The Contractile Vacuole as a Key Regulator of Cellular Water Flow in Chlamydomonas reinhardtii

Most freshwater flagellates use contractile vacuoles (CVs) to expel excess water. We have used Chlamydomonas reinhardtii as a green model system to investigate CV function during adaptation to osmotic changes in culture medium. We show that the contractile vacuole in Chlamydomonas is regulated in two different ways. The size of the contractile vacuoles increases during cell growth, with the contraction interval strongly depending on the osmotic strength of the medium. In contrast, there are only small fluctuations in cytosolic osmolarity and plasma membrane permeability. Modeling of the CV membrane permeability indicates that only a small osmotic gradient is necessary for water flux into the CV, which most likely is facilitated by the aquaporin major intrinsic protein 1 (MIP1). We show that MIP1 is localized to the contractile vacuole, and that the expression rate and protein level of MIP1 exhibit only minor fluctuations under different osmotic conditions. In contrast, SEC6, a protein of the exocyst complex that is required for the water expulsion step, and a dynamin-like protein are upregulated under strong hypotonic conditions. The overexpression of a CreMIP1-GFP construct did not change the physiology of the CV. The functional implications of these results are discussed.