Highly efficient split Photosystem I complex in alga Chromera velia

Ondřej Prášila, Erica Belgioa, Stefano Santabarbarab, David Bínac, Miroslava Herbstovác, Eliška Trskováa
a
Centre Algatech, Institute of Microbiology ASCR, Třeboň, Czech Republic; bInstituto di Biofisica CNR, Via Celoria 26, Milano, Italy; cInstitute of Plant Molecular Biology, Biology Centre ASCR, Branišovská 31, České Budějovice, Czech Republic

The trapping efficiency of Photosystem I (PS1) remains extraordinarily high (> 95%) even in the presence of significant energy spread in the antenna. Recently, we have reported that PS1 in apicomplexan-related alga Chromera velia departs from the “canonical” organization of PS1 in many ways [1]. Core reaction centre protein PsaA has been broken into two fragments, which are independently transcribed, translated and assembled into functional PS1. Comparative structure alignment suggested that the putative interface between PsaA1 and PsaA2 is rich in chlorophyll molecules that are coordinated by both parts of the split PsaA. Here we have determined the functional consequences of the modified PS1 structure by quantifying the photochemical trapping. We have measured fluorescence decay lifetimes across the emission band (660-760 nm) in the isolated PS1-LHC1 supercomplex of C. velia. The results are compared with those obtained in the isolated PS1-LHC1 supercomplex of diatom Phaeodactylum tricornutum, which is a phylogenetic relative of C. velia but possess a “canonical”, i.e. non-split, PS1 core complex. In both organism the excited state decay is dominated by lifetime in the range of 16-18 ps, and the average lifetime displays a very small wavelength dependence. It is therefore concluded that the scission of PsaA has no impact on the overall efficiency of charge separation in the PS1.

Figure: Left panel: Decay Associated Spectra of the excited state decay kinetics of PS1-LHCI supercomplex of C. velia. Right panel: Spectral dependence of the average lifetime in the PS1-LHCI. Solid Circles – when considering only the fastest components. Open squares – when considering also the 176 ps component.