The cell structure that would prevent bacteria living in salty seawater from rupturing in a freshwater environment is the cell wall.

Here's why:

* Osmosis: When bacteria move from a high salt concentration (seawater) to a low salt concentration (freshwater), water moves into the cell due to osmosis. This is because the cell's internal environment is now hypertonic compared to the surrounding water, meaning it has a higher concentration of dissolved solutes.

* Cell Wall's Role: The cell wall, a rigid outer layer, provides structural support and helps maintain the cell's shape. It acts as a barrier, preventing the cell from swelling excessively and bursting due to the influx of water.

* Other Structures: While the cell membrane also plays a role in regulating what enters and leaves the cell, it's not strong enough to withstand the pressure from the influx of water.

Therefore, the cell wall is the primary structure responsible for preventing the bacteria from rupturing in this scenario.