Bioenergetic aspects of halophilism

Examination of microbial diversity in environments of increasing salt conce ntrations indicates that certain types of dissimilatory metabolism do not o ccur at the highest salinities. Examples are methanogenesis for H-2 + CO2 o r fi om acetate, dissimilatory sulfate reduction with oxidation of acetate, and autotrophic nitrification. Occurrence of the different metabolic types is cor is elated with the free-energy change associated with the dissimila tory reactions. Life at high salt concentrations is energetically expensive . Most bacteria and also the methanogenic archaea produce high intracellula r concentrations of organic osmotic solutes at a high energetic cost. All h alophilic microorganisms expend large amounts of energy to maintain steep g radients of NA(+) and K+ concentrations across their cytoplasmic membrane. The energetic cost of salt adaptation probably dictates what types of metab olism can support life at the highest salt concentrations. Use of KCI as an intracellular solute, while requiring far-reaching adaptations of the intr acellular machinery, is energetically more favorable than production of org anic-compatible solutes. This may explain why the anaerobic halophilic ferm entative bacteria (order Haloanaerobiales) use this strategy and also why h alophilic homoacetogenic bacteria that produce acetate from H-2 + CO2 exist whereas methanogens that use the same substrates in a reaction with a simi lar free-energy yield do not.