Abstract
Low-temperature fluorescence measurements are frequently used in photosynthesis research to assess photosynthetic processes. Upon illumination of photosystem II (PSII) frozen to 77 K, fluorescence quenching is observed. In this work, we studied the light-induced quenching in intact cells of Chlamydomonas reinhardtii at 77 K using time-resolved fluorescence spectroscopy with a streak camera setup. In agreement with previous studies, global analysis of the data shows that prolonged illumination of the sample affects the nanosecond decay component of the PSII emission. Using target analysis, we resolved the quenching on the PSII-684 compartment which describes bulk chlorophyll molecules of the PSII core antenna. Further, we quantified the quenching rate constant and observed that as the illumination proceeds the accumulation of the quencher leads to a speed up of the fluorescence decay of the PSII-684 compartment as the decay rate constant increases from about 3 to 4 ns− 1. The quenching on PSII-684 leads to indirect quenching of the compartments PSII-690 and PSII-695 which represent the red chlorophyll of the PSII core. These results explain past and current observations of light-induced quenching in 77 K steady-state and time-resolved fluorescence spectra.
Highlights
Photosynthesis starts with the absorption of a photon by a light-sensitive molecule, typically chlorophyll
Exposure of the C. reinhardtii cells frozen to 77 K to prolonged excitation light decreases their fluorescence emission between 683 and 704 nm which primarily originates from Photosystem II (PSII) (Fig. 2)
Our results present in the current work are in line with these previous findings: upon prolonged illumination of C. reinhardtii cells frozen to 77 K, we observe that the yield of the PSII-dominated emission band in the component characterized by a lifetime of 2.7 ns drops (Fig. 2)
Summary
Photosynthesis starts with the absorption of a photon by a light-sensitive molecule, typically chlorophyll. In the late 1950s, these studies were extended to cryogenic temperatures (Brody 1958). The resulting higher spectral resolution revealed the existence of emission bands which were not detected in photosynthetic samples at ambient temperatures and which originated from Photosystem I (PSI) (Brody 1958) and Photosystem II (PSII) (Litvin et al 1960; Govindjee and Yang 1966), two major photosynthetic pigment–protein complexes that convert light. The excitation energy leads to a charge separation between a chlorophyll molecule, the PSII primary donor P680, and a pheophytin molecule, Pheo. The primary electron-transfer pathway is completed when the oxidized P680 (P680+) is reduced by the S-state cycle via the redox-active tyrosine residue YZ (Schweitzer and Brudvig 1997).
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