Over 70% of the Earth’s surface is covered by saline environments. While the salinity of the open ocean is fairly stable, in coastal waters and estuaries, where river freshwater mixes with marine water bodies, salinity is usually highly variable, and, in some situations, such as lagoons or rock pools, evaporation of water can lead to hypersaline conditions. Changes in salinity directly affect water potential and turgor pressure in walled cells. Furthermore, salinity changes alter the intracellular concentration of inorganic ions such as sodium, which can have deleterious effects on processes such as photosynthesis and respiration. Salinity can therefore pose challenges for the physiology and growth of aquatic phototrophs. Algae respond to differences in salinity through a range of physiological mechanisms, including osmotic adjustment involving inorganic ion fluxes and the production of organically compatible solutes. In some cases, acclimation to salinity involves ultrastructural plasticity. Horizontal salinity gradients, found in environments including estuaries, lagoons or semi-isolated systems such as the Baltic Sea, promote the development of physiologically distinct variants of algal species, known as ecotypes, and eventually speciation in algae.