting glass

“…The average fluorescence lifetime decay, τ av , increases from 13.8 to 16.7 ps (Table 1). Interestingly, these two values are almost identical to those found for algal PSI core in the open and closed state, respectively (Giera et al 2014). On the other hand, immobilization of probably mostly closed monomeric PSI on FTO (see below) leads to a significant acceleration of the decay ( τ av  = 10.6 ps).…”

“…3 and 5. In the brackets, there are molecular lifetimes reported in Gobets et al (2001; brackets with symbol “*”), in Giera et al (2010; with symbol “#”; algal PSI core) and in van Stokkum et al (2013; with symbol “&”) as well as average lifetimes reported for algal PSI core in Giera et al (2014; with symbol “$”). Δ λ is a difference between the maxima of red and bulk Chls SASes.…”

“…Both the monomeric and trimeric forms of PSI were measured in two cases: a) P700 in the neutral state (open RC)—addition of 20 µM phenazine methasulfate (PMS) supplemented with 20 mM ascorbic acid; b) P700 in the oxidized state (closed RC)—addition of 3 mM potassium ferricyanide. Examples of using those agents to modulate the RC state can be found in the literature (Byrdin et al 2000; Gobets et al 2001; Giera et al 2014). Before deposition of PSI complexes onto FTO, they were diluted in buffer D without addition of any reducing or oxidizing agents.…”

“…Each experiment was corrected by background subtraction and then analyzed with Glotaran software (Snellenburg et al 2012) using global and target analysis. It was previously demonstrated how the excitation dynamics in PSI can be described using those two analysis approaches (Giera et al 2010, 2014). Mathematical principles of these approaches can be found in van Stokkum et al (2004).…”

“…These forms are the so-called red or long-wavelength Chls (“red” Chls) due to their redshifted absorption. While all red Chls are present in cyanobacterial PSI core complexes, in green algae and plants most of the red Chls are located in LHCI (Croce et al 1998; Giera et al 2014). The number of red Chls and their spectroscopic characteristics are species-dependent (Gobets et al 2001), and they constitute 3–10 % of the total amount of Chls.…”