What happen when a marine water protozoan is kept in fresh water?

When a marine water protozoan is placed in fresh water, it experiences osmotic stress due to the difference in solute concentration. Freshwater has a lower concentration of solutes compared to the protozoan's internal environment, leading to water entering the cell through osmosis. This can cause the protozoan to swell and potentially burst, as they are not adapted to cope with such a drastic change in osmotic pressure. To survive, they would need to actively regulate their internal conditions or find a way to return to a saltier environment.