STRUCTURE AND BEHAVIOR OF CONTRACTILE VACUOLES IN CHLAMYDOMONAS-REINHARDTII

The contractile vacuole (CV) cycle of Chlamydomonas reinhardtii has be en investigated by videomicroscopy and electron microscopy. Correlatio n of the two kinds of observation indicates that the total cycle (15 s under the hypo-osmotic conditions used for videomicroscopy) can be di vided into early, middle, and late stages. In the early stage (early d iastole, about 3 s long) numerous small vesicles about 70-120 nm in di ameter are present. In the middle stage (mid-diastole, about 6 s long) , the vesicles appear to fuse with one another to form the contractile vacuole proper. In the late stage (late diastole, also about 6 s long ), the CV increases in diameter by the continued fusion of small vesic les with the vacuole, and makes contact with the plasma membrane. The CV then rapidly decreases in size (systole, about 0.2 s). In isosmotic media, CVs do not appear to be functioning; under these conditions, t he CV regions contain numerous small vesicles typical of the earliest stage of diastole. Fine structure observations have provided no eviden ce for a two-component CV system such as has been observed in some oth er cell types. Electron microscopy of cryofixed and freeze-substituted cells suggests that the irregularity of the profiles of larger vesicl es and vacuoles and some other morphological details seen in conventio nally fixed cells may be shrinkage artefacts. This study thus defines some of the membrane events in the normal contractile vacuole cycle of Chlamydomonas, and provides a morphological and temporal basis for th e study of membrane fusion and fluid transport across membranes in a c ell favorable for genetic analysis.