Phototactic gliding movement of cyanobacterium Synechocystis sp. PCC 6803

Abstract

The photomovements of two Synechocystis sp. PCC 6803 (Syn6803) and ATCC 27184 (Syn27184) were examined in detail using the computer-aided cyanobacterial motion analysis combined with Pathfinder tracking software. The phototactic and photokinetic speeds of Syn6803 in liquid system were 5 ㎛/min and 15.8㎛/min, respectively, at 3μmol/㎡/s of stimulant white light. Syn6803 senses light direction rather than intensity for phototaxis. Syn27184 showed a weak photokinesis but no phototaxis. Analysis of Syn27184 suggests that the loss of phototaxis results from spontaneous mutation during several years of subculture. When directional irradiation was applied, the cell population of Syn6803 began to deviate from random movement and reached maximum orientation at 5 min after the onset of stimulant white light. Syn6803 showed high sensitivity to the stimulant white light of fluence rates as low as 0.002μmol/㎡/s. Neither DCMU nor DBMIB affected phototactic orientation whereas cyanide and azide inhibited only gliding speed. This result indicates that the phototaxis of Syn6803 is independent of photosynthetic phosphorylation and that its gliding movement is primarily powered by oxidative phosphorylation. When calcium was removed from the solid medium by EGTA, the phototactic gliding was inhibited. The depletion effect of calcium on phototaxis was partially recovered by the addition of divalent cations following the order: $Ca^{2+}, Mg^{2+}, Mn^{2+}, Sr^{2+}$, and $Ba^{2+}$. The phototactic gliding on soft agar was also inhibited by calcium ionophore, A23187, and channel blockers such as lanthanum and verapamil. This result implies that calcium ion is essential for the phototaxis in Syn6803. In the visible wavelength region, 520 nm, 660 nm, and even 730 nm caused positive phototaxis, but 360 nm light induced strikingly negative phototaxis. Therefore, at least two independent photoreceptors may exist to control phototaxis. The photoreceptor for positive phototaxis appears likely t...