Why can some freshwater organisms adapt to highly saline lakes?

Organisms that inhabit highly saline lakes have evolved specialized adaptations to manage the significant physiological stress caused by a concentrated external medium. Because these environments are often more stressful than freshwater, the biota in inland saline waters typically derive from freshwater ancestors that possess a high capacity for adaptation.1

The primary challenge in saline environments is maintaining internal water and salt balance, as organisms face constant water loss and an influx of ions. To survive, these organisms employ osmoregulation, a physiological process that maintains a stable internal environment despite external fluctuations. Key mechanisms include:23

• Active Ion Transport: Organisms use energy to actively pump inorganic ions, such as sodium and chloride, across cell membranes to maintain salinity balance. This process is vital but energy-intensive, often consuming a large portion of an organism's energy budget.2
• Osmoregulatory Flexibility: Many species are euryhaline, meaning they can tolerate a wide range of external ion concentrations.2
• Behavioral and Structural Adaptations: Beyond physiology, organisms may use behaviors like burrowing to avoid dehydration or develop life cycles that align with seasonally favorable conditions.1

These adaptations allow organisms to thrive in environments where the osmotic pressure would otherwise cause lethal cellular dehydration.21