Light distribution, transfer and utilization in the marine red alga Porphyra perforata from photoacoustic energy-storage measurements

Abstract

Light energy utilization in Porphyra perforata was monitored by the photoacoustic method in different conditions of illumination. Auxiliary chlorophyll a fluorescence measurements were made to estimate the fraction of open photosystem II (PS II) reaction centers. These measurements allowed a consistent quantitation of excitation distribution and transfer from PS II to PS I under the physiological conditions used. Maximum energy storage was obtained with modulated light absorbed almost exclusively by the phycobilins (light 2). Modulated light absorbed by chlorophyll a (light I) gave much smaller energy storage (about 1 3 of the maximum), which could be enhanced to the maximum by addition of background light 2. Addition of increasing intensities of background light 1 to modulated light 2 did not initially induce any effect and then decreased the energy storage to about half of the maximum. From the above results and with simple mathematical modelling, numbers were obtained for light distribution and energy transfer parameters. From the enhancement saturation curves of the effect of background light 2 on the energy storage in modulated light 1 we conclude that in state 1 light 2 is exclusively absorbed in PS II and that there is no energy transfer to PS I from open PS II reaction centers. From the value of the energy storage for light 2 in state 1 and the degree of openness of PS II reaction centers it is possible to conclude that energy transfer to PS I occurs from closed PS II reaction centers with a probability approaching 1. In state 2 light 2 is distributed more evenly (approximately in a ratio PS II PS I of 0.55:0.45) either by energy transfer via PS II from open PS II reaction centers, or by direct interactions of the phycobilins and PS I. Comparison of the maximum fluorescence values in the two states favors the second possibility. Energy transfer from PS II units with closed reaction centers occurs again in state 2 with a probability approaching 1. Comparison of energy utilization and oxygen evolution in light 1 relative to light 2 and the inhibitory effect of DCMU, which is complete in light 2 but only partial in light 1, suggests the existence of two types of PS I units: one type is engaged in electron transfer from PS II and the other type specializes in cyclic electron flow. The above quantitative analysis allows to estimate the ratio of the two types of PS I unit to be roughly about 0.3:0.7, respectively.