|MadSci Network: Zoology|
Thank you for your question. Protozoa are very interesting organisms. (In case you're interested, there's a neat page called The Protist Image Database that has great pictures of different protozoa and descriptions of how they live.)
One problem that all aquatic organisms have to deal with is balancing their internal salt concentration with the salt concentration of the outside water. Salt concentration is important because water flows from areas of low salt concentration into areas of high salt concentration by a method called osmosis. Rolf Marteijn has written how destructive osmosis can be in another MAD Scientist essay.
To prevent changes in water volume due to osmosis, aquatic organisms have evolved methods to move water and salts into or out of their cells. If you placed a freshwater protozoa into water with just a little salt, the protozoa will still live. Osmosis would cause water to flow from inside the protozoa (low salt) out into the surrounding salty water (high salt). The protozoa would begin pumping in water from outside of the cell to keep its water volume from decreasing. Because the water being pumped in is high in salt, the concentration of salt within the protozoa would begin to increase. The protist's salt regulation mechanisms would begin excreting salt to keep the salt concentration from rising too high. So, to adjust to its new salty environment, the freshwater protozoa would begin pumping in water and excreting salt.
However, if you place the freshwater protozoa into a very salty environment, then the protozoa would die. The protozoa could not pump water in fast enough to prevent dehydration. The cell would shrink up, and stop functioning due to the loss of too much water.
Have you ever wondered why humans don't shrink up when they swim in the ocean? If you want help figuring that out, or have any other questions, please keep asking MadSci Network!
Try the links in the MadSci Library for more information on Zoology.