Probing connection of PBS with the photosystems in intact cells of Spirulina platensis by temperature-induced fluorescence fluctuation

Fluorescence inner filters of Arthrospira platensis: Novel perspective for precise fluorescence-based sensors

Microalgae have been reputed as novel biological materials due to their unique structure, surface functionality and optical activity, making them worthwhile agents in biosensing and theranostic applications. However, further scrutiny is required for utilizing them in routine optical techniques due to their complex structure and diverse chemical components. Here, laser induced fluorescence (LIF) features of a bio-compatible microalgae i. e. Arthrospira platensis (Spirulina) have been assessed. Typical fluorescence properties as well as the inner filter effects (IFEs) were examined and revealed to be strongly dependent on concentration, excitation wavelength, and detection geometry as well. IFEs and resulting spectral shifts have been analyzed considering various SP chromophores, reabsorption processes, and resonance energy transfer (RET) mainly from "Carotenoids to Phycobilisomes" as well as "Phycobilisomes to Chlorophyll-a". As a result, LIF spectral shift due to the re-absorption events (secondary-IFE) is introduced as a credible parameter for design of precise fluorescence-based sensors, due to being less dependent on ambient noises. We hope that the findings provide novel features regarding the LIF of Spirulina (SP) that could be utilized to design and develop optical sensors in the field of photonics, material diagnosis and biomedical theranostics.

Revisiting cyanobacterial state transitions

Critical evaluation of “new” and “old” models of cyanobacterial state transitions. Phycobilisome and membrane contributions to this mechanism are addressed. The signaling transduction pathway is discussed.

Modulating energy arriving at photochemical reaction centers: orange carotenoid protein-related photoprotection and state transitions

Photosynthetic organisms tightly regulate the energy arriving to the reaction centers in order to avoid photodamage or imbalance between the photosystems. To this purpose, cyanobacteria have developed mechanisms involving relatively rapid (seconds to minutes) changes in the photosynthetic apparatus. In this review, two of these processes will be described: orange carotenoid protein(OCP)-related photoprotection and state transitions which optimize energy distribution between the two photosystems. The photoactive OCP is a light intensity sensor and an energy dissipater. Photoactivation depends on light intensity and only the red-active OCP form, by interacting with phycobilisome cores, increases thermal energy dissipation at the level of the antenna. A second protein, the "fluorescence recovery protein", is needed to recover full antenna capacity under low light conditions. This protein accelerates OCP conversion to the inactive orange form and plays a role in dislodging the red OCP protein from the phycobilisome. The mechanism of state transitions is still controversial. Changes in the redox state of the plastoquinone pool induce movement of phycobilisomes and/or photosystems leading to redistribution of energy absorbed by phycobilisomes between PSII and PSI and/or to changes in excitation energy spillover between photosystems. The different steps going from the induction of redox changes to movement of phycobilisomes or photosystems remain to be elucidated.

Estimation of relative contribution of "mobile phycobilisome" and "energy spillover" in the light-dark induced state transition in Spirulina platensis

Previously, it was clarified that phycobilisome (PBS) mobility and energy spillover were both involved in light-to-dark induced state transitions of intact Spirulina platensis cells. In this work, by taking advantage of the characteristic fluorescence spectra of photosystem I (PSI) trimers and monomers as indicators, the relative contributions for the "mobile PBS" and "energy spillover" are quantitatively estimated by separating the fluorescence contribution of PBS mobility from that of PSI oligomeric change. Above the phase transition temperature (TPT) of the membrane lipids, the relative proportion of the contributions is invariable with 65% of "mobile PBS" and 35% of "energy spillover". Below TPT, the proportion for the "mobile PBS" becomes larger under lowering temperature even reaching 95% with 5% "energy spillover" at 0 degrees C. It is known that lower temperature leads to a further light state due to a more reduced or oxidized PQ pool. Based on the current result, it can be deduced that disequilibrium of the redox state of the PQ pool will trigger PBS movement instead of change in the PSI oligomeric state.

The origin of the temperature-induced fluorescence fluctuation in Spirulina platensis: temperature-sensitive mobility of PQ molecules

Temperature effects on state transitions have been studied in the cyanobacterium Spirulina platensis. At lower temperatures the time to reach completion took longer and the extent of the state transitions was larger. Effects were limited to the temperature range below the phase transition temperature of the membrane lipids. In the presence of the artificial electron acceptor phenyl-1,4-benzoquinone (PBQ) state transitions became completely temperature-independent. State transitions induced by a change in the light climate or in darkness by a switch from aerobic to anaerobic conditions responded similar to temperature; the occurrence of state transitions solely by a change of the temperature has been excluded. Our conclusion is that the temperature-dependent mobility of plastoquinone molecules in the thylakoid membranes is the intrinsic cause of temperature effects on state transitions.

The state transition mechanism—simply depending on light-on and -off in Spirulina platensis

The state transition in cyanobacteria is a long-discussed topic of how the photosynthetic machine regulates the excitation energy distribution in balance between the two photosystems. In the current work, whether the state transition is realized by "mobile phycobilisome (PBS)" or "energy spillover" has been clearly answered by monitoring the spectral responses of the intact cells of the cyanobacterium Spirulina platensis. Firstly, light-induced state transition depends completely on a movement of PBSs toward PSI or PSII while the redox-induced one on not only the "mobile PBS" but also an "energy spillover". Secondly, the "energy spillover" is triggered by dissociation of PSI trimers into the monomers which specially occurs under a case from light to dark, while the PSI monomers will re-aggregate into the trimers under a case from dark to light, i.e., the PSI oligomerization is reversibly regulated by light switch on and off. Thirdly, PSI oligomerization is regulated by the local H(+) concentration on the cytosol side of the thylakoid membranes, which in turn is regulated by light switch on and off. Fourthly, PSI oligomerization change is the only mechanism for the "energy spillover". Thus, it can be concluded that the "mobile PBS" is a common rule for light-induced state transition while the "energy spillover" is only a special case when dark condition is involved.