Uphill energy transfer in photosystem I from Chlamydomonas reinhardtii. Time-resolved fluorescence measurements at 77 K

• Published in: Photosynthesis research Vol. 137; no. 2; pp. 321 - 335
• Main Authors: Giera, Wojciech, Szewczyk, Sebastian, McConnell, Michael D., Redding, Kevin E., van Grondelle, Rienk, Gibasiewicz, Krzysztof
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.08.2018; Springer; Springer Nature B.V
• Subjects: Algae; Analysis; Antennas (Electronics); Biochemistry; Biomedical and Life Sciences; Chlamydomonas reinhardtii; Chlorophyll; Energy; Fluorescence; Life Sciences; Light-harvesting complex; Original Article; Photosynthesis; Photosystem I; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Proteins; Trapping; Photosystem I; Excitation energy transfer; Light-harvesting complex I; Time-resolved fluorescence; Red chlorophylls; Chlamydomonas reinhardtii
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/s11120-018-0506-z

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI–LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI–LHCI complexes isolated from the green alga Chlamydomonas reinhardtii . The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI–LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI–LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~ 12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~ 675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.