Electrical nature of the taxis signal in cyanobacteria.

Electrical events after a light-dark stimulus were studied in the multicellular organism Phormidium uncinatum. Normally, such a stimulus causes the gliding trichome to reverse direction. By directing a large light spot on the end of a batch of trichomes and then switching it off, we achieved synchronization of the trichomes, since the "head" is much more sensitive than the "tail." The abrupt disappearance of a uniform light produced a depolarization wave which initiated at the head, as registered by externally applied electrodes. The second stimulus produced a depolarization of the opposite direction, reflecting the reorientation of the trichomes. No electrical response was observed at Ca2+ concentrations less than or equal to 10(-8) M. Factors causing oscillatory reversals, i.e., a combination of Ca2+ and A23187, or a viscous environment also abolished the electrical signal. Changes in an externally applied electrical field (4 V/cm2) had little effect on the motile behavior of P. uncinatum or Oscillatoria princeps. However, in the presence of 5 microM Ca2+-1 microM A23187, all the trichomes reversed synchronously to the anode after a change in polarity of an externally applied electrical field. We suggest that an increased Ca2+ concentration together with a change in delta psi (or delta mu H+) represents the taxis signal in cyanobacteria.