Conrad W. Mullineaux

State 1-State 2 transitions in the cyanobacterium Synechococcus 6301 are controlled by the redox state of electron carriers between Photosystems I and II

The mechanism by which state 1-state 2 transitions in the cyanobacterium Synechococcus 6301 are controlled was investigated by examining the effects of a variety of chemical and illumination treatments which modify the redox state of the plastoquinone pool. The extent to which these treatments modify excitation energy distribution was determined by 77K fluorescence emission spectroscopy. It was found that treatment which lead to the oxidation of the plastoquinone pool induce a shift towards state 1 whereas treatments which lead to the reduction of the plastoquinone pool induce a shift towards state 2. We therefore propose that state transitions in cyanobacteria are triggered by changes in the redox state of plastoquinone or a closely associated electron carrier. Alternative proposals have included control by the extent of cyclic electron transport around PS I and control by localised electrochemical gradients around PS I and PS II. Neither of these proposals is consistent with the results reported here.

The state 2 transition in the cyanobacterium Synechococcus 6301 can be driven by respiratory electron flow into the plastoquinone pool

State 1‐state 2 transitions in the cyanobacterium Synechococcus 6301 were observed using a modulated fluorescence measurement system. State 1 transitions could be reversed even when PS II turnover was completely inhibited by DCMU. In starved cells, in which the rate of respiratory electron flow was greatly decreased, the extent of this reversion to state 2 was reduced. This suggests that state 2 transitions can be driven by respiratory electron flow as well as by PS II turnover and provides further evidence for the intersection of photosynthetic and respiratory electron transport in cyanobacteria. We propose that excitation energy distribution in this organism is controlled by the redox level of plastoquinone or a closely associated component of the electron transport chain.

Picosecond time-resolved fluorescence emission spectra indicate decreased energy transfer from the phycobilisome to Photosystem II in light-state 2 in the cyanobacterium Synechococcus 6301

Abstract Picosecond time-resolved fluorescence emission spectra were recorded for cells of the cyanobacterium Synechococcus 6301. Fluorescence decay was measured by single-photon timing and kinetic components were resolved by global data analysis. Time-resolved fluoresence decay components were assigned to PS I, PS II and the phycobilisome terminal emitter by comparing spectra for cells with open PS II centres (fluorescence at Fo) with those for cells with closed PS II centres (fluorescence at Fm) for excitation wavelengths of 620 nm and 670 nm. Time-resolved spectra were recorded under these conditions for cells adapted to state 1 or to state 2. The state 2 transition reduced the amplitude of the PS II fluorescence emission by about 60%, with a complementary increase in the apparent amplitude of the emission from the phycobilisome terminal emitter. Similar changes in amplitude were observed for cells at Fo and at Fm. State transitions had no significant effect on the lifetime of PS II fluorescence decay. These results indicate that state transitions alter the extent of energy transfer from the phycobilisome to PS II.

Fluorescence induction transients indicate dissociation of Photosystem II from the phycobilisome during the State-2 transition in the cyanobacterium Synechococcus 6301

Cells of Synechococcus 6301 were adapted to State 1 or to State 2. Fluorescence induction transients on a millisecond time-scale were then recorded in the presence of DCMU, using excitation of different wavelengths primarily in order to excite either the phycobilisome or the chlorophyll a antenna of Photosystem II (PS II). The transients recorded with the phycobilisome-absorbed light indicate a reduction in PS II absorption cross-section during the transition to State 2. The transients recorded using chlorophyll a-absorbed light indicate increased energy transfer away from PS II in State 2, possibly due to increased spill-over of energy from PS II to PS I in State 2. We suggest that the basis of the State-2 transition in phycobilisome-containing organisms is a decreased excitation-energy transfer to PS II reaction centres that results from detachment of PS II from the phycobilisome.

Kinetics of excitation energy transfer in the cyanobacterial phycobilisome-Photosystem II complex

Abstract Fluorescence decays on a picosecond timescale were recorded at various excitation and emission wavelengths for intact cells of the cyanobacterium Synechococcus 6301. Fluorescence decays were measured by single photon timing and resolved into exponential components by global data analysis. Exponential fluorescence decay components were assigned to PS II, PS I and components of the phycobilisome according to their excitation spectra as determined from a time-resolved excitation-emission matrix and according to the effects of PS II reaction centre closure on the lifetimes and relative amplitudes. We have used the experimentally observed decay components from PS II and the phycobilisome core to estimate the effective rate constants for energy transfer within the phycobilisome/PS II complex and for charge separation at PS II. We show that the rate-limiting step for energy migration in the phycobilisome/PS II complex is a relatively slow energy transfer step from the phycobilisome terminal emitters to the PS II core complex. These results also support the idea that adaptation of cells to light-state 2 decreases the extent of energy transfer from phycobilisomes to PS II.

State transitions, photosystem stoichiometry adjustment and non-photochemical quenching in cyanobacterial cells acclimated to light absorbed by photosystem I or photosystem II

Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.

A proportion of photosystem II core complexes are decoupled from the phycobilisome in light-state 2 in the cyanobacterium Synechococcus 6301

Cells of Synechococcus 6301 were briefly exposed to a phycocyanin‐absorbed light in the presence of DCMU. PS II trap closure was then estimated from fluorescence induction measurements with excitation light absorbed predominantly either by chlorophyll or by phycocyanin. In cells adapted to light‐state 2, the exposure to light absorbed by phycocyanin closed only a proportion of the PS II centres that could be closed by exposure to light absorbed by chlorophyll. This distinction was reduced in cells adapted to light‐state 1. We conclude that a proportion of PS II core complexes become decoupled from the phycobilisomes during the transition to light‐state 2.

Photosynthetic energy storage in cyanobacterial cells adapted to light-states 1 and 2. A laser-induced optoacoustic study

We have used laser-induced optoacoustic spectroscopy to investigate photosynthetic energy storage during the first 1.4 μs after a laser excitation flash in cells of the cyanobacterium Synechococcus 6301 adapted to light-states 1 and 2. We find no decrease i energy storage on transition to light-state 2, indicating that energy diverted away from Photosystem II is not quenched but may be transferred to Photosystem I.

Fluorescence induction transients indicate altered absorption cross-section during light-state transitions in the cyanobacterium Synechococcus 6301

Abstract State 1–State 2 transitions in the cyanobacterium Synechococcus 6301 were observed using a lock-in amplifier to detect the fluorescence generated by a modulated excitation beam. Millisecond fluorescence induction transients were recorded for cells in State 1 and State 2. Comparison of the transients suggests that excitation energy distribution in this cyanobacterium is regulated by changes in the absorption cross-section of Photosystem II.

Effect of photosystem II reaction centre closure on fluorescence decay kinetics in a cyanobacterium

Fluorescence decays on a picosecond timescale were recorded by single-photon timing and resolved into exponential components by global data analysis for intact cells of the cyanobacterium Synechococcus 6301 with excitation at 670 nm. Fluorescence decays were measured for cells at F0 (all PS II reaction centres open), at Fm (all PS II reaction centres closed) and at two intermediate levels of PS II reaction centre closure. We found the same set of fluorescence lifetimes to be present at all levels of PS II trap closure; only the relative amplitudes of the various lifetime components changed. We conclude that there is little energy transfer among PS II complexes in this organism and that the variable fluorescence component Fv is principally the consequence of the decreased probability for exciton trapping at individual closed PS II reaction centres.