Galvanic Stimulation of Volvox globator II. Mechanism of Galvanic Response, an Effect of Calcium Displacement

SYNOPSIS Slight increases or decreases in calcium ions in solutions which supported the growth of Volvox globator colonies caused the colonies to fall to the bottoms of their containers. High speed cinematography (600 frames/sec) showed that the flagella beat normally (21/sec) in balanced electrolyte solutions which have calcium concentrations between 0.5 and 1.0 mM. When colonies were placed in 10.0 or 0.0 mM CaCl2 solutions, flagellar beating disappeared within 1 hr. The cessation of flagellar beating was reversible when colonies were replaced in the balanced solution. The Volvox cell wall has been shown to be a fairly good cation‐exchanger with calcium ions acting as the counterion to the fixed negative change. Colonies that were photopositive and gave a cathodal galvanotaxis responded to DC electrical potentials by producing solution patterns that were indicators of colony electronegativity. Colony resistance to electroosmotic flow was compared in potassium and calcium solutions. When colonies were placed in darkness for 24 hr and stimulated by DC electrical potentials, their cation‐exchange properties became reduced and the cell walls appeared thinner. Application of a high DC electrical potential to dark‐adapted colonies caused the colonies to shrink on their anode sides (anodal contraction). Other workers have found that the flagella on the anodal sides of dark‐adapted colonies ceased beating during DC electrical stimulation. It is hypothesized that the electric current caused an increase of calcium ions on the anodal side of the colony that inhibited the flagellar mechanism of beating on that side. It is also hypothesized that the galvanotaxis associated with light‐adapted (photopositive) colonies was due to calcium displacements in the colony cell walls that affected the flagellar beating on both sides of the colony.