Examination of the microbial diversity in hypersaline lakes of increasing salt concentrations shows that certain types of dissimilatory metabolism do not occur at the highest salinities. Examples are methanogenesis from hydrogen and carbon dioxide or from acetate, dissimilatory sulfate reduction with oxidation of acetate, and autotrophic nitrification. The observations can be explained on the basis of the energetic cost of haloadaptation used by the different metabolic groups and the free-energy change associated with the dissimilatory reactions. All halophilic microorganisms spend large amounts of energy to maintain steep gradients of Na+ and K+ concentrations across their cytoplasmic membrane. Most Bacteria and also the methanogenic Archaea produce high intracellular concentrations of organic osmotic solutes at a high energetic cost. The halophilic aerobic Archaea (order Halobacteriales) and the halophilic fermentative Bacteria (order Halanaerobiales) use KCl as the main intracellular solute. This strategy, while requiring far-reaching adaptations of the intracellular machinery, is energetically more favorable than production of organic compatible solutes. By combining information on the amount of energy available to each physiological group and the strategy used to cope with salt stress, a coherent model emerges that provides explanations for the upper salinity limit at which the different microbial conversions occur in hypersaline lakes.
Bibliographical noteFunding Information:
I thank Carol D. Litchfield (George Mason University, Fairfax, VA.) for helpful comments. This study was supported by a grant from the Israel Science Foundation founded by the Israel Academy of Sciences and Humanities. The study of the Alicante salterns by the author was supported by a grant from the Israel Ministry of Science and the Spanish Ministry of Foreign Affairs – The General Directorate of Cultural and Scientific Relations.
- Salt lakes
- Sulfate reduction