Freshwater algae have an intracellular osmolarity in excess of that of the medium. The tendency for water to enter the cells is resisted, in cells with a mechanically functional cell wall, by the ability of the wall to withstand a turgor pressure equivalent to the intracellular osmolarity; in cells lacking such a mechanically functional wall the water which enters is expelled by a contractile vacuole (or its functional equivalent). A comparison of the energy costs of cell volume regulation by these two mechanisms has been carried out making energy costings based on the minimum thermodynamic requirement and on plausible mechanisms for the two processes (wall synthesis and contractile vacuole function). This analysis yielded the following conclusions.

(1) Wall synthesis is a ‘growth’ process, while contractile vacuole operation is a’maintenance’process; other things being equal, a cell wall is a preferable mechanism of volume regulation in non-growing cells.

(2) For a 5 μm radius cell with a specific growth rate of 8 × 10−6 s−1 and a 10 μm radius cell with a rate of 4 × 10−6 s−1 the energy input rate for contractile vacuole operation is considerably lower than for wall synthesis on the basis of minimum thermodynamic requirement; the plausible mechanism approach makes the two energy costs quite similar. Decreased growth rates favours the wall over the contractile vacuole mechanism.

(3) Increased cell size (10 μm radius rather than 5 μm) has similar effects on the energy costs of both volume regulation mechanisms, provided a (cell organic weight)−0.32 dependence of intrinsic growth rate is incorporated into the computations.

(4) Increased intracellular osmolarity (at constant extracellular osmolarity) causes a directly proportional increase in energy input rate for the cell wall mechanism, and a more than proportional increase in energy cost for the contractile vacuole mechanism.

(5) A non-spherical shape increases the energy cost (at constant cell volume) for both the cell wall and the contractile vacuole mechanisms.

These conclusions may be used in the interpretation of such phenomena as the lower osmolarity of wall-less than walled cells and the tendency for the resting stages of normally wall-less and flagellate cells to have walls. It appears that the majority of freshwater algae have genetic capabilities to produce both walls and contractile vacuoles for cell volume regulation (e.g, the production of walled cysts by organisms which are normally flagellate, and the production of wall-less motile spores and gametes by normally walled organisms).