Effects of sodium azide on phototaxis of the blue-green alga Anabaena variabilis and consequences to the two-photoreceptor systems-hypothesis.

Experiments with sodium azide support the earlier report that two different photoreceptor systems participate in the absorption of the phototactically active light in Anabaena variabilis. The one of them, represented by the phycobiliproteins and chlorophyll a, is responsible for positive and negative phototaxis around 440 nm and between 580 and 700 nm. This system is sensitive to sodium azide which is able to reverse the negative reaction at high fluence rates to a positive one. The second one which absorbs light between 500 and 560 nm and above 700 nm is insensitive to azide. It triggers only negative responses in absence and presence of azide as well. P750 is obviously not a photoreceptor pigment of this system, since there is no indication for its occurrence in Anabaena. Even photobleaching of the photosynthetic pigments at high fluence rates is prevented by azide. The noncyclic photosynthetic electron transport is not severely inhibited by azide because photokinesis is only in part impaired. Therefore, the hypothesis is suggested that the phototactic reaction-sign reversal generator of Anabaena is controlled by the level of an active oxygen species, probably singlet oxygen, which is quenched by azide.