Kinetic and spectral resolution of two components of delayed emission from Chlorella pyrenoidosa

Abstract

1. Decay kinetics of delayed emission from Chlorella pyrenoidosa have been determined with a high degree of precision. The decay in the msec-to-sec interval after excitation can be represented accurately by the sum of two exponential decays—a “fast component” whose intensity-dependent lifetime ranges between 3 msec and 10 msec and a “slow component” whose lifetime ranges between 170 msec and 215 msec. 2. The emission spectrum of the slow component has a peak at 685 nm but lacks the shoulder between 710 and 730 nm seen in the fluorescence emission spectrum. The slow component can be isolated by monitoring the emission at 685 nm for long times (>30 msec) after high or low intensity excitation, or at any time in the msec to sec interval following low intensity (< 0.9 mW/cm 2) excitation. Saturation of the slow component occurs in the low intensity region where oxygen evolution is linear with light intensity. The temperature sensitivity of the slow component indicates involvement of an enzymatic and/or diffusion-limited process. 3. The emission spectrum of the fast component is identical to the fluorescence emission spectrum of the cells, with a peak at 685 nm and a shoulder between 710 and 730 nm. The fast component can be isolated by monitoring the emission at 740 nm. The intensity of the fast component parallels the reduction of System 2 electron acceptors and reaches a maximum level when photochemistry is light saturated. This fact was established by simultaneous measurements of oxygen evolution and delayed emission. 4. The fast component can be selectively inhibited by 3 (3,4-dichlorophenyl)-1,1-dimethylurea, heat or ultraviolet irradiation. The slow component can be selectively inhibited by hydroxylamine or low temperatures. 5. Possible mechanisms resulting in fast and slow component delayed emission are discussed in view of the experimental results.