On the polyphasic quenching kinetics of chlorophyll a fluorescence in algae after light pulses of variable length

Lipid-polymorphism of plant thylakoid membranes. Enhanced non-bilayer lipid phases associated with increased membrane permeability

Earlier experiments, using ³¹ P-NMR and time-resolved merocyanine fluorescence spectroscopy, have shown that isolated intact, fully functional plant thylakoid membranes, in addition to the bilayer phase, contain three non-bilayer (or non-lamellar) lipid phases. It has also been shown that the lipid polymorphism of thylakoid membranes can be characterized by remarkable plasticity, i.e. by significant variations in ³¹ P-NMR signatures. However, changes in the lipid-phase behaviour of thylakoids could not be assigned to changes in the overall membrane organization and the photosynthetic activity, as tested by circular dichroism and 77 K fluorescence emission spectroscopy and the magnitude of the variable fluorescence of photosystem II, which all showed only marginal variations. In this work, we investigated in more detail the temporal stability of the different lipid phases by recording ³¹ P-NMR spectra on isolated thylakoid membranes that were suspended in sorbitol- or NaCl-based media. We observed, at 5°C during 8 h in the dark, substantial gradual enhancement of the isotropic lipid phases and diminishment of the bilayer phase in the sorbitol-based medium. These changes compared well with the gradually increasing membrane permeability, as testified by the gradual acceleration of the decay of flash-induced electrochromic absorption changes and characteristic changes in the kinetics of fast chlorophyll a-fluorescence transients; all variations were much less pronounced in the NaCl-based medium. These observations suggest that non-bilayer lipids and non-lamellar lipid phases play significant roles in the structural dynamics and functional plasticity of thylakoid membranes.

The ratio of single-turnover to multiple-turnover fluorescence varies predictably with growth rate and cellular chlorophyll in the green alga Dunaliella tertiolecta

Marine phytoplankton experience a wide range of nutrient and light conditions in nature and respond to these conditions through changes in growth rate, chlorophyll concentration, and other physiological properties. Chlorophyll fluorescence is a non-invasive and efficient tool for characterizing changes in these physiological properties. In particular, the introduction of fast repetition rate fluorometry (FRRf) into studies of phytoplankton physiology has enabled detailed studies of photosynthetic components and kinetics. One property retrieved with an FRRf is the 'single-turnover' maximum fluorescence (Fm^ST) when the primary electron acceptor, Qa, is reduced but the plastoquinone (PQ) pool is oxidized. A second retrieved property is the 'multiple-turnover' fluorescence (F^MT) when both Qa and PQ are reduced. Here, variations in Fm^ST and F^MT were measured in the green alga Dunaliella tertiolecta grown under nitrate-limited, light-limited, and replete conditions. The ratio of Fm^ST to F^MT (ST/MT) showed a consistent relationship with cellular chlorophyll in D. tertiolecta across all growth conditions. However, the ST/MT ratio decreased with growth rate under nitrate-limited conditions but increased with growth rate under light-limited conditions. In addition, cells from light-limited conditions showed a high accumulation of Qb-nonreducing centers, while cells from nitrate-limited conditions showed little to none. We propose that these findings reflect differences in the reduction and oxidation rates of plastoquinone due to the unique impacts of light and nitrate limitation on the stoichiometry of light-harvesting components and downstream electron acceptors.

On the origin of the slow M-T chlorophyll a fluorescence decline in cyanobacteria: interplay of short-term light-responses

