A model of the phototactic reaction chain of the cyanobacterium Anabaena variabilis

The hypothesis has been proposed that in Anabaena variabilis the phototactic reaction sign is regulated by an unknown reaction sign reversal generator which is controlled by the intracellular level of singlet molecular oxygen (¹O₂). This hypothesis is supported by the following findings presented in this paper: Gassing with N₂ and Ar shifts the phototactic transition point at which the positive reaction becomes negative to higher fluence rates. Surprisingly this is true also for gassing with molecular oxygen ³O₂. Since ¹O₂ is produced in photosynthesis, the availability of external molecular oxygen seems not to be important. Apparently, a stream of any gas which is fast enough to remove ¹O₂ from the surface of the Anabaena trichomes decreases the internal ¹O₂ concentration and this way acts on the reaction sign reversal generator. Moreover, several carotenoids such as the water-soluble crocetin and preparations of solubilized -carotene, canthaxanthine and the C₃₀-ester ethyl--apo-8-carotenoate shift the transition point of phototaxis to higher fluence rates by about one order of magnitude. Several tested furan derivatives, such as dimethylfuran, diphenylisobenzofuran, and furfuryl ethanol, are either cytotoxic or not water-soluble at the concentrations necessary for an effective ¹O₂ quenching. Based one these results a model of the phototactic reaction chain of A. variabilis is proposed.Abbreviations DABCO 1,4-diazabicyclo(2.2.2)octane - DMF dimethylfuran - DPBF diphenylisobenzofuranDedicated to Prof. Dr. Gerhart Drews on the occasion of his 60th birthday