Life in High-Salinity Environments

Abstract

A great variety of hypersaline environments exist, here defined as environments with salt concentrations exceeding seawater salinity. These include natural inland salt lakes such as Great Salt Lake, Utah, and the Dead Sea, alkaline soda lakes with salt concentrations often close to saturation, and man-made saltern ponds for the production of salt from seawater. There are thalassohaline (seawater-derived) and athalassohaline brines with ionic compositions very different from that of seawater. High salt environments are inhabited by diverse halophilic and/or halotolerant microorganisms belonging to all three domains of life: Archaea, Bacteria, and Eukarya. Many halophilic microorganisms are 'polyextremophiles', able to thrive in environments stressed not only by high salt concentrations but also by extremes of pH, temperature, or both. There are two different strategies that enable microorganisms to live at high salt: some maintain a low-salt cytoplasm and produce organic 'compatible' solutes to provide osmotic balance, while others accumulate molar concentration of KCl intracellularly. Not all physiological types of microorganisms are found up to the highest salinities, and this is probably due to the high energetic cost of osmotic adaptation. At the highest salinities most aerobic heterotrophic activity is due to Archaea of the family Halobacteriaceae, but extremely halophilic Bacteria also exist. The microbial communities inhabiting high salt environments can be studied by culture-dependent and by molecular, culture-independent techniques. Different groups of halophiles have characteristic lipids and pigments, and these can be used as biomarkers for the qualitative and quantitative characterization of the biota of salt lakes, saltern ponds and other hypersaline environments.