The requirement of high salt (NaCl) concentrations for growth of microorganisms. Microorganisms (mainly bacteria) can be classified by their physiological tolerance to salt. Most normal eubacteria, such as Escherichia coli and Pseudomonas fluorescens, and most fresh-water microorganisms, are nonhalophiles (best growth at less than 1.2% NaCl). Slight halophiles (1.2–3% NaCl) include many marine microorganisms. Moderate halophiles (3–15% NaCl) include Vibrio costicola, Paracoccus halodenitrificans, and many others. Borderline extreme halophiles (9–25% NaCl) include the photosynthetic bacterium Ectothiorhodospira halophila, the actinomycete Actinopolyspora halophila, and the halophylic archaebacteria Halobacterium volcanii and H. mediterranei. Extreme halophiles (require at least 10% NaCl; optima 15–30% NaCl) are Halobacterium salinarium and Halococcus morrhuae. See Methanogenesis (bacteria)
The halophilic aerobic archaebacteria give a striking red color to hypersaline waters. They are found in the Dead Sea, the Great Salt Lake, Lake Magadi in Kenya, and other alkaline salt lakes, and in solar salterns where salt is prepared by evaporating seawater. Their red color is due to carotenoid pigments (bacterioruberins), which seem to protect them from strong sunlight in their natural environments. See Carotenoid
Microorganisms that live in high concentrations of salt or other solutes do not exclude solutes from the interior of the cell. However, the internal solute composition is quite different from the outside composition. Dunaliella species have internal glycerol concentrations corresponding to the external concentration of NaCl. Other salt-tolerant algae and yeasts also have high internal concentrations of glycerol, or other polyols. Solutes which maintain osmotic equilibrium between inside and outside of the cell without interfering with the cell's physiological processes are called compatible solutes. See Osmoregulatory mechanisms
Mechanisms of adaptation to a highly saline environment have been best characterized in aerobic halophilic bacteria whose enzymes are able to function in high salt concentrations; indeed, most of them require such salt concentrations for activity, stability, or both. For a number of enzyme systems, KCl rather than NaCl is required. Other parts of the cells of these bacteria also require high salt concentrations for function or stability. Halobacteria lyse and their cell walls may completely dissolve unless salt concentrations are high. NaCl is specifically required for active transport of ions and nutrients in all halophilic bacteria.