6.2.5: Water availability

Water is the solvent of life (Madigan et al., 2003). A solvent is a substance that can dissolve other molecules or compounds, which we refer to as solutes. Many of the reactants and products in microbial reactions are soluble in water, the main component of a cell’s cytoplasm. Water also helps microbial reactions occur by serving as transport agent for reactants and products (Konhauser, 2007).

The availability of water for these needs depends not only on the amount of water present (i.e., how wet or how dry) but also the salinity of the water (i.e., the concentration of solutes that are dissolved in the water) (Madigan et al., 2003). Thus, environmental processes such as evaporation, mixing of water masses, and addition of salt mass to aquatic habitats (e.g., road salt) all have the potential to alter the availability of water for microbial populations.

The salinity of an environment affects pressure within cells. Typically, the cytoplasm of cells has higher salinity than the environment (Madigan et al., 2003). This difference in solute concentrations favors flow of water into the cell through a process known as osmosis, which gives the cell a positive pressure. As the salinity of an environment increases, however, it can eventually become more saline than the cytoplasm, which makes it favorable for water to flow out of the cell.

Most microorganisms are not able to grow in environments with high salinity (Madigan et al., 2003), and accordingly humans have long used salt as a food preservative. Halotolerant microorganisms are those that can tolerate some increase in salinity but typically grow best at low salinity (Madigan et al., 2003). Halophilic microorganisms require elevated salinity, with the amount depending on the species (Madigan et al., 2003). These microorganisms can adapt to high salinity by making their cytoplasm saltier, an approach known as the salt-in strategy (Oren, 1999). Cells can also maintain low salt concentrations in their cytoplasm and balance osmotic pressure with organic-compatible solutes (Oren, 1999).

---