Revealing the diversity of light-harvesting antenna of in vivo photosystem I

The photosynthetic reaction is driven by the cooperation of two light-excited pigment-protein supercomplexes: photosystem II (PSII) and photosystem I (PSI). The efficiency of the excitation of the two PSs relies on the exquisite organization of their light-harvesting antenna under environmental fluctuations. However, since the antenna-protein composition within cells remains elusive, the in vivo events arising from antenna variations cannot be accurately explored. Here, we implemented the single-pixel excitation-emission spectroscopy of Chlamydomonas cells under 80 K using a cryogenic optical microscope. The antenna variations of in vivo PSI can be exclusively evaluated via this low-temperature spectro-imaging method. The simultaneous acquisition of two types of fluorescence spectra enables the analysis of the intracellular correlation between the PSII/PSI intensity ratio and the chlorophyll-b/a (Chl-b/a) concentration ratio. We found that the Chl-b/a ratio hardly correlated with the PSII/PSI intensity ratio in most cases, suggesting that the in vivo PSI intensity ratio reflects the relative PSI stoichiometry rather than their antenna sizes. More importantly, the analysis of the PSI antenna-related Chl-b concentration within cells reveals a mega-antenna system that is much larger than the antenna sizes of the PSI supercomplexes whose structures have been resolved so far. Such PSI megacomplexes tended to be enriched in the region surrounding the pyrenoids of Chlamydomonas cells. We anticipate the present investigation to be a starting point for directly estimating the arrangements of antenna systems of photosystems at the single-cell scale, which is necessary for a deeper understanding of dynamic in vivo events related to the photosynthetic light-harvesting process.