A New Mechanism for Adaptation to Changes in Light Intensity and Quality in the Red Alga Porphyra perforata

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Abstract
In the red alga P. perforata, the level of chlorophyll fluorescence in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) decreased during illumination of the thallus. This decay was related to the photooxidative activity of photosystem [PS]I. Q, the primary electron acceptor of PS II, became oxidized during the decay of the fluorescence. Reagents which inhibit the back reaction of PS II inhibited the decay. Evidently, when conditions in the chloroplasts of this red alga become too oxidative, excess light energy can be converted to heat as a result of an accelerated back reaction of PS II. This may be one of the mechanisms by which this alga can cope with the high salt and high light conditions that can occur in its natural habitat.