The Role of Potassium in the Control of Turgor Pressure in a Gas-Vacuolate Blue-Green Alga

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Abstract
The contribution of K+ accumulation to cell turgor pressure was investigated in the gas-vacuolate blue-green alga Anabaena flos-aquae. The cell turgor pressure, measured by the gas vesicle method, drops in cells suspended in culture medium depleted of K+ but rapidly rises again, by 100 kPa or more, when K+ is resupplied. A similar though rather slower rise in turgor pressure is supported by an equivalent concentration of Rb+. The internal K+ concentration rose from 66 to 91 mM when K+ was supplied at an external concentration of 0.4 mM. This rise was light-dependent. Greater increases in internal K+ concentration and turgor pressure occurred when K+ was supplied at a higher concentration, 3.6 mM. In both cases over 60% of the observed turgor pressure rise could be accounted for by accumulation of K+. The turgor pressure rise supported by light-stimulated K+ uptake can cause collapse of enough of the alga's gas vesicles to destroy its buoyancy. The effect of K+ availability on buoyancy regulation by planktonic blue-green algae is discussed.

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