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Ozone Alteration of Membrane Permeability in Chlorella

I. Permeability of Potassium Ion as Measured by ⁸⁶Rubidium Tracer ¹

^a Department of Botany and Plant Sciences, University of California, Riverside, California 92521

The addition of ozone to a suspension of Chlorella sorokiniana causes a rapid loss of K⁺, as measured by efflux of ⁸⁶Rb from prelabeled cells. The efflux of the tracer is stimulated some 15 to 20 times over that of the control. For about 100 microliters per liter ozone, about 25 minutes (6 x 10^–8 moles O₃ delivered per minute) of exposure are required for a 50% depletion of the intracellular K⁺. The stimulation of K⁺ efflux is nearly linearly dependent upon the amount of ozone delivered into the solution. Following short pulses of ozone (lasting 1 to 5 minutes), efflux rates return to the control level but only after about 15 minutes.

While influx of K⁺ is ultimately inhibited by ozone, at low concentrations or for short exposure times the tracer influex is stimulated 100 to 200%. Ozone stimulation of an active pump mechanism is unlikely in view of a concomitant decrease in respiration. Thus, this influx may represent movement of K⁺ along its electrochemical gradient. Assuming that influx and efflux are in steady-state according to the Goldman equation, it was calculated that the membrane potential for K⁺ of –80 to –90 millivolts in control cells drops to –40 millivolts with ozone exposure and is accompanied by a calculated increased permeability to K⁺ of 2- to 3-fold.