Protoplasmic Potentials in Halicystis. VI. Rôle of Ammonia in Potential Reversal by Perfusion. ∗

Abstract

Earlier papers 1 have described the reversal of the normally positive potential difference across the protoplasm of impaled cells of Halicystis osterhoutii (Bermuda) both by exposure to ammonia (or other weak bases) and by perfusion of the vacuole with more alkaline sap or sea water. The author was then inclined to explain the former effect by the latter, since penetration of ammonia raised the vacuolar pH. One difficulty of this interpretation was that the Californian species, H. ovalis, did not show such reversal on perfusion with sea water at pH 8.0 or higher, although reversing well on exposure to ammonia. This difference between the species now seems reconciled by referring the effects in both cases to the penetration of ammonia into the protoplasm. This is shown in H. ovalis by introducing into the perfusing fluid a trace of NH₄Cl; at pH 5.0, the normal acidity of the sap, no effect is observed, but if sea water now is perfused at pH 8.0, reversal of potential often results. Then as perfusion is continued, and removes the added ammonia, recovery of positive P.D. may result, even at pH 8.0.

Reëxamination of H. osterhoutii in Bermuda shows similar results. Very clear, pale cells, with no evidence of recent reproduction, also do not show reversal of P.D. when perfused with sea water at pH 8.0. This behavior was sometimes found in earlier work, but was then ascribed to imperfect perfusion; such is now definitely not the case, since rapid flow, good mixing in the vacuole, (as shown by tracer dyes), and long continued perfusion were carefully maintained. Certain other cells at first may reverse on such treatment, but later recover positive P.D. on continued perfusion. In both of these cases, addition of at trace of NH₄Cl to the perfusing fluid promptly causes reversal, which can be counteracted by lowering the pH to 6.0 or less:—the effect originally ascribed to pH alone. In such a state also, the P.D. responds to light, in a manner analogous to that produced when ammonia is externally applied.