Site of Ca²⁺ action in triggering motility in the cyanobacterium spirulina subsalsa

Motility of the marine filamentous cyanobacterium Spirulina subsalsa is both Ca²⁺ and Na⁺ dependent, and replacement of Na⁺ by mannitol arrests it. The data presented suggest that Ca²⁺ interacts with sites on the surface of the cell membrane. The inhibitory effect of dicyclohexylcarbodiimide (DCCD) hints at the possibility that the role of Ca²⁺ may be associated with a membrane bound Ca‐ATPase. Motility is pH dependent, being nil at pH < 6.5 and > 10.0, with an optimum at 8.5. Norepinephrine abolishes most of the inhibitory effect of low pH on motility. Ca²⁺ has an “all‐or‐none” effect on motility that is triggered at 5 mM. Acetylcholine lowers the threshold of Ca²⁺ necessary for triggering motility.