Final answer:
Single-celled eukaryotes in fresh water survive by using contractile vacuoles and salt pumping mechanisms to regulate water intake and osmotic pressure.
Step-by-step explanation:
Single-celled eukaryotes in fresh water avoid bursting due to the influx of water by using certain organelles called contractile vacuoles. These vacuoles collect excess water and then contract to expel it from the cell.
Another strategy employed by some protists is to pump salts in or out of the cell to adjust solute concentrations and osmotic pressure. This allows the organism to regulate water uptake and avoid bursting in fresh water.