Protozoans that live in freshwater have greater water potential outside than inside their cells. This means water may enter the cell by osmosis and create internal pressures too great for survival. In naked cells or cells with flexible cell walls, this problem may be solved by the action of the contractile vacuole, which pumps excess water out of cells (2, 4). Contractile vacuoles are sometimes difficult to observe because their action can be hard to see against a background of other organelles, there are multiple planes of focus, and their action can make them periodically invisible. This movie shows several methods for making contractile vacuoles clearly visible.
Some protozoans are inherently easier to observe and photograph. They are larger cells with slower motions; one may be seen in this movie. However, having an eye for spotting contractile vacuoles is still an advantage, i.e., knowing what one is looking for gives one a better chance of seeing it. Having a video camera attachment adds to this advantage because the action of contractile vacuoles may be viewed as slow motion. The protozoan mentioned above is presented at 0.5x natural speed to make the action of the contractile vacuole more prominent. Media which increase viscosity significantly but do not fatally alter water potential are a third useful method for observing microbial actions that are otherwise too rapid to observe. These media include methyl cellulose (1) and polyethylene oxide (3), but the preparations Protoslo (Carolina Biological Supply Company, Burlington, NC) or Detain (Ward's Natural Science, Rochester, NY) may be useful and convenient options. The action of flagella, cilia, and contractile vacuoles can be observed clearly using these materials. The best results are seen when these three methods are combined, i.e., when an organism with a prominent contractile vacuole is immobilized in a viscose mounting medium and video images are recorded. Because the protozoan is generally immobile, images can be presented in fast motion making the action of the contractile vacuole very prominent, as shown in the last frames of this video.
Figure 1: Observing Contractile Vacuoles (video).
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