The effect of CO2on potassium transport byChlorella fusca

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Abstract
The effect of CO2on net K+uptake byChlorella fuscagrown on high CO2levels was examined by passing 1.5% CO2through algal suspensions gassed previously with air or CO2‐free air Addition of CO2in the light caused a large net uptake of K+(initial velocity 4.2–9.2 mmol s−1m−3cells) which decreased the concentration of K+in the supernatant from 0.1–0.2 mol m−3to 3–10 mmol m−3. In the dark and in the presence of 30 mmol m−3DCMU, no effects were found.Measurement or the unidirectional K+fluxes by using86Rb+as a label showed that in the presence of 1.5% CO2, influx of K+was increased by a factor of 2–4 while efflux was inhibited completely.CO2hyperpolarized the membrane potential (determined through TPP+uptake) from –120mV to –130 mV which could not explain the more than 15,000‐fold K+accumulations.In the light, CO2lowered the intracellular pH (determined with DMO) by 0.5 units. In the dark and in the presence of DCMU only, a small acidification of 0.1 units was found.During the first 15 min after addition of CO2the malate content of the cells increased from 0.7 to 1.5 mol m−3packed cells.On the basis of these and earlier results, CO2‐induced net K+uptake is interpreted as a stimulation of an electroneutral ATP‐dependent K+/H+exchange at the plasmalemma. This exchange acts as a ‘pHstat’ by reducing the intracellular acidification caused by production of acidic assimilation products.