Microbiol Mol Biol Rev 63(2): 334-348

Bioenergetic Aspects of Halophilism

Division of Microbial and Molecular Ecology, Institute of Life Sciences, and the Moshe Shilo Minerva Center for Marine Biogeochemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

^{Mailing address: Division of Microbial and Molecular Ecology, Institute of Life Sciences, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel. Phone: 972-2-6584951. Fax: 972-2-6528008. E-mail:}li.ca.ijuh.cc.muhs@anero.

Abstract

Examinination of microbial diversity in environments of increasing salt concentrations indicates that certain types of dissimilatory metabolism do not occur at the highest salinities. Examples are methanogenesis for H₂ + CO₂ or from acetate, dissimilatory sulfate reduction with oxidation of acetate, and autotrophic nitrification. Occurrence of the different metabolic types is correlated with the free-energy change associated with the dissimilatory reactions. Life at high salt concentrations is energetically expensive. Most bacteria and also the methanogenic archaea produce high intracellular concentrations of organic osmotic solutes at a high energetic cost. All halophilic microorganisms expend large amounts of energy to maintain steep gradients of NA^{and K}^{concentrations across their cytoplasmic membrane. The energetic cost of salt adaptation probably dictates what types of metabolism can support life at the highest salt concentrations. Use of KCl as an intracellular solute, while requiring far-reaching adaptations of the intracellular machinery, is energetically more favorable than production of organic-compatible solutes. This may explain why the anaerobic halophilic fermentative bacteria (order Haloanaerobiales) use this strategy and also why halophilic homoacetogenic bacteria that produce acetate from H}₂ + CO₂ exist whereas methanogens that use the same substrates in a reaction with a similar free-energy yield do not.

Abstract

ACKNOWLEDGMENTS

I thank Carol D. Litchfield (George Mason University, Fairfax, Va.), Etana Padan and Shimon Schuldiner (The Hebrew University of Jerusalem), and Karlheinz Altendorf and Evert Bakker (University of Osnabrück) for critically reading the manuscript and for invaluable comments.

This study was supported in part by grant 95-00027 from the United States–Israel Binational Science Foundation (BSF, Jerusalem, Israel).

ACKNOWLEDGMENTS

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