Originally Posted by anmldr
Ok, I learned this a long time ago but will give it a shot - the osmolality of the bact that live in FW and SW are completely different. If you put a FW cell into SW then it will shrink and die because the water will move through the membrane to try and make the surrounding water the same as what is in the cell. It is just the opposite with a SW cell, it would expand and bust.
It would be like putting a FW fish into a SW tank. The fish would die.
You are correct. This is the exact reason why freshwater species (including single cell organisms) are unable to tolerate the high salinity of a saltwater tank. This will also explain why inverts have such a hard time adjusting to salinity changes. Great bit of information, and obviously you have a good memory!