PROTOPLASMIC POTENTIALS IN HALICYSTIS : III. THE EFFECTS OF AMMONIA

Understanding protoplasmic potentials in marine organisms requires controlled manipulation of internal and external conditions. Prior research has shown that ionic concentrations and pH influence membrane potentials. However, the specific role of ammonia in altering these potentials remains unclear. This gap motivated a detailed investigation into how ammonia affects Halicystis cells. No prior work had resolved the threshold concentrations or mechanisms of potential reversal. The study addresses this by examining the effects of ammonium chloride and pH on protoplasmic potentials. The focus is on how ammonia penetration alters internal pH and membrane behavior. This paper's contribution is to clarify the pH-dependent mechanisms behind potential reversal. The findings may help distinguish ionic effects from pH-driven changes in cellular physiology.

This study aimed to determine how ammonia affects protoplasmic potentials in Halicystis cells. The specific problem is the lack of understanding about the threshold concentrations and mechanisms of potential reversal. The motivation comes from the need to distinguish ionic effects from pH-driven changes. The authors propose that ammonia may influence membrane potentials through pH changes. The study tests this hypothesis by varying ammonium chloride concentrations and pH. The goal is to identify the conditions under which potential reversal occurs. The researchers also seek to determine if the reversal is reversible and repeatable. The study focuses on the relationship between pH, ammonia concentration, and protoplasmic potential.

The researchers altered external and internal conditions of Halicystis cells using ammonium chloride solutions. They varied concentrations from 0.00001 M to 0.01 M in seawater. The experiments were conducted at pH 8.1 to observe potential changes. They measured protoplasmic potentials using standard electrophysiological techniques. The cells were exposed to different concentrations to identify threshold values. The study also tested the effects of pH changes by adjusting seawater to pH 5 and 10.3. The researchers recorded potential differences and time curves during exposure. They repeated the process multiple times on the same cells to assess reversibility and repeatability.

At pH 8.1, no significant effect was observed below 0.0005 M NH(4)Cl. At 0.001 M, a sudden reversal of potential occurred from 68 mV outside positive to 30–40 mV outside negative. The reversal was S-shaped with a slow onset and rapid change. Higher concentrations caused faster reversals and more negative potentials. Recovery occurred when NH(4)Cl was reduced, with a lower threshold than the reversal point. Recovery often showed a positive overshoot. The potential reversal was repeatable without cell damage. The plot of potential against log ammonium concentration showed a 100 mV break at the threshold.

The authors propose that potential reversal is due to undissociated ammonia (NH(3)) rather than ammonium ions. The pH of the sap at the threshold is between 6.0 and 6.5. This matches the pH required for reversal by direct vacuole perfusion. The study shows that pH is the governing factor, not ammonium salt concentration. The linear relationship between sap pH and log NH(3) supports this. The sudden change in potential must be ascribed to a pH-sensitive system in the cell. The findings suggest that ammonia enters cells until equilibrium is reached. The results clarify the distinction between ionic and pH effects in membrane potentials.