Light harvesting and state transitions in cyanobacteria

In oxygenic photosynthetic organisms two photosystems, namely photosystem I (PSI) and photosystem II (PSII) independently harvest the sunlight and drive the primary photochemistry. The two photosystems work in series and for the optimal/maximum photosynthetic efficiency their excitations must be balanced. State transitions form a very important regulatory mechanism in oxygenic photosynthetic organism (green plants, algae, cyanobacteria) which contributes to an optimal photosynthetic performance in constantly changing light conditions. Despite the rigorous research on cyanobacterial state transitions for many decades, the systematic understanding of the process is still elusive. The main purpose of this thesis was to resolve the mechanism and structural basis of phycobilisome (PBS)-absorbed and chlorophyll (Chl) a – absorbed excitation-energy (EE) regulation during state transitions in two different species of cyanobacteria, i.e., Synechococcus and Synechocystis. Consequently, a mechanism for the state transitions in cyanobacteria is determined in this work. Furthermore, a salt-induced-quenching method is introduced to probe the interaction of PBSs with membrane and photosystems in vivo and to characterize the conformational states of PBSs and PSII. Time-resolved fluorescence spectroscopy and fluorescence-lifetime-imaging microscopy were used as main techniques. The main findings from all chapters are summarized below.

In chapter 2 light harvesting, excitation-energy transfer (EET) and state transitions in wild-type (WT), ΔApcD and ΔApcF strains of Synechococcus and Synechocystis were probed at physiological conditions. In the ΔApcF strains EET from PBSs to PSII (or probably both photosystems) was found to be significantly impaired as compared to ΔApcD strains. In Synechococcus in both ΔApcD and ΔApcF strains the state transitions were pronounced (slightly stronger than in the WT strain), showing that the impairment of excitation-energy transfer (EET) form PBS to photosystems due to the deletion of ΔApcD and ΔApcF units does not make the state transitions process inefficient. From the analysis of the time-resolved fluorescence data it was concluded that in cyanobacteria, the light harvesting by PSII remains similar in both state I and state II and the PSII fluorescence decrease in state II can be explained by PSII quenching.

In chapter 3 the physiological basis of PSII quenching and the (non-)involvement of PBSs during state transitions was investigated. It was hypothesized that certain subtle differences in the molecular details of PBSs to PSII interaction between Synechococcus and Synechocystis can explain as to how the fluorescence changes attributed to state transitions are inhibited in Synechocystis ΔApcD but not in Synechococcus ΔApcD. An unstable/ transient interaction between PBS and PSII was proposed to allow the reconfiguration of PBS-PSII complexes and reversible conformational changes in PSII during state transitions. Reversible ultrastructural changes in thylakoid membranes were also proposed to induce reversible conformational and organizational changes in PSII complexes. From the kinetic study of the state I to state II transition in pure and potassium-phosphate-incubated cells of Synechococcus it was demonstrated that at the basis of PSII quenching is structural flexibility of PSII core antenna complexes. The potassium-phosphate-induced quenching was shown to be very sensitive to the interaction details of PBSs with PSII and the conformational state of the PSII complexes. With the help of the salt-induced-quenching method it was demonstrated that the apparent differences regarding the mechanism of state transitions between Synechococcus and Synechocystis suggested by their ΔApcD strains are mere artefacts of these mutants.

In chapter 4 the global distribution of photosystems and state transitions were mapped in individual cells of cyanobacterium Synechococcus. In thylakoid membranes, the microdomains relatively rich either in PSI or PSII were found to be stable when a transition from state I to state II was induced in cells. No large scale (sub-micrometre) changes in the distribution of photosystems were observed during state transitions. The ratio of unquenched PSII/quenched PSII was found nearly homogenous throughout the cells despite an inhomogeneous distribution of photosystems. PSI was concluded to have a higher concentration in the inner thylakoids as compared to the outer ones.

In Chapter 5 the characterization of white-light (WL) and far-red light (FRL) photoacclimated (FaRLip) strains of Chlorogloepsis fritschi (CF) and Chroococcidiopsis thermalis PCC 7203 (CT) has shown that the photosynthetic machinery of the FaRLip strains of these species is very heterogeneous. After several weeks of acclimation to far-red light, FRL-photosystems incorporating red-shifted Chls and FRL-bi-cylinderical cores (FRL-BCs) containing red-shifted APC are synthesized. The analysis of the time-resolved fluorescence data shows that the incorporation of far-red APC increases both the absorption cross-section and photochemical efficiency of chlorophyll f-containing PSII.