The potential difference across the protoplasm of impaled cells of Halicystis is not affected by increase of oxygen tension in equilibrium with the sea water, nor with decrease down to about 1/10 its tension in the air (2 per cent O2 in N2). When bubbling of 2 per cent O2 is stopped, the P.D. drifts downward, to be restored on stirring the sea water, or rebubbling the gas. Bubbling 0.2 per cent O2 causes the P.D. to drop to 20 mv. or less; 1.1 per cent O2 to about 50 mv. Restoration of 2 per cent or higher O2 causes recovery to 70 or 80 mv. often with a preliminary cusp which decreases the P.D. before it rises. Perfusion of aerated sea water through the vacuole is just as effective in restoring the P.D. as external aeration, indicating that the direction of the oxygen gradient is not significant.

Low O2 tension also inhibits the reversed, negative P.D. produced by adding NH4Cl to sea water, 0.2 per cent O2 bringing this P.D. back to the same low positive values found without ammonia. Restoration of 2 per cent O2 or air, restores this latent negativity. At slightly below the threshold for ammonia reversal, low O2 may induce a temporary negativity when first bubbled, and a negative cusp may occur on aeration before positive P.D. is regained. This may be due to a decreased consumption of ammonia, or to intermediate pH changes.

The locus of the P.D. alteration was tested by applying increased KCl concentrations to the cell exterior; the large cusps produced in aerated solutions become greatly decreased when the P.D. has fallen in 0.2 per cent O2. This indicates that the originally high relative mobility or concentration of K+ ion has approached that of Na+ in the external protoplasmic surface under reduced O2 tension. Results obtained with sulfate sea water indicate that Na+ mobility approaches that of SO4 in 0.2 per cent O2. P.D. measurements alone cannot tell whether this is due to an increase of the slower ion or a decrease of the faster ion. A decrease of all ionic permeability is indicated, however, by a greatly increased effective resistance to direct current during low O2. Low resistance is regained on aeration.

The resistance increase resembles that produced by weak acids, cresol, etc. Acids or other substances produced in anaerobiosis may be responsible for the alteration. Or a deficiency of some surface constituent may develop.

In addition to the surface changes there may be alterations in gradients of inorganic or organic ions within the protoplasm, but there is at present no evidence on this point. The surface changes are probably sufficient to account for the phenomena.

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