A Mutant of Chlamydomonas moewusii lacking contractile vacuoles*

SYNOPSIS. An X‐ray induced mutant (S‐16) of Chlamydomonas moewusii Gerloff lacked observable contractile vacuoles. Unlike the wild strain with vacuoles, S‐16 survived only when the osmotic pressure of the medium was above 1.5 atmospheres and was provided by compounds that penetrate cell membranes slowly, like NaCl or sucrose. In solutions of such compounds, when the osmotic pressure was above 2.1 atmospheres, growth of S‐16 was similar to that of the wild strain. S‐16 did not grow at all in isosmotic solutions of ethylene glycol cr other rapidly penetrating compounds. Urea and glycerol, which penetrate only moderately rapidly, allowed an intermediate growth. Thus, S‐16 cells are dependent upon the prevention of net inward diffusion of water. Contractile vacuoles of the wild strain were not seen when cells were suspended in solutions that allowed growth of S‐16. but were visible in isosmotic solutions cf ethylene glycol or ethanol. S‐16 cells ultimately burst in solutions of low osmotic pressure; large vacuole‐like areas were formed but did not contract. The pulsation rate of vacuoles of the wild strain was relatively independent of osmetic pressure; vacuoles became smaller and finally invisible at higher osmotic pressures. It is concluded that the sole essential function of contractile vacuoles in C. moewusii is elimination of water.