Structure and behavior of contractile vacuoles inChlamydomonas reinhardtii

Abstract

Summary.The contractile vacuole (CV) cycle of Chlamydomonas reinhardtii has been investigated by videomicroscopy and electron microscopy. Correlation of the two kinds of observation indicates that the total cycle (15 s under the hypo-osmotic conditions used for videomicroscopy) can be divided into early, middle, and late stages, In the early stage (early diastole, about 3 s long) numerous small vesicles about 70-120 nm in diameter 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 vesicles 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, the CV regions contain numerous small vesicles typical of the earliest stage of diastole. Fine structure observations have provided no evidence for a two-component CV system such as has been observed in some other cell types. Electron microscopy of cryofixed and freeze-substituted cells suggests that the irregularity of the profiles of larger vesicles and vacuoles and some other morphological details seen in conventionally fixed cells may be shrinkage artefacts. This study thus defines some of the membrane events in the normal contractile vacuole cycle of Chlamydornonas, and provides a morphological and temporal basis for the study of membrane fusion and fluid transport across membranes in a cell favorable for genetic analysis.

Mating-Induced Shedding of Cell Walls, Removal of Walls from Vegetative Cells, and Osmotic Stress Induce Presumed Cell Wall Genes in Chlamydomonas

“…The intracellular osmotic status is expected to be affected by removal of the wall when performed in TAP medium since this medium with 53 osmol has a lower osmotic strength than that of the C. reinhardtii interior, which is isosmotic with a medium of 120 osmol (Luykx et al, 1997a). Cell wall removal is expected to lead to higher activity of the contractile vacuole and/or cell expansion through water influx, possibly causing the activation of osmosensors and linked signaling pathways.…”

“…For the cytosol of C. reinhardtii, a value of 120 osmol has been determined (Luykx et al, 1997a). Removal of the cell wall in TAP medium (53 osmol) is expected to change the osmotic conditions within the cells due to water influx.…”

“…This, under hypoosmotic conditions, will result in water influx due to turgor reduction. A crucial role for maintenance of cell integrity under these conditions may be assigned to the contractile vacuole that primarily appears to serve as a pump to remove excess water entering the cells by osmosis (Luykx et al, 1997a(Luykx et al, , 1997b.…”

The SEC6 protein is required for function of the contractile vacuole inChlamydomonas reinhardtii

“…1C). During diastole these smaller vacuoles swell and fuse with each other to form again the spherical vacuole at the end of a cycle (Luykx et al, 1997b) (Fig. 1A).…”

“…The structure and function of the CV in Chlamydomonas have been investigated in some detail (Luykx et al, 1997a;Luykx et al, 1997b;Robinson et al, 1998). At the end of diastole the contractile vacuole of Chlamydomonas is spherical, expels the liquid into the medium and the CV fragments into smaller vacuoles (systolic phase; Fig.…”