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.