The slow kinetic phases of the chlorophyll a fluorescence transient (induction) are valuable tools in studying dynamic regulation of light harvesting, light energy distribution between photosystems, and heat dissipation in photosynthetic organisms. However, the origin of these phases are not yet fully understood. This is especially true in the case of prokaryotic oxygenic photoautotrophs, the cyanobacteria. To understand the origin of the slowest (tens of minutes) kinetic phase, the M-T fluorescence decline, in the context of light acclimation of these globally important microorganisms, we have compared spectrally resolved fluorescence induction data from the wild type Synechocystis sp. PCC 6803 cells, using orange (λ = 593 nm) actinic light, with those of mutants, ΔapcD and ΔOCP, that are unable to perform either state transition or fluorescence quenching by orange carotenoid protein (OCP), respectively. Our results suggest a multiple origin of the M-T decline and reveal a complex interplay of various known regulatory processes in maintaining the redox homeostasis of a cyanobacterial cell. In addition, they lead us to suggest that a new type of regulatory process, operating on the timescale of minutes to hours, is involved in dissipating excess light energy in cyanobacteria.

Characterization of thylakoid membrane in a heterocystous cyanobacterium and green alga with dual-detector fluorescence lifetime imaging microscopy with a systematic change of incident laser power

Fluorescence Lifetime Imaging Microscopy (FLIM) has been applied to plants, algae and cyanobacteria, in which excitation laser conditions affect the chlorophyll fluorescence lifetime due to several mechanisms. However, the dependence of FLIM data on input laser power has not been quantitatively explained by absolute excitation probabilities under actual imaging conditions. In an effort to distinguish between photosystem I and photosystem II (PSI and PSII) in microscopic images, we have obtained dependence of FLIM data on input laser power from a filamentous cyanobacterium Anabaena variabilis and single cellular green alga Parachlorella kessleri. Nitrogen-fixing cells in A. variabilis, heterocysts, are mostly visualized as cells in which short-lived fluorescence (≤0.1 ns) characteristic of PSI is predominant. The other cells in A. variabilis (vegetative cells) and P. kessleri cells show a transition in the status of PSII from an open state with the maximal charge separation rate at a weak excitation limit to a closed state in which charge separation is temporarily prohibited by previous excitation(s) at a relatively high laser power. This transition is successfully reproduced by a computer simulation with a high fidelity to the actual imaging conditions. More details in the fluorescence from heterocysts were examined to assess possible functions of PSII in the anaerobic environment inside the heterocysts for the nitrogen-fixing enzyme, nitrogenase. Photochemically active PSII:PSI ratio in heterocysts is tentatively estimated to be typically below our detection limit or at most about 5% in limited heterocysts in comparison with that in vegetative cells.

A simple routine for quantitative analysis of light and dark kinetics of photochemical and non-photochemical quenching of chlorophyll fluorescence in intact leaves

Paper describes principles and application of a novel routine that enables the quantitative analysis of the photochemical O-J phase of the variable fluorescence F v associated with the reversible photo-reduction of the secondary electron acceptor QA of photosystem II (PSII) in algae and intact leaves. The kinetic parameters that determine the variable fluorescence F (PP)(t) associated with the release of photochemical quenching are estimated from 10 µs time-resolved light-on and light-off responses of F v induced by two subsequent light pulses of 0.25 (default) and 1000 ms duration, respectively. Application of these pulses allows estimations of (i) the actual value of the rate constants k L and k AB of the light excitation (photoreduction of QA) and of the dark re-oxidation of photoreduced QA ([Formula: see text]), respectively, (ii) the actual maximal normalized variable fluorescence [nF v] associated with 100 % photoreduction of QA of open RCs, and (iii) the actual size β of RCs in which the re-oxidation of [Formula: see text] is largely suppressed (QB-nonreducing RC with k AB ~ 0). The rate constants of the dark reversion of Fv associated with the release of photo-electrochemical quenching F (PE) and photo-electric stimulation F (CET) in the successive J-I and I-P parts of the thermal phase are in the range of (100 ms)(-1) and (1 s)(-1), respectively. The kinetics of fluorescence changes during and after the I-P phase are given special attention in relation to the hypothesis on the involvement of a Δµ H+-dependent effect during this phase and thereafter. Paper closes with author's personal view on the demands that should be fulfilled for chlorophyll fluorescence methods being a correct and unchallenged signature of photosynthesis in algae and plants.