Specificity of Photochemical Energy Conversion in Photosystem I from the Green Microalga <i>Chlorella ohadii</i>

Primary excitation energy transfer and charge separation reactions in photosystem I (PSI) from the extremophile desert green alga Chlorella ohadii, grown in low light, were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850 nm and in the time range of 50 fs–500 ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band, and compared with similar processes in PSI from the cyanobacterium Synechocystis sp. PCC 6803. When PSI from C. ohadii was excited at a wavelength of 660 nm, the processes of energy redistribution in the light-harvesting antenna of the complex were observed in a time interval of up to 25 ps, while the formation of a stable ion-radical pair P700+A1− was kinetically heterogeneous with characteristic times of 25 and 120 ps. With an alternative variant of excitation into the red edge of the Qy band at a wavelength of 715 nm, in half of the complexes, primary charge separation reactions were observed in the time range of 7 ps. In the rest of the complexes, the formation of the ion-radical pair P700+A1− was limited by energy transfer and occurred with a characteristic time of 70 ps. Similar photochemical reactions in PSI from Synechocystis 6803 were significantly faster: upon excitation at a wavelength of 680 nm, in ~30% of the complexes, the formation of primary ion-radical pairs occurred with a time of 3 ps. Upon excitation at 720 nm, kinetically unresolvable ultrafast primary charge separation was observed in half of the complexes, and the subsequent formation of a P700+A1− ion-radical pair was observed at 25 ps. The photodynamics of PSI from C. ohadii had a noticeable similarity with the processes of excitation energy transfer and charge separation in PSI from the microalga Chlamydomonas reinhardtii; however, in the PSI from C. ohadii slower components in the energy transfer dynamics were also observed.