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The contractile vacuole allows free-living protists to expel extra water. <b>The contractile vacuole allows free-living protists to expel extra water <br /></b>.
The vacuole contains high concentrations of nitrate ions and is therefore thought to be a storage organelle. Gas vacuoles, which are freely permeable to gas, are present in some species of Cyanobacteria. They allow the bacteria to control their buoyancy.