Revealing the diversity of in vivo photosystem I light-harvesting antennae.

Abstract

The photosynthetic reaction is driven by the 2 light-excited pigment-protein supercomplexes: photosystem II (PSII) and photosystem I (PSI). Due to the low excitation probability of chlorophylls (Chls), the efficient excitation of the 2 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 reinhardtii 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 2 types of fluorescence spectra enables the analysis of the intracellular association between the PSII/PSI intensity ratio and the Chl-b/a intensity ratio. We found that the Chl-b/a intensity ratio hardly correlated with the PSII/PSI intensity ratio, suggesting that the in vivo PSII/PSI fluorescence intensity ratio reflects the relative PSII/PSI stoichiometry rather than their antenna sizes. More importantly, the analysis of the PSI antenna-related Chl-b contribution within cells reveals a mega-antenna system that has much larger antenna sizes than those of the PSI supercomplexes whose structures have been resolved so far. Such PSI megacomplexes tended to be enriched in the region surrounding the pyrenoids. 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